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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
39static 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. */
59static 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
72void 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
162static 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
200void *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
235void 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}
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
39static 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. */
59static 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
72void 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
162static 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
200void *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
235void 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}