xref: /openbmc/linux/arch/arm/kernel/module-plts.c (revision 82003e04)
1 /*
2  * Copyright (C) 2014 Linaro Ltd. <ard.biesheuvel@linaro.org>
3  *
4  * This program is free software; you can redistribute it and/or modify
5  * it under the terms of the GNU General Public License version 2 as
6  * published by the Free Software Foundation.
7  */
8 
9 #include <linux/elf.h>
10 #include <linux/kernel.h>
11 #include <linux/module.h>
12 #include <linux/sort.h>
13 
14 #include <asm/cache.h>
15 #include <asm/opcodes.h>
16 
17 #define PLT_ENT_STRIDE		L1_CACHE_BYTES
18 #define PLT_ENT_COUNT		(PLT_ENT_STRIDE / sizeof(u32))
19 #define PLT_ENT_SIZE		(sizeof(struct plt_entries) / PLT_ENT_COUNT)
20 
21 #ifdef CONFIG_THUMB2_KERNEL
22 #define PLT_ENT_LDR		__opcode_to_mem_thumb32(0xf8dff000 | \
23 							(PLT_ENT_STRIDE - 4))
24 #else
25 #define PLT_ENT_LDR		__opcode_to_mem_arm(0xe59ff000 | \
26 						    (PLT_ENT_STRIDE - 8))
27 #endif
28 
29 struct plt_entries {
30 	u32	ldr[PLT_ENT_COUNT];
31 	u32	lit[PLT_ENT_COUNT];
32 };
33 
34 u32 get_module_plt(struct module *mod, unsigned long loc, Elf32_Addr val)
35 {
36 	struct plt_entries *plt = (struct plt_entries *)mod->arch.plt->sh_addr;
37 	int idx = 0;
38 
39 	/*
40 	 * Look for an existing entry pointing to 'val'. Given that the
41 	 * relocations are sorted, this will be the last entry we allocated.
42 	 * (if one exists).
43 	 */
44 	if (mod->arch.plt_count > 0) {
45 		plt += (mod->arch.plt_count - 1) / PLT_ENT_COUNT;
46 		idx = (mod->arch.plt_count - 1) % PLT_ENT_COUNT;
47 
48 		if (plt->lit[idx] == val)
49 			return (u32)&plt->ldr[idx];
50 
51 		idx = (idx + 1) % PLT_ENT_COUNT;
52 		if (!idx)
53 			plt++;
54 	}
55 
56 	mod->arch.plt_count++;
57 	BUG_ON(mod->arch.plt_count * PLT_ENT_SIZE > mod->arch.plt->sh_size);
58 
59 	if (!idx)
60 		/* Populate a new set of entries */
61 		*plt = (struct plt_entries){
62 			{ [0 ... PLT_ENT_COUNT - 1] = PLT_ENT_LDR, },
63 			{ val, }
64 		};
65 	else
66 		plt->lit[idx] = val;
67 
68 	return (u32)&plt->ldr[idx];
69 }
70 
71 #define cmp_3way(a,b)	((a) < (b) ? -1 : (a) > (b))
72 
73 static int cmp_rel(const void *a, const void *b)
74 {
75 	const Elf32_Rel *x = a, *y = b;
76 	int i;
77 
78 	/* sort by type and symbol index */
79 	i = cmp_3way(ELF32_R_TYPE(x->r_info), ELF32_R_TYPE(y->r_info));
80 	if (i == 0)
81 		i = cmp_3way(ELF32_R_SYM(x->r_info), ELF32_R_SYM(y->r_info));
82 	return i;
83 }
84 
85 static bool is_zero_addend_relocation(Elf32_Addr base, const Elf32_Rel *rel)
86 {
87 	u32 *tval = (u32 *)(base + rel->r_offset);
88 
89 	/*
90 	 * Do a bitwise compare on the raw addend rather than fully decoding
91 	 * the offset and doing an arithmetic comparison.
92 	 * Note that a zero-addend jump/call relocation is encoded taking the
93 	 * PC bias into account, i.e., -8 for ARM and -4 for Thumb2.
94 	 */
95 	switch (ELF32_R_TYPE(rel->r_info)) {
96 		u16 upper, lower;
97 
98 	case R_ARM_THM_CALL:
99 	case R_ARM_THM_JUMP24:
100 		upper = __mem_to_opcode_thumb16(((u16 *)tval)[0]);
101 		lower = __mem_to_opcode_thumb16(((u16 *)tval)[1]);
102 
103 		return (upper & 0x7ff) == 0x7ff && (lower & 0x2fff) == 0x2ffe;
104 
105 	case R_ARM_CALL:
106 	case R_ARM_PC24:
107 	case R_ARM_JUMP24:
108 		return (__mem_to_opcode_arm(*tval) & 0xffffff) == 0xfffffe;
109 	}
110 	BUG();
111 }
112 
113 static bool duplicate_rel(Elf32_Addr base, const Elf32_Rel *rel, int num)
114 {
115 	const Elf32_Rel *prev;
116 
117 	/*
118 	 * Entries are sorted by type and symbol index. That means that,
119 	 * if a duplicate entry exists, it must be in the preceding
120 	 * slot.
121 	 */
122 	if (!num)
123 		return false;
124 
125 	prev = rel + num - 1;
126 	return cmp_rel(rel + num, prev) == 0 &&
127 	       is_zero_addend_relocation(base, prev);
128 }
129 
130 /* Count how many PLT entries we may need */
131 static unsigned int count_plts(const Elf32_Sym *syms, Elf32_Addr base,
132 			       const Elf32_Rel *rel, int num)
133 {
134 	unsigned int ret = 0;
135 	const Elf32_Sym *s;
136 	int i;
137 
138 	for (i = 0; i < num; i++) {
139 		switch (ELF32_R_TYPE(rel[i].r_info)) {
140 		case R_ARM_CALL:
141 		case R_ARM_PC24:
142 		case R_ARM_JUMP24:
143 		case R_ARM_THM_CALL:
144 		case R_ARM_THM_JUMP24:
145 			/*
146 			 * We only have to consider branch targets that resolve
147 			 * to undefined symbols. This is not simply a heuristic,
148 			 * it is a fundamental limitation, since the PLT itself
149 			 * is part of the module, and needs to be within range
150 			 * as well, so modules can never grow beyond that limit.
151 			 */
152 			s = syms + ELF32_R_SYM(rel[i].r_info);
153 			if (s->st_shndx != SHN_UNDEF)
154 				break;
155 
156 			/*
157 			 * Jump relocations with non-zero addends against
158 			 * undefined symbols are supported by the ELF spec, but
159 			 * do not occur in practice (e.g., 'jump n bytes past
160 			 * the entry point of undefined function symbol f').
161 			 * So we need to support them, but there is no need to
162 			 * take them into consideration when trying to optimize
163 			 * this code. So let's only check for duplicates when
164 			 * the addend is zero.
165 			 */
166 			if (!is_zero_addend_relocation(base, rel + i) ||
167 			    !duplicate_rel(base, rel, i))
168 				ret++;
169 		}
170 	}
171 	return ret;
172 }
173 
174 int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
175 			      char *secstrings, struct module *mod)
176 {
177 	unsigned long plts = 0;
178 	Elf32_Shdr *s, *sechdrs_end = sechdrs + ehdr->e_shnum;
179 	Elf32_Sym *syms = NULL;
180 
181 	/*
182 	 * To store the PLTs, we expand the .text section for core module code
183 	 * and for initialization code.
184 	 */
185 	for (s = sechdrs; s < sechdrs_end; ++s) {
186 		if (strcmp(".plt", secstrings + s->sh_name) == 0)
187 			mod->arch.plt = s;
188 		else if (s->sh_type == SHT_SYMTAB)
189 			syms = (Elf32_Sym *)s->sh_addr;
190 	}
191 
192 	if (!mod->arch.plt) {
193 		pr_err("%s: module PLT section missing\n", mod->name);
194 		return -ENOEXEC;
195 	}
196 	if (!syms) {
197 		pr_err("%s: module symtab section missing\n", mod->name);
198 		return -ENOEXEC;
199 	}
200 
201 	for (s = sechdrs + 1; s < sechdrs_end; ++s) {
202 		Elf32_Rel *rels = (void *)ehdr + s->sh_offset;
203 		int numrels = s->sh_size / sizeof(Elf32_Rel);
204 		Elf32_Shdr *dstsec = sechdrs + s->sh_info;
205 
206 		if (s->sh_type != SHT_REL)
207 			continue;
208 
209 		/* ignore relocations that operate on non-exec sections */
210 		if (!(dstsec->sh_flags & SHF_EXECINSTR))
211 			continue;
212 
213 		/* sort by type and symbol index */
214 		sort(rels, numrels, sizeof(Elf32_Rel), cmp_rel, NULL);
215 
216 		plts += count_plts(syms, dstsec->sh_addr, rels, numrels);
217 	}
218 
219 	mod->arch.plt->sh_type = SHT_NOBITS;
220 	mod->arch.plt->sh_flags = SHF_EXECINSTR | SHF_ALLOC;
221 	mod->arch.plt->sh_addralign = L1_CACHE_BYTES;
222 	mod->arch.plt->sh_size = round_up(plts * PLT_ENT_SIZE,
223 					  sizeof(struct plt_entries));
224 	mod->arch.plt_count = 0;
225 
226 	pr_debug("%s: plt=%x\n", __func__, mod->arch.plt->sh_size);
227 	return 0;
228 }
229