1 /*
2  * parse_vdso.c: Linux reference vDSO parser
3  * Written by Andrew Lutomirski, 2011-2014.
4  *
5  * This code is meant to be linked in to various programs that run on Linux.
6  * As such, it is available with as few restrictions as possible.  This file
7  * is licensed under the Creative Commons Zero License, version 1.0,
8  * available at http://creativecommons.org/publicdomain/zero/1.0/legalcode
9  *
10  * The vDSO is a regular ELF DSO that the kernel maps into user space when
11  * it starts a program.  It works equally well in statically and dynamically
12  * linked binaries.
13  *
14  * This code is tested on x86.  In principle it should work on any
15  * architecture that has a vDSO.
16  */
17 
18 #include <stdbool.h>
19 #include <stdint.h>
20 #include <string.h>
21 #include <limits.h>
22 #include <elf.h>
23 
24 #include "parse_vdso.h"
25 
26 /* And here's the code. */
27 #ifndef ELF_BITS
28 # if ULONG_MAX > 0xffffffffUL
29 #  define ELF_BITS 64
30 # else
31 #  define ELF_BITS 32
32 # endif
33 #endif
34 
35 #define ELF_BITS_XFORM2(bits, x) Elf##bits##_##x
36 #define ELF_BITS_XFORM(bits, x) ELF_BITS_XFORM2(bits, x)
37 #define ELF(x) ELF_BITS_XFORM(ELF_BITS, x)
38 
39 static struct vdso_info
40 {
41 	bool valid;
42 
43 	/* Load information */
44 	uintptr_t load_addr;
45 	uintptr_t load_offset;  /* load_addr - recorded vaddr */
46 
47 	/* Symbol table */
48 	ELF(Sym) *symtab;
49 	const char *symstrings;
50 	ELF(Word) *bucket, *chain;
51 	ELF(Word) nbucket, nchain;
52 
53 	/* Version table */
54 	ELF(Versym) *versym;
55 	ELF(Verdef) *verdef;
56 } vdso_info;
57 
58 /* Straight from the ELF specification. */
elf_hash(const unsigned char * name)59 static unsigned long elf_hash(const unsigned char *name)
60 {
61 	unsigned long h = 0, g;
62 	while (*name)
63 	{
64 		h = (h << 4) + *name++;
65 		if (g = h & 0xf0000000)
66 			h ^= g >> 24;
67 		h &= ~g;
68 	}
69 	return h;
70 }
71 
vdso_init_from_sysinfo_ehdr(uintptr_t base)72 void vdso_init_from_sysinfo_ehdr(uintptr_t base)
73 {
74 	size_t i;
75 	bool found_vaddr = false;
76 
77 	vdso_info.valid = false;
78 
79 	vdso_info.load_addr = base;
80 
81 	ELF(Ehdr) *hdr = (ELF(Ehdr)*)base;
82 	if (hdr->e_ident[EI_CLASS] !=
83 	    (ELF_BITS == 32 ? ELFCLASS32 : ELFCLASS64)) {
84 		return;  /* Wrong ELF class -- check ELF_BITS */
85 	}
86 
87 	ELF(Phdr) *pt = (ELF(Phdr)*)(vdso_info.load_addr + hdr->e_phoff);
88 	ELF(Dyn) *dyn = 0;
89 
90 	/*
91 	 * We need two things from the segment table: the load offset
92 	 * and the dynamic table.
93 	 */
94 	for (i = 0; i < hdr->e_phnum; i++)
95 	{
96 		if (pt[i].p_type == PT_LOAD && !found_vaddr) {
97 			found_vaddr = true;
98 			vdso_info.load_offset =	base
99 				+ (uintptr_t)pt[i].p_offset
100 				- (uintptr_t)pt[i].p_vaddr;
101 		} else if (pt[i].p_type == PT_DYNAMIC) {
102 			dyn = (ELF(Dyn)*)(base + pt[i].p_offset);
103 		}
104 	}
105 
106 	if (!found_vaddr || !dyn)
107 		return;  /* Failed */
108 
109 	/*
110 	 * Fish out the useful bits of the dynamic table.
111 	 */
112 	ELF(Word) *hash = 0;
113 	vdso_info.symstrings = 0;
114 	vdso_info.symtab = 0;
115 	vdso_info.versym = 0;
116 	vdso_info.verdef = 0;
117 	for (i = 0; dyn[i].d_tag != DT_NULL; i++) {
118 		switch (dyn[i].d_tag) {
119 		case DT_STRTAB:
120 			vdso_info.symstrings = (const char *)
121 				((uintptr_t)dyn[i].d_un.d_ptr
122 				 + vdso_info.load_offset);
123 			break;
124 		case DT_SYMTAB:
125 			vdso_info.symtab = (ELF(Sym) *)
126 				((uintptr_t)dyn[i].d_un.d_ptr
127 				 + vdso_info.load_offset);
128 			break;
129 		case DT_HASH:
130 			hash = (ELF(Word) *)
131 				((uintptr_t)dyn[i].d_un.d_ptr
132 				 + vdso_info.load_offset);
133 			break;
134 		case DT_VERSYM:
135 			vdso_info.versym = (ELF(Versym) *)
136 				((uintptr_t)dyn[i].d_un.d_ptr
137 				 + vdso_info.load_offset);
138 			break;
139 		case DT_VERDEF:
140 			vdso_info.verdef = (ELF(Verdef) *)
141 				((uintptr_t)dyn[i].d_un.d_ptr
142 				 + vdso_info.load_offset);
143 			break;
144 		}
145 	}
146 	if (!vdso_info.symstrings || !vdso_info.symtab || !hash)
147 		return;  /* Failed */
148 
149 	if (!vdso_info.verdef)
150 		vdso_info.versym = 0;
151 
152 	/* Parse the hash table header. */
153 	vdso_info.nbucket = hash[0];
154 	vdso_info.nchain = hash[1];
155 	vdso_info.bucket = &hash[2];
156 	vdso_info.chain = &hash[vdso_info.nbucket + 2];
157 
158 	/* That's all we need. */
159 	vdso_info.valid = true;
160 }
161 
vdso_match_version(ELF (Versym)ver,const char * name,ELF (Word)hash)162 static bool vdso_match_version(ELF(Versym) ver,
163 			       const char *name, ELF(Word) hash)
164 {
165 	/*
166 	 * This is a helper function to check if the version indexed by
167 	 * ver matches name (which hashes to hash).
168 	 *
169 	 * The version definition table is a mess, and I don't know how
170 	 * to do this in better than linear time without allocating memory
171 	 * to build an index.  I also don't know why the table has
172 	 * variable size entries in the first place.
173 	 *
174 	 * For added fun, I can't find a comprehensible specification of how
175 	 * to parse all the weird flags in the table.
176 	 *
177 	 * So I just parse the whole table every time.
178 	 */
179 
180 	/* First step: find the version definition */
181 	ver &= 0x7fff;  /* Apparently bit 15 means "hidden" */
182 	ELF(Verdef) *def = vdso_info.verdef;
183 	while(true) {
184 		if ((def->vd_flags & VER_FLG_BASE) == 0
185 		    && (def->vd_ndx & 0x7fff) == ver)
186 			break;
187 
188 		if (def->vd_next == 0)
189 			return false;  /* No definition. */
190 
191 		def = (ELF(Verdef) *)((char *)def + def->vd_next);
192 	}
193 
194 	/* Now figure out whether it matches. */
195 	ELF(Verdaux) *aux = (ELF(Verdaux)*)((char *)def + def->vd_aux);
196 	return def->vd_hash == hash
197 		&& !strcmp(name, vdso_info.symstrings + aux->vda_name);
198 }
199 
vdso_sym(const char * version,const char * name)200 void *vdso_sym(const char *version, const char *name)
201 {
202 	unsigned long ver_hash;
203 	if (!vdso_info.valid)
204 		return 0;
205 
206 	ver_hash = elf_hash(version);
207 	ELF(Word) chain = vdso_info.bucket[elf_hash(name) % vdso_info.nbucket];
208 
209 	for (; chain != STN_UNDEF; chain = vdso_info.chain[chain]) {
210 		ELF(Sym) *sym = &vdso_info.symtab[chain];
211 
212 		/* Check for a defined global or weak function w/ right name. */
213 		if (ELF64_ST_TYPE(sym->st_info) != STT_FUNC)
214 			continue;
215 		if (ELF64_ST_BIND(sym->st_info) != STB_GLOBAL &&
216 		    ELF64_ST_BIND(sym->st_info) != STB_WEAK)
217 			continue;
218 		if (sym->st_shndx == SHN_UNDEF)
219 			continue;
220 		if (strcmp(name, vdso_info.symstrings + sym->st_name))
221 			continue;
222 
223 		/* Check symbol version. */
224 		if (vdso_info.versym
225 		    && !vdso_match_version(vdso_info.versym[chain],
226 					   version, ver_hash))
227 			continue;
228 
229 		return (void *)(vdso_info.load_offset + sym->st_value);
230 	}
231 
232 	return 0;
233 }
234 
vdso_init_from_auxv(void * auxv)235 void vdso_init_from_auxv(void *auxv)
236 {
237 	ELF(auxv_t) *elf_auxv = auxv;
238 	for (int i = 0; elf_auxv[i].a_type != AT_NULL; i++)
239 	{
240 		if (elf_auxv[i].a_type == AT_SYSINFO_EHDR) {
241 			vdso_init_from_sysinfo_ehdr(elf_auxv[i].a_un.a_val);
242 			return;
243 		}
244 	}
245 
246 	vdso_info.valid = false;
247 }
248