xref: /openbmc/linux/tools/lib/bpf/btf_dump.c (revision 5a158981)

// SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause)

/*
 * BTF-to-C type converter.
 *
 * Copyright (c) 2019 Facebook
 */

#include <stdbool.h>
#include <stddef.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <linux/err.h>
#include <linux/btf.h>
#include "btf.h"
#include "hashmap.h"
#include "libbpf.h"
#include "libbpf_internal.h"

static const char PREFIXES[] = "\t\t\t\t\t\t\t\t\t\t\t\t\t";
static const size_t PREFIX_CNT = sizeof(PREFIXES) - 1;

static const char *pfx(int lvl)
{
	return lvl >= PREFIX_CNT ? PREFIXES : &PREFIXES[PREFIX_CNT - lvl];
}

enum btf_dump_type_order_state {
	NOT_ORDERED,
	ORDERING,
	ORDERED,
};

enum btf_dump_type_emit_state {
	NOT_EMITTED,
	EMITTING,
	EMITTED,
};

/* per-type auxiliary state */
struct btf_dump_type_aux_state {
	/* topological sorting state */
	enum btf_dump_type_order_state order_state: 2;
	/* emitting state used to determine the need for forward declaration */
	enum btf_dump_type_emit_state emit_state: 2;
	/* whether forward declaration was already emitted */
	__u8 fwd_emitted: 1;
	/* whether unique non-duplicate name was already assigned */
	__u8 name_resolved: 1;
	/* whether type is referenced from any other type */
	__u8 referenced: 1;
};

struct btf_dump {
	const struct btf *btf;
	const struct btf_ext *btf_ext;
	btf_dump_printf_fn_t printf_fn;
	struct btf_dump_opts opts;

	/* per-type auxiliary state */
	struct btf_dump_type_aux_state *type_states;
	/* per-type optional cached unique name, must be freed, if present */
	const char **cached_names;

	/* topo-sorted list of dependent type definitions */
	__u32 *emit_queue;
	int emit_queue_cap;
	int emit_queue_cnt;

	/*
	 * stack of type declarations (e.g., chain of modifiers, arrays,
	 * funcs, etc)
	 */
	__u32 *decl_stack;
	int decl_stack_cap;
	int decl_stack_cnt;

	/* maps struct/union/enum name to a number of name occurrences */
	struct hashmap *type_names;
	/*
	 * maps typedef identifiers and enum value names to a number of such
	 * name occurrences
	 */
	struct hashmap *ident_names;
};

static size_t str_hash_fn(const void *key, void *ctx)
{
	const char *s = key;
	size_t h = 0;

	while (*s) {
		h = h * 31 + *s;
		s++;
	}
	return h;
}

static bool str_equal_fn(const void *a, const void *b, void *ctx)
{
	return strcmp(a, b) == 0;
}

static const char *btf_name_of(const struct btf_dump *d, __u32 name_off)
{
	return btf__name_by_offset(d->btf, name_off);
}

static void btf_dump_printf(const struct btf_dump *d, const char *fmt, ...)
{
	va_list args;

	va_start(args, fmt);
	d->printf_fn(d->opts.ctx, fmt, args);
	va_end(args);
}

struct btf_dump *btf_dump__new(const struct btf *btf,
			       const struct btf_ext *btf_ext,
			       const struct btf_dump_opts *opts,
			       btf_dump_printf_fn_t printf_fn)
{
	struct btf_dump *d;
	int err;

	d = calloc(1, sizeof(struct btf_dump));
	if (!d)
		return ERR_PTR(-ENOMEM);

	d->btf = btf;
	d->btf_ext = btf_ext;
	d->printf_fn = printf_fn;
	d->opts.ctx = opts ? opts->ctx : NULL;

	d->type_names = hashmap__new(str_hash_fn, str_equal_fn, NULL);
	if (IS_ERR(d->type_names)) {
		err = PTR_ERR(d->type_names);
		d->type_names = NULL;
		btf_dump__free(d);
		return ERR_PTR(err);
	}
	d->ident_names = hashmap__new(str_hash_fn, str_equal_fn, NULL);
	if (IS_ERR(d->ident_names)) {
		err = PTR_ERR(d->ident_names);
		d->ident_names = NULL;
		btf_dump__free(d);
		return ERR_PTR(err);
	}

	return d;
}

void btf_dump__free(struct btf_dump *d)
{
	int i, cnt;

	if (!d)
		return;

	free(d->type_states);
	if (d->cached_names) {
		/* any set cached name is owned by us and should be freed */
		for (i = 0, cnt = btf__get_nr_types(d->btf); i <= cnt; i++) {
			if (d->cached_names[i])
				free((void *)d->cached_names[i]);
		}
	}
	free(d->cached_names);
	free(d->emit_queue);
	free(d->decl_stack);
	hashmap__free(d->type_names);
	hashmap__free(d->ident_names);

	free(d);
}

static int btf_dump_mark_referenced(struct btf_dump *d);
static int btf_dump_order_type(struct btf_dump *d, __u32 id, bool through_ptr);
static void btf_dump_emit_type(struct btf_dump *d, __u32 id, __u32 cont_id);

/*
 * Dump BTF type in a compilable C syntax, including all the necessary
 * dependent types, necessary for compilation. If some of the dependent types
 * were already emitted as part of previous btf_dump__dump_type() invocation
 * for another type, they won't be emitted again. This API allows callers to
 * filter out BTF types according to user-defined criterias and emitted only
 * minimal subset of types, necessary to compile everything. Full struct/union
 * definitions will still be emitted, even if the only usage is through
 * pointer and could be satisfied with just a forward declaration.
 *
 * Dumping is done in two high-level passes:
 *   1. Topologically sort type definitions to satisfy C rules of compilation.
 *   2. Emit type definitions in C syntax.
 *
 * Returns 0 on success; <0, otherwise.
 */
int btf_dump__dump_type(struct btf_dump *d, __u32 id)
{
	int err, i;

	if (id > btf__get_nr_types(d->btf))
		return -EINVAL;

	/* type states are lazily allocated, as they might not be needed */
	if (!d->type_states) {
		d->type_states = calloc(1 + btf__get_nr_types(d->btf),
					sizeof(d->type_states[0]));
		if (!d->type_states)
			return -ENOMEM;
		d->cached_names = calloc(1 + btf__get_nr_types(d->btf),
					 sizeof(d->cached_names[0]));
		if (!d->cached_names)
			return -ENOMEM;

		/* VOID is special */
		d->type_states[0].order_state = ORDERED;
		d->type_states[0].emit_state = EMITTED;

		/* eagerly determine referenced types for anon enums */
		err = btf_dump_mark_referenced(d);
		if (err)
			return err;
	}

	d->emit_queue_cnt = 0;
	err = btf_dump_order_type(d, id, false);
	if (err < 0)
		return err;

	for (i = 0; i < d->emit_queue_cnt; i++)
		btf_dump_emit_type(d, d->emit_queue[i], 0 /*top-level*/);

	return 0;
}

/*
 * Mark all types that are referenced from any other type. This is used to
 * determine top-level anonymous enums that need to be emitted as an
 * independent type declarations.
 * Anonymous enums come in two flavors: either embedded in a struct's field
 * definition, in which case they have to be declared inline as part of field
 * type declaration; or as a top-level anonymous enum, typically used for
 * declaring global constants. It's impossible to distinguish between two
 * without knowning whether given enum type was referenced from other type:
 * top-level anonymous enum won't be referenced by anything, while embedded
 * one will.
 */
static int btf_dump_mark_referenced(struct btf_dump *d)
{
	int i, j, n = btf__get_nr_types(d->btf);
	const struct btf_type *t;
	__u16 vlen;

	for (i = 1; i <= n; i++) {
		t = btf__type_by_id(d->btf, i);
		vlen = btf_vlen(t);

		switch (btf_kind(t)) {
		case BTF_KIND_INT:
		case BTF_KIND_ENUM:
		case BTF_KIND_FWD:
			break;

		case BTF_KIND_VOLATILE:
		case BTF_KIND_CONST:
		case BTF_KIND_RESTRICT:
		case BTF_KIND_PTR:
		case BTF_KIND_TYPEDEF:
		case BTF_KIND_FUNC:
		case BTF_KIND_VAR:
			d->type_states[t->type].referenced = 1;
			break;

		case BTF_KIND_ARRAY: {
			const struct btf_array *a = btf_array(t);

			d->type_states[a->index_type].referenced = 1;
			d->type_states[a->type].referenced = 1;
			break;
		}
		case BTF_KIND_STRUCT:
		case BTF_KIND_UNION: {
			const struct btf_member *m = btf_members(t);

			for (j = 0; j < vlen; j++, m++)
				d->type_states[m->type].referenced = 1;
			break;
		}
		case BTF_KIND_FUNC_PROTO: {
			const struct btf_param *p = btf_params(t);

			for (j = 0; j < vlen; j++, p++)
				d->type_states[p->type].referenced = 1;
			break;
		}
		case BTF_KIND_DATASEC: {
			const struct btf_var_secinfo *v = btf_var_secinfos(t);

			for (j = 0; j < vlen; j++, v++)
				d->type_states[v->type].referenced = 1;
			break;
		}
		default:
			return -EINVAL;
		}
	}
	return 0;
}
static int btf_dump_add_emit_queue_id(struct btf_dump *d, __u32 id)
{
	__u32 *new_queue;
	size_t new_cap;

	if (d->emit_queue_cnt >= d->emit_queue_cap) {
		new_cap = max(16, d->emit_queue_cap * 3 / 2);
		new_queue = realloc(d->emit_queue,
				    new_cap * sizeof(new_queue[0]));
		if (!new_queue)
			return -ENOMEM;
		d->emit_queue = new_queue;
		d->emit_queue_cap = new_cap;
	}

	d->emit_queue[d->emit_queue_cnt++] = id;
	return 0;
}

/*
 * Determine order of emitting dependent types and specified type to satisfy
 * C compilation rules.  This is done through topological sorting with an
 * additional complication which comes from C rules. The main idea for C is
 * that if some type is "embedded" into a struct/union, it's size needs to be
 * known at the time of definition of containing type. E.g., for:
 *
 *	struct A {};
 *	struct B { struct A x; }
 *
 * struct A *HAS* to be defined before struct B, because it's "embedded",
 * i.e., it is part of struct B layout. But in the following case:
 *
 *	struct A;
 *	struct B { struct A *x; }
 *	struct A {};
 *
 * it's enough to just have a forward declaration of struct A at the time of
 * struct B definition, as struct B has a pointer to struct A, so the size of
 * field x is known without knowing struct A size: it's sizeof(void *).
 *
 * Unfortunately, there are some trickier cases we need to handle, e.g.:
 *
 *	struct A {}; // if this was forward-declaration: compilation error
 *	struct B {
 *		struct { // anonymous struct
 *			struct A y;
 *		} *x;
 *	};
 *
 * In this case, struct B's field x is a pointer, so it's size is known
 * regardless of the size of (anonymous) struct it points to. But because this
 * struct is anonymous and thus defined inline inside struct B, *and* it
 * embeds struct A, compiler requires full definition of struct A to be known
 * before struct B can be defined. This creates a transitive dependency
 * between struct A and struct B. If struct A was forward-declared before
 * struct B definition and fully defined after struct B definition, that would
 * trigger compilation error.
 *
 * All this means that while we are doing topological sorting on BTF type
 * graph, we need to determine relationships between different types (graph
 * nodes):
 *   - weak link (relationship) between X and Y, if Y *CAN* be
 *   forward-declared at the point of X definition;
 *   - strong link, if Y *HAS* to be fully-defined before X can be defined.
 *
 * The rule is as follows. Given a chain of BTF types from X to Y, if there is
 * BTF_KIND_PTR type in the chain and at least one non-anonymous type
 * Z (excluding X, including Y), then link is weak. Otherwise, it's strong.
 * Weak/strong relationship is determined recursively during DFS traversal and
 * is returned as a result from btf_dump_order_type().
 *
 * btf_dump_order_type() is trying to avoid unnecessary forward declarations,
 * but it is not guaranteeing that no extraneous forward declarations will be
 * emitted.
 *
 * To avoid extra work, algorithm marks some of BTF types as ORDERED, when
 * it's done with them, but not for all (e.g., VOLATILE, CONST, RESTRICT,
 * ARRAY, FUNC_PROTO), as weak/strong semantics for those depends on the
 * entire graph path, so depending where from one came to that BTF type, it
 * might cause weak or strong ordering. For types like STRUCT/UNION/INT/ENUM,
 * once they are processed, there is no need to do it again, so they are
 * marked as ORDERED. We can mark PTR as ORDERED as well, as it semi-forces
 * weak link, unless subsequent referenced STRUCT/UNION/ENUM is anonymous. But
 * in any case, once those are processed, no need to do it again, as the
 * result won't change.
 *
 * Returns:
 *   - 1, if type is part of strong link (so there is strong topological
 *   ordering requirements);
 *   - 0, if type is part of weak link (so can be satisfied through forward
 *   declaration);
 *   - <0, on error (e.g., unsatisfiable type loop detected).
 */
static int btf_dump_order_type(struct btf_dump *d, __u32 id, bool through_ptr)
{
	/*
	 * Order state is used to detect strong link cycles, but only for BTF
	 * kinds that are or could be an independent definition (i.e.,
	 * stand-alone fwd decl, enum, typedef, struct, union). Ptrs, arrays,
	 * func_protos, modifiers are just means to get to these definitions.
	 * Int/void don't need definitions, they are assumed to be always
	 * properly defined.  We also ignore datasec, var, and funcs for now.
	 * So for all non-defining kinds, we never even set ordering state,
	 * for defining kinds we set ORDERING and subsequently ORDERED if it
	 * forms a strong link.
	 */
	struct btf_dump_type_aux_state *tstate = &d->type_states[id];
	const struct btf_type *t;
	__u16 vlen;
	int err, i;

	/* return true, letting typedefs know that it's ok to be emitted */
	if (tstate->order_state == ORDERED)
		return 1;

	t = btf__type_by_id(d->btf, id);

	if (tstate->order_state == ORDERING) {
		/* type loop, but resolvable through fwd declaration */
		if (btf_is_composite(t) && through_ptr && t->name_off != 0)
			return 0;
		pr_warn("unsatisfiable type cycle, id:[%u]\n", id);
		return -ELOOP;
	}

	switch (btf_kind(t)) {
	case BTF_KIND_INT:
		tstate->order_state = ORDERED;
		return 0;

	case BTF_KIND_PTR:
		err = btf_dump_order_type(d, t->type, true);
		tstate->order_state = ORDERED;
		return err;

	case BTF_KIND_ARRAY:
		return btf_dump_order_type(d, btf_array(t)->type, through_ptr);

	case BTF_KIND_STRUCT:
	case BTF_KIND_UNION: {
		const struct btf_member *m = btf_members(t);
		/*
		 * struct/union is part of strong link, only if it's embedded
		 * (so no ptr in a path) or it's anonymous (so has to be
		 * defined inline, even if declared through ptr)
		 */
		if (through_ptr && t->name_off != 0)
			return 0;

		tstate->order_state = ORDERING;

		vlen = btf_vlen(t);
		for (i = 0; i < vlen; i++, m++) {
			err = btf_dump_order_type(d, m->type, false);
			if (err < 0)
				return err;
		}

		if (t->name_off != 0) {
			err = btf_dump_add_emit_queue_id(d, id);
			if (err < 0)
				return err;
		}

		tstate->order_state = ORDERED;
		return 1;
	}
	case BTF_KIND_ENUM:
	case BTF_KIND_FWD:
		/*
		 * non-anonymous or non-referenced enums are top-level
		 * declarations and should be emitted. Same logic can be
		 * applied to FWDs, it won't hurt anyways.
		 */
		if (t->name_off != 0 || !tstate->referenced) {
			err = btf_dump_add_emit_queue_id(d, id);
			if (err)
				return err;
		}
		tstate->order_state = ORDERED;
		return 1;

	case BTF_KIND_TYPEDEF: {
		int is_strong;

		is_strong = btf_dump_order_type(d, t->type, through_ptr);
		if (is_strong < 0)
			return is_strong;

		/* typedef is similar to struct/union w.r.t. fwd-decls */
		if (through_ptr && !is_strong)
			return 0;

		/* typedef is always a named definition */
		err = btf_dump_add_emit_queue_id(d, id);
		if (err)
			return err;

		d->type_states[id].order_state = ORDERED;
		return 1;
	}
	case BTF_KIND_VOLATILE:
	case BTF_KIND_CONST:
	case BTF_KIND_RESTRICT:
		return btf_dump_order_type(d, t->type, through_ptr);

	case BTF_KIND_FUNC_PROTO: {
		const struct btf_param *p = btf_params(t);
		bool is_strong;

		err = btf_dump_order_type(d, t->type, through_ptr);
		if (err < 0)
			return err;
		is_strong = err > 0;

		vlen = btf_vlen(t);
		for (i = 0; i < vlen; i++, p++) {
			err = btf_dump_order_type(d, p->type, through_ptr);
			if (err < 0)
				return err;
			if (err > 0)
				is_strong = true;
		}
		return is_strong;
	}
	case BTF_KIND_FUNC:
	case BTF_KIND_VAR:
	case BTF_KIND_DATASEC:
		d->type_states[id].order_state = ORDERED;
		return 0;

	default:
		return -EINVAL;
	}
}

static void btf_dump_emit_struct_fwd(struct btf_dump *d, __u32 id,
				     const struct btf_type *t);
static void btf_dump_emit_struct_def(struct btf_dump *d, __u32 id,
				     const struct btf_type *t, int lvl);

static void btf_dump_emit_enum_fwd(struct btf_dump *d, __u32 id,
				   const struct btf_type *t);
static void btf_dump_emit_enum_def(struct btf_dump *d, __u32 id,
				   const struct btf_type *t, int lvl);

static void btf_dump_emit_fwd_def(struct btf_dump *d, __u32 id,
				  const struct btf_type *t);

static void btf_dump_emit_typedef_def(struct btf_dump *d, __u32 id,
				      const struct btf_type *t, int lvl);

/* a local view into a shared stack */
struct id_stack {
	const __u32 *ids;
	int cnt;
};

static void btf_dump_emit_type_decl(struct btf_dump *d, __u32 id,
				    const char *fname, int lvl);
static void btf_dump_emit_type_chain(struct btf_dump *d,
				     struct id_stack *decl_stack,
				     const char *fname, int lvl);

static const char *btf_dump_type_name(struct btf_dump *d, __u32 id);
static const char *btf_dump_ident_name(struct btf_dump *d, __u32 id);
static size_t btf_dump_name_dups(struct btf_dump *d, struct hashmap *name_map,
				 const char *orig_name);

static bool btf_dump_is_blacklisted(struct btf_dump *d, __u32 id)
{
	const struct btf_type *t = btf__type_by_id(d->btf, id);

	/* __builtin_va_list is a compiler built-in, which causes compilation
	 * errors, when compiling w/ different compiler, then used to compile
	 * original code (e.g., GCC to compile kernel, Clang to use generated
	 * C header from BTF). As it is built-in, it should be already defined
	 * properly internally in compiler.
	 */
	if (t->name_off == 0)
		return false;
	return strcmp(btf_name_of(d, t->name_off), "__builtin_va_list") == 0;
}

/*
 * Emit C-syntax definitions of types from chains of BTF types.
 *
 * High-level handling of determining necessary forward declarations are handled
 * by btf_dump_emit_type() itself, but all nitty-gritty details of emitting type
 * declarations/definitions in C syntax  are handled by a combo of
 * btf_dump_emit_type_decl()/btf_dump_emit_type_chain() w/ delegation to
 * corresponding btf_dump_emit_*_{def,fwd}() functions.
 *
 * We also keep track of "containing struct/union type ID" to determine when
 * we reference it from inside and thus can avoid emitting unnecessary forward
 * declaration.
 *
 * This algorithm is designed in such a way, that even if some error occurs
 * (either technical, e.g., out of memory, or logical, i.e., malformed BTF
 * that doesn't comply to C rules completely), algorithm will try to proceed
 * and produce as much meaningful output as possible.
 */
static void btf_dump_emit_type(struct btf_dump *d, __u32 id, __u32 cont_id)
{
	struct btf_dump_type_aux_state *tstate = &d->type_states[id];
	bool top_level_def = cont_id == 0;
	const struct btf_type *t;
	__u16 kind;

	if (tstate->emit_state == EMITTED)
		return;

	t = btf__type_by_id(d->btf, id);
	kind = btf_kind(t);

	if (tstate->emit_state == EMITTING) {
		if (tstate->fwd_emitted)
			return;

		switch (kind) {
		case BTF_KIND_STRUCT:
		case BTF_KIND_UNION:
			/*
			 * if we are referencing a struct/union that we are
			 * part of - then no need for fwd declaration
			 */
			if (id == cont_id)
				return;
			if (t->name_off == 0) {
				pr_warn("anonymous struct/union loop, id:[%u]\n",
					id);
				return;
			}
			btf_dump_emit_struct_fwd(d, id, t);
			btf_dump_printf(d, ";\n\n");
			tstate->fwd_emitted = 1;
			break;
		case BTF_KIND_TYPEDEF:
			/*
			 * for typedef fwd_emitted means typedef definition
			 * was emitted, but it can be used only for "weak"
			 * references through pointer only, not for embedding
			 */
			if (!btf_dump_is_blacklisted(d, id)) {
				btf_dump_emit_typedef_def(d, id, t, 0);
				btf_dump_printf(d, ";\n\n");
			};
			tstate->fwd_emitted = 1;
			break;
		default:
			break;
		}

		return;
	}

	switch (kind) {
	case BTF_KIND_INT:
		tstate->emit_state = EMITTED;
		break;
	case BTF_KIND_ENUM:
		if (top_level_def) {
			btf_dump_emit_enum_def(d, id, t, 0);
			btf_dump_printf(d, ";\n\n");
		}
		tstate->emit_state = EMITTED;
		break;
	case BTF_KIND_PTR:
	case BTF_KIND_VOLATILE:
	case BTF_KIND_CONST:
	case BTF_KIND_RESTRICT:
		btf_dump_emit_type(d, t->type, cont_id);
		break;
	case BTF_KIND_ARRAY:
		btf_dump_emit_type(d, btf_array(t)->type, cont_id);
		break;
	case BTF_KIND_FWD:
		btf_dump_emit_fwd_def(d, id, t);
		btf_dump_printf(d, ";\n\n");
		tstate->emit_state = EMITTED;
		break;
	case BTF_KIND_TYPEDEF:
		tstate->emit_state = EMITTING;
		btf_dump_emit_type(d, t->type, id);
		/*
		 * typedef can server as both definition and forward
		 * declaration; at this stage someone depends on
		 * typedef as a forward declaration (refers to it
		 * through pointer), so unless we already did it,
		 * emit typedef as a forward declaration
		 */
		if (!tstate->fwd_emitted && !btf_dump_is_blacklisted(d, id)) {
			btf_dump_emit_typedef_def(d, id, t, 0);
			btf_dump_printf(d, ";\n\n");
		}
		tstate->emit_state = EMITTED;
		break;
	case BTF_KIND_STRUCT:
	case BTF_KIND_UNION:
		tstate->emit_state = EMITTING;
		/* if it's a top-level struct/union definition or struct/union
		 * is anonymous, then in C we'll be emitting all fields and
		 * their types (as opposed to just `struct X`), so we need to
		 * make sure that all types, referenced from struct/union
		 * members have necessary forward-declarations, where
		 * applicable
		 */
		if (top_level_def || t->name_off == 0) {
			const struct btf_member *m = btf_members(t);
			__u16 vlen = btf_vlen(t);
			int i, new_cont_id;

			new_cont_id = t->name_off == 0 ? cont_id : id;
			for (i = 0; i < vlen; i++, m++)
				btf_dump_emit_type(d, m->type, new_cont_id);
		} else if (!tstate->fwd_emitted && id != cont_id) {
			btf_dump_emit_struct_fwd(d, id, t);
			btf_dump_printf(d, ";\n\n");
			tstate->fwd_emitted = 1;
		}

		if (top_level_def) {
			btf_dump_emit_struct_def(d, id, t, 0);
			btf_dump_printf(d, ";\n\n");
			tstate->emit_state = EMITTED;
		} else {
			tstate->emit_state = NOT_EMITTED;
		}
		break;
	case BTF_KIND_FUNC_PROTO: {
		const struct btf_param *p = btf_params(t);
		__u16 vlen = btf_vlen(t);
		int i;

		btf_dump_emit_type(d, t->type, cont_id);
		for (i = 0; i < vlen; i++, p++)
			btf_dump_emit_type(d, p->type, cont_id);

		break;
	}
	default:
		break;
	}
}

static int btf_align_of(const struct btf *btf, __u32 id)
{
	const struct btf_type *t = btf__type_by_id(btf, id);
	__u16 kind = btf_kind(t);

	switch (kind) {
	case BTF_KIND_INT:
	case BTF_KIND_ENUM:
		return min(sizeof(void *), t->size);
	case BTF_KIND_PTR:
		return sizeof(void *);
	case BTF_KIND_TYPEDEF:
	case BTF_KIND_VOLATILE:
	case BTF_KIND_CONST:
	case BTF_KIND_RESTRICT:
		return btf_align_of(btf, t->type);
	case BTF_KIND_ARRAY:
		return btf_align_of(btf, btf_array(t)->type);
	case BTF_KIND_STRUCT:
	case BTF_KIND_UNION: {
		const struct btf_member *m = btf_members(t);
		__u16 vlen = btf_vlen(t);
		int i, align = 1;

		for (i = 0; i < vlen; i++, m++)
			align = max(align, btf_align_of(btf, m->type));

		return align;
	}
	default:
		pr_warn("unsupported BTF_KIND:%u\n", btf_kind(t));
		return 1;
	}
}

static bool btf_is_struct_packed(const struct btf *btf, __u32 id,
				 const struct btf_type *t)
{
	const struct btf_member *m;
	int align, i, bit_sz;
	__u16 vlen;

	align = btf_align_of(btf, id);
	/* size of a non-packed struct has to be a multiple of its alignment*/
	if (t->size % align)
		return true;

	m = btf_members(t);
	vlen = btf_vlen(t);
	/* all non-bitfield fields have to be naturally aligned */
	for (i = 0; i < vlen; i++, m++) {
		align = btf_align_of(btf, m->type);
		bit_sz = btf_member_bitfield_size(t, i);
		if (bit_sz == 0 && m->offset % (8 * align) != 0)
			return true;
	}

	/*
	 * if original struct was marked as packed, but its layout is
	 * naturally aligned, we'll detect that it's not packed
	 */
	return false;
}

static int chip_away_bits(int total, int at_most)
{
	return total % at_most ? : at_most;
}

static void btf_dump_emit_bit_padding(const struct btf_dump *d,
				      int cur_off, int m_off, int m_bit_sz,
				      int align, int lvl)
{
	int off_diff = m_off - cur_off;
	int ptr_bits = sizeof(void *) * 8;

	if (off_diff <= 0)
		/* no gap */
		return;
	if (m_bit_sz == 0 && off_diff < align * 8)
		/* natural padding will take care of a gap */
		return;

	while (off_diff > 0) {
		const char *pad_type;
		int pad_bits;

		if (ptr_bits > 32 && off_diff > 32) {
			pad_type = "long";
			pad_bits = chip_away_bits(off_diff, ptr_bits);
		} else if (off_diff > 16) {
			pad_type = "int";
			pad_bits = chip_away_bits(off_diff, 32);
		} else if (off_diff > 8) {
			pad_type = "short";
			pad_bits = chip_away_bits(off_diff, 16);
		} else {
			pad_type = "char";
			pad_bits = chip_away_bits(off_diff, 8);
		}
		btf_dump_printf(d, "\n%s%s: %d;", pfx(lvl), pad_type, pad_bits);
		off_diff -= pad_bits;
	}
}

static void btf_dump_emit_struct_fwd(struct btf_dump *d, __u32 id,
				     const struct btf_type *t)
{
	btf_dump_printf(d, "%s %s",
			btf_is_struct(t) ? "struct" : "union",
			btf_dump_type_name(d, id));
}

static void btf_dump_emit_struct_def(struct btf_dump *d,
				     __u32 id,
				     const struct btf_type *t,
				     int lvl)
{
	const struct btf_member *m = btf_members(t);
	bool is_struct = btf_is_struct(t);
	int align, i, packed, off = 0;
	__u16 vlen = btf_vlen(t);

	packed = is_struct ? btf_is_struct_packed(d->btf, id, t) : 0;

	btf_dump_printf(d, "%s%s%s {",
			is_struct ? "struct" : "union",
			t->name_off ? " " : "",
			btf_dump_type_name(d, id));

	for (i = 0; i < vlen; i++, m++) {
		const char *fname;
		int m_off, m_sz;

		fname = btf_name_of(d, m->name_off);
		m_sz = btf_member_bitfield_size(t, i);
		m_off = btf_member_bit_offset(t, i);
		align = packed ? 1 : btf_align_of(d->btf, m->type);

		btf_dump_emit_bit_padding(d, off, m_off, m_sz, align, lvl + 1);
		btf_dump_printf(d, "\n%s", pfx(lvl + 1));
		btf_dump_emit_type_decl(d, m->type, fname, lvl + 1);

		if (m_sz) {
			btf_dump_printf(d, ": %d", m_sz);
			off = m_off + m_sz;
		} else {
			m_sz = max(0, btf__resolve_size(d->btf, m->type));
			off = m_off + m_sz * 8;
		}
		btf_dump_printf(d, ";");
	}

	/* pad at the end, if necessary */
	if (is_struct) {
		align = packed ? 1 : btf_align_of(d->btf, id);
		btf_dump_emit_bit_padding(d, off, t->size * 8, 0, align,
					  lvl + 1);
	}

	if (vlen)
		btf_dump_printf(d, "\n");
	btf_dump_printf(d, "%s}", pfx(lvl));
	if (packed)
		btf_dump_printf(d, " __attribute__((packed))");
}

static void btf_dump_emit_enum_fwd(struct btf_dump *d, __u32 id,
				   const struct btf_type *t)
{
	btf_dump_printf(d, "enum %s", btf_dump_type_name(d, id));
}

static void btf_dump_emit_enum_def(struct btf_dump *d, __u32 id,
				   const struct btf_type *t,
				   int lvl)
{
	const struct btf_enum *v = btf_enum(t);
	__u16 vlen = btf_vlen(t);
	const char *name;
	size_t dup_cnt;
	int i;

	btf_dump_printf(d, "enum%s%s",
			t->name_off ? " " : "",
			btf_dump_type_name(d, id));

	if (vlen) {
		btf_dump_printf(d, " {");
		for (i = 0; i < vlen; i++, v++) {
			name = btf_name_of(d, v->name_off);
			/* enumerators share namespace with typedef idents */
			dup_cnt = btf_dump_name_dups(d, d->ident_names, name);
			if (dup_cnt > 1) {
				btf_dump_printf(d, "\n%s%s___%zu = %d,",
						pfx(lvl + 1), name, dup_cnt,
						(__s32)v->val);
			} else {
				btf_dump_printf(d, "\n%s%s = %d,",
						pfx(lvl + 1), name,
						(__s32)v->val);
			}
		}
		btf_dump_printf(d, "\n%s}", pfx(lvl));
	}
}

static void btf_dump_emit_fwd_def(struct btf_dump *d, __u32 id,
				  const struct btf_type *t)
{
	const char *name = btf_dump_type_name(d, id);

	if (btf_kflag(t))
		btf_dump_printf(d, "union %s", name);
	else
		btf_dump_printf(d, "struct %s", name);
}

static void btf_dump_emit_typedef_def(struct btf_dump *d, __u32 id,
				     const struct btf_type *t, int lvl)
{
	const char *name = btf_dump_ident_name(d, id);

	/*
	 * Old GCC versions are emitting invalid typedef for __gnuc_va_list
	 * pointing to VOID. This generates warnings from btf_dump() and
	 * results in uncompilable header file, so we are fixing it up here
	 * with valid typedef into __builtin_va_list.
	 */
	if (t->type == 0 && strcmp(name, "__gnuc_va_list") == 0) {
		btf_dump_printf(d, "typedef __builtin_va_list __gnuc_va_list");
		return;
	}

	btf_dump_printf(d, "typedef ");
	btf_dump_emit_type_decl(d, t->type, name, lvl);
}

static int btf_dump_push_decl_stack_id(struct btf_dump *d, __u32 id)
{
	__u32 *new_stack;
	size_t new_cap;

	if (d->decl_stack_cnt >= d->decl_stack_cap) {
		new_cap = max(16, d->decl_stack_cap * 3 / 2);
		new_stack = realloc(d->decl_stack,
				    new_cap * sizeof(new_stack[0]));
		if (!new_stack)
			return -ENOMEM;
		d->decl_stack = new_stack;
		d->decl_stack_cap = new_cap;
	}

	d->decl_stack[d->decl_stack_cnt++] = id;

	return 0;
}

/*
 * Emit type declaration (e.g., field type declaration in a struct or argument
 * declaration in function prototype) in correct C syntax.
 *
 * For most types it's trivial, but there are few quirky type declaration
 * cases worth mentioning:
 *   - function prototypes (especially nesting of function prototypes);
 *   - arrays;
 *   - const/volatile/restrict for pointers vs other types.
 *
 * For a good discussion of *PARSING* C syntax (as a human), see
 * Peter van der Linden's "Expert C Programming: Deep C Secrets",
 * Ch.3 "Unscrambling Declarations in C".
 *
 * It won't help with BTF to C conversion much, though, as it's an opposite
 * problem. So we came up with this algorithm in reverse to van der Linden's
 * parsing algorithm. It goes from structured BTF representation of type
 * declaration to a valid compilable C syntax.
 *
 * For instance, consider this C typedef:
 *	typedef const int * const * arr[10] arr_t;
 * It will be represented in BTF with this chain of BTF types:
 *	[typedef] -> [array] -> [ptr] -> [const] -> [ptr] -> [const] -> [int]
 *
 * Notice how [const] modifier always goes before type it modifies in BTF type
 * graph, but in C syntax, const/volatile/restrict modifiers are written to
 * the right of pointers, but to the left of other types. There are also other
 * quirks, like function pointers, arrays of them, functions returning other
 * functions, etc.
 *
 * We handle that by pushing all the types to a stack, until we hit "terminal"
 * type (int/enum/struct/union/fwd). Then depending on the kind of a type on
 * top of a stack, modifiers are handled differently. Array/function pointers
 * have also wildly different syntax and how nesting of them are done. See
 * code for authoritative definition.
 *
 * To avoid allocating new stack for each independent chain of BTF types, we
 * share one bigger stack, with each chain working only on its own local view
 * of a stack frame. Some care is required to "pop" stack frames after
 * processing type declaration chain.
 */
static void btf_dump_emit_type_decl(struct btf_dump *d, __u32 id,
				    const char *fname, int lvl)
{
	struct id_stack decl_stack;
	const struct btf_type *t;
	int err, stack_start;

	stack_start = d->decl_stack_cnt;
	for (;;) {
		err = btf_dump_push_decl_stack_id(d, id);
		if (err < 0) {
			/*
			 * if we don't have enough memory for entire type decl
			 * chain, restore stack, emit warning, and try to
			 * proceed nevertheless
			 */
			pr_warn("not enough memory for decl stack:%d", err);
			d->decl_stack_cnt = stack_start;
			return;
		}

		/* VOID */
		if (id == 0)
			break;

		t = btf__type_by_id(d->btf, id);
		switch (btf_kind(t)) {
		case BTF_KIND_PTR:
		case BTF_KIND_VOLATILE:
		case BTF_KIND_CONST:
		case BTF_KIND_RESTRICT:
		case BTF_KIND_FUNC_PROTO:
			id = t->type;
			break;
		case BTF_KIND_ARRAY:
			id = btf_array(t)->type;
			break;
		case BTF_KIND_INT:
		case BTF_KIND_ENUM:
		case BTF_KIND_FWD:
		case BTF_KIND_STRUCT:
		case BTF_KIND_UNION:
		case BTF_KIND_TYPEDEF:
			goto done;
		default:
			pr_warn("unexpected type in decl chain, kind:%u, id:[%u]\n",
				btf_kind(t), id);
			goto done;
		}
	}
done:
	/*
	 * We might be inside a chain of declarations (e.g., array of function
	 * pointers returning anonymous (so inlined) structs, having another
	 * array field). Each of those needs its own "stack frame" to handle
	 * emitting of declarations. Those stack frames are non-overlapping
	 * portions of shared btf_dump->decl_stack. To make it a bit nicer to
	 * handle this set of nested stacks, we create a view corresponding to
	 * our own "stack frame" and work with it as an independent stack.
	 * We'll need to clean up after emit_type_chain() returns, though.
	 */
	decl_stack.ids = d->decl_stack + stack_start;
	decl_stack.cnt = d->decl_stack_cnt - stack_start;
	btf_dump_emit_type_chain(d, &decl_stack, fname, lvl);
	/*
	 * emit_type_chain() guarantees that it will pop its entire decl_stack
	 * frame before returning. But it works with a read-only view into
	 * decl_stack, so it doesn't actually pop anything from the
	 * perspective of shared btf_dump->decl_stack, per se. We need to
	 * reset decl_stack state to how it was before us to avoid it growing
	 * all the time.
	 */
	d->decl_stack_cnt = stack_start;
}

static void btf_dump_emit_mods(struct btf_dump *d, struct id_stack *decl_stack)
{
	const struct btf_type *t;
	__u32 id;

	while (decl_stack->cnt) {
		id = decl_stack->ids[decl_stack->cnt - 1];
		t = btf__type_by_id(d->btf, id);

		switch (btf_kind(t)) {
		case BTF_KIND_VOLATILE:
			btf_dump_printf(d, "volatile ");
			break;
		case BTF_KIND_CONST:
			btf_dump_printf(d, "const ");
			break;
		case BTF_KIND_RESTRICT:
			btf_dump_printf(d, "restrict ");
			break;
		default:
			return;
		}
		decl_stack->cnt--;
	}
}

static void btf_dump_emit_name(const struct btf_dump *d,
			       const char *name, bool last_was_ptr)
{
	bool separate = name[0] && !last_was_ptr;

	btf_dump_printf(d, "%s%s", separate ? " " : "", name);
}

static void btf_dump_emit_type_chain(struct btf_dump *d,
				     struct id_stack *decls,
				     const char *fname, int lvl)
{
	/*
	 * last_was_ptr is used to determine if we need to separate pointer
	 * asterisk (*) from previous part of type signature with space, so
	 * that we get `int ***`, instead of `int * * *`. We default to true
	 * for cases where we have single pointer in a chain. E.g., in ptr ->
	 * func_proto case. func_proto will start a new emit_type_chain call
	 * with just ptr, which should be emitted as (*) or (*<fname>), so we
	 * don't want to prepend space for that last pointer.
	 */
	bool last_was_ptr = true;
	const struct btf_type *t;
	const char *name;
	__u16 kind;
	__u32 id;

	while (decls->cnt) {
		id = decls->ids[--decls->cnt];
		if (id == 0) {
			/* VOID is a special snowflake */
			btf_dump_emit_mods(d, decls);
			btf_dump_printf(d, "void");
			last_was_ptr = false;
			continue;
		}

		t = btf__type_by_id(d->btf, id);
		kind = btf_kind(t);

		switch (kind) {
		case BTF_KIND_INT:
			btf_dump_emit_mods(d, decls);
			name = btf_name_of(d, t->name_off);
			btf_dump_printf(d, "%s", name);
			break;
		case BTF_KIND_STRUCT:
		case BTF_KIND_UNION:
			btf_dump_emit_mods(d, decls);
			/* inline anonymous struct/union */
			if (t->name_off == 0)
				btf_dump_emit_struct_def(d, id, t, lvl);
			else
				btf_dump_emit_struct_fwd(d, id, t);
			break;
		case BTF_KIND_ENUM:
			btf_dump_emit_mods(d, decls);
			/* inline anonymous enum */
			if (t->name_off == 0)
				btf_dump_emit_enum_def(d, id, t, lvl);
			else
				btf_dump_emit_enum_fwd(d, id, t);
			break;
		case BTF_KIND_FWD:
			btf_dump_emit_mods(d, decls);
			btf_dump_emit_fwd_def(d, id, t);
			break;
		case BTF_KIND_TYPEDEF:
			btf_dump_emit_mods(d, decls);
			btf_dump_printf(d, "%s", btf_dump_ident_name(d, id));
			break;
		case BTF_KIND_PTR:
			btf_dump_printf(d, "%s", last_was_ptr ? "*" : " *");
			break;
		case BTF_KIND_VOLATILE:
			btf_dump_printf(d, " volatile");
			break;
		case BTF_KIND_CONST:
			btf_dump_printf(d, " const");
			break;
		case BTF_KIND_RESTRICT:
			btf_dump_printf(d, " restrict");
			break;
		case BTF_KIND_ARRAY: {
			const struct btf_array *a = btf_array(t);
			const struct btf_type *next_t;
			__u32 next_id;
			bool multidim;
			/*
			 * GCC has a bug
			 * (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=8354)
			 * which causes it to emit extra const/volatile
			 * modifiers for an array, if array's element type has
			 * const/volatile modifiers. Clang doesn't do that.
			 * In general, it doesn't seem very meaningful to have
			 * a const/volatile modifier for array, so we are
			 * going to silently skip them here.
			 */
			while (decls->cnt) {
				next_id = decls->ids[decls->cnt - 1];
				next_t = btf__type_by_id(d->btf, next_id);
				if (btf_is_mod(next_t))
					decls->cnt--;
				else
					break;
			}

			if (decls->cnt == 0) {
				btf_dump_emit_name(d, fname, last_was_ptr);
				btf_dump_printf(d, "[%u]", a->nelems);
				return;
			}

			next_id = decls->ids[decls->cnt - 1];
			next_t = btf__type_by_id(d->btf, next_id);
			multidim = btf_is_array(next_t);
			/* we need space if we have named non-pointer */
			if (fname[0] && !last_was_ptr)
				btf_dump_printf(d, " ");
			/* no parentheses for multi-dimensional array */
			if (!multidim)
				btf_dump_printf(d, "(");
			btf_dump_emit_type_chain(d, decls, fname, lvl);
			if (!multidim)
				btf_dump_printf(d, ")");
			btf_dump_printf(d, "[%u]", a->nelems);
			return;
		}
		case BTF_KIND_FUNC_PROTO: {
			const struct btf_param *p = btf_params(t);
			__u16 vlen = btf_vlen(t);
			int i;

			btf_dump_emit_mods(d, decls);
			if (decls->cnt) {
				btf_dump_printf(d, " (");
				btf_dump_emit_type_chain(d, decls, fname, lvl);
				btf_dump_printf(d, ")");
			} else {
				btf_dump_emit_name(d, fname, last_was_ptr);
			}
			btf_dump_printf(d, "(");
			/*
			 * Clang for BPF target generates func_proto with no
			 * args as a func_proto with a single void arg (e.g.,
			 * `int (*f)(void)` vs just `int (*f)()`). We are
			 * going to pretend there are no args for such case.
			 */
			if (vlen == 1 && p->type == 0) {
				btf_dump_printf(d, ")");
				return;
			}

			for (i = 0; i < vlen; i++, p++) {
				if (i > 0)
					btf_dump_printf(d, ", ");

				/* last arg of type void is vararg */
				if (i == vlen - 1 && p->type == 0) {
					btf_dump_printf(d, "...");
					break;
				}

				name = btf_name_of(d, p->name_off);
				btf_dump_emit_type_decl(d, p->type, name, lvl);
			}

			btf_dump_printf(d, ")");
			return;
		}
		default:
			pr_warn("unexpected type in decl chain, kind:%u, id:[%u]\n",
				kind, id);
			return;
		}

		last_was_ptr = kind == BTF_KIND_PTR;
	}

	btf_dump_emit_name(d, fname, last_was_ptr);
}

/* return number of duplicates (occurrences) of a given name */
static size_t btf_dump_name_dups(struct btf_dump *d, struct hashmap *name_map,
				 const char *orig_name)
{
	size_t dup_cnt = 0;

	hashmap__find(name_map, orig_name, (void **)&dup_cnt);
	dup_cnt++;
	hashmap__set(name_map, orig_name, (void *)dup_cnt, NULL, NULL);

	return dup_cnt;
}

static const char *btf_dump_resolve_name(struct btf_dump *d, __u32 id,
					 struct hashmap *name_map)
{
	struct btf_dump_type_aux_state *s = &d->type_states[id];
	const struct btf_type *t = btf__type_by_id(d->btf, id);
	const char *orig_name = btf_name_of(d, t->name_off);
	const char **cached_name = &d->cached_names[id];
	size_t dup_cnt;

	if (t->name_off == 0)
		return "";

	if (s->name_resolved)
		return *cached_name ? *cached_name : orig_name;

	dup_cnt = btf_dump_name_dups(d, name_map, orig_name);
	if (dup_cnt > 1) {
		const size_t max_len = 256;
		char new_name[max_len];

		snprintf(new_name, max_len, "%s___%zu", orig_name, dup_cnt);
		*cached_name = strdup(new_name);
	}

	s->name_resolved = 1;
	return *cached_name ? *cached_name : orig_name;
}

static const char *btf_dump_type_name(struct btf_dump *d, __u32 id)
{
	return btf_dump_resolve_name(d, id, d->type_names);
}

static const char *btf_dump_ident_name(struct btf_dump *d, __u32 id)
{
	return btf_dump_resolve_name(d, id, d->ident_names);
}