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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 __SIZEOF_LONG__ >= 8
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#ifdef __s390x__
40#define ELF_HASH_ENTRY ELF(Xword)
41#else
42#define ELF_HASH_ENTRY ELF(Word)
43#endif
44
45static struct vdso_info
46{
47 bool valid;
48
49 /* Load information */
50 uintptr_t load_addr;
51 uintptr_t load_offset; /* load_addr - recorded vaddr */
52
53 /* Symbol table */
54 ELF(Sym) *symtab;
55 const char *symstrings;
56 ELF(Word) *gnu_hash, *gnu_bucket;
57 ELF_HASH_ENTRY *bucket, *chain;
58 ELF_HASH_ENTRY nbucket, nchain;
59
60 /* Version table */
61 ELF(Versym) *versym;
62 ELF(Verdef) *verdef;
63} vdso_info;
64
65/*
66 * Straight from the ELF specification...and then tweaked slightly, in order to
67 * avoid a few clang warnings.
68 */
69static unsigned long elf_hash(const char *name)
70{
71 unsigned long h = 0, g;
72 const unsigned char *uch_name = (const unsigned char *)name;
73
74 while (*uch_name)
75 {
76 h = (h << 4) + *uch_name++;
77 g = h & 0xf0000000;
78 if (g)
79 h ^= g >> 24;
80 h &= ~g;
81 }
82 return h;
83}
84
85static uint32_t gnu_hash(const char *name)
86{
87 const unsigned char *s = (void *)name;
88 uint32_t h = 5381;
89
90 for (; *s; s++)
91 h += h * 32 + *s;
92 return h;
93}
94
95void vdso_init_from_sysinfo_ehdr(uintptr_t base)
96{
97 size_t i;
98 bool found_vaddr = false;
99
100 vdso_info.valid = false;
101
102 vdso_info.load_addr = base;
103
104 ELF(Ehdr) *hdr = (ELF(Ehdr)*)base;
105 if (hdr->e_ident[EI_CLASS] !=
106 (ELF_BITS == 32 ? ELFCLASS32 : ELFCLASS64)) {
107 return; /* Wrong ELF class -- check ELF_BITS */
108 }
109
110 ELF(Phdr) *pt = (ELF(Phdr)*)(vdso_info.load_addr + hdr->e_phoff);
111 ELF(Dyn) *dyn = 0;
112
113 /*
114 * We need two things from the segment table: the load offset
115 * and the dynamic table.
116 */
117 for (i = 0; i < hdr->e_phnum; i++)
118 {
119 if (pt[i].p_type == PT_LOAD && !found_vaddr) {
120 found_vaddr = true;
121 vdso_info.load_offset = base
122 + (uintptr_t)pt[i].p_offset
123 - (uintptr_t)pt[i].p_vaddr;
124 } else if (pt[i].p_type == PT_DYNAMIC) {
125 dyn = (ELF(Dyn)*)(base + pt[i].p_offset);
126 }
127 }
128
129 if (!found_vaddr || !dyn)
130 return; /* Failed */
131
132 /*
133 * Fish out the useful bits of the dynamic table.
134 */
135 ELF_HASH_ENTRY *hash = 0;
136 vdso_info.symstrings = 0;
137 vdso_info.gnu_hash = 0;
138 vdso_info.symtab = 0;
139 vdso_info.versym = 0;
140 vdso_info.verdef = 0;
141 for (i = 0; dyn[i].d_tag != DT_NULL; i++) {
142 switch (dyn[i].d_tag) {
143 case DT_STRTAB:
144 vdso_info.symstrings = (const char *)
145 ((uintptr_t)dyn[i].d_un.d_ptr
146 + vdso_info.load_offset);
147 break;
148 case DT_SYMTAB:
149 vdso_info.symtab = (ELF(Sym) *)
150 ((uintptr_t)dyn[i].d_un.d_ptr
151 + vdso_info.load_offset);
152 break;
153 case DT_HASH:
154 hash = (ELF_HASH_ENTRY *)
155 ((uintptr_t)dyn[i].d_un.d_ptr
156 + vdso_info.load_offset);
157 break;
158 case DT_GNU_HASH:
159 vdso_info.gnu_hash =
160 (ELF(Word) *)((uintptr_t)dyn[i].d_un.d_ptr +
161 vdso_info.load_offset);
162 break;
163 case DT_VERSYM:
164 vdso_info.versym = (ELF(Versym) *)
165 ((uintptr_t)dyn[i].d_un.d_ptr
166 + vdso_info.load_offset);
167 break;
168 case DT_VERDEF:
169 vdso_info.verdef = (ELF(Verdef) *)
170 ((uintptr_t)dyn[i].d_un.d_ptr
171 + vdso_info.load_offset);
172 break;
173 }
174 }
175 if (!vdso_info.symstrings || !vdso_info.symtab ||
176 (!hash && !vdso_info.gnu_hash))
177 return; /* Failed */
178
179 if (!vdso_info.verdef)
180 vdso_info.versym = 0;
181
182 /* Parse the hash table header. */
183 if (vdso_info.gnu_hash) {
184 vdso_info.nbucket = vdso_info.gnu_hash[0];
185 /* The bucket array is located after the header (4 uint32) and the bloom
186 * filter (size_t array of gnu_hash[2] elements).
187 */
188 vdso_info.gnu_bucket = vdso_info.gnu_hash + 4 +
189 sizeof(size_t) / 4 * vdso_info.gnu_hash[2];
190 } else {
191 vdso_info.nbucket = hash[0];
192 vdso_info.nchain = hash[1];
193 vdso_info.bucket = &hash[2];
194 vdso_info.chain = &hash[vdso_info.nbucket + 2];
195 }
196
197 /* That's all we need. */
198 vdso_info.valid = true;
199}
200
201static bool vdso_match_version(ELF(Versym) ver,
202 const char *name, ELF(Word) hash)
203{
204 /*
205 * This is a helper function to check if the version indexed by
206 * ver matches name (which hashes to hash).
207 *
208 * The version definition table is a mess, and I don't know how
209 * to do this in better than linear time without allocating memory
210 * to build an index. I also don't know why the table has
211 * variable size entries in the first place.
212 *
213 * For added fun, I can't find a comprehensible specification of how
214 * to parse all the weird flags in the table.
215 *
216 * So I just parse the whole table every time.
217 */
218
219 /* First step: find the version definition */
220 ver &= 0x7fff; /* Apparently bit 15 means "hidden" */
221 ELF(Verdef) *def = vdso_info.verdef;
222 while(true) {
223 if ((def->vd_flags & VER_FLG_BASE) == 0
224 && (def->vd_ndx & 0x7fff) == ver)
225 break;
226
227 if (def->vd_next == 0)
228 return false; /* No definition. */
229
230 def = (ELF(Verdef) *)((char *)def + def->vd_next);
231 }
232
233 /* Now figure out whether it matches. */
234 ELF(Verdaux) *aux = (ELF(Verdaux)*)((char *)def + def->vd_aux);
235 return def->vd_hash == hash
236 && !strcmp(name, vdso_info.symstrings + aux->vda_name);
237}
238
239static bool check_sym(ELF(Sym) *sym, ELF(Word) i, const char *name,
240 const char *version, unsigned long ver_hash)
241{
242 /* Check for a defined global or weak function w/ right name. */
243 if (ELF64_ST_TYPE(sym->st_info) != STT_FUNC)
244 return false;
245 if (ELF64_ST_BIND(sym->st_info) != STB_GLOBAL &&
246 ELF64_ST_BIND(sym->st_info) != STB_WEAK)
247 return false;
248 if (strcmp(name, vdso_info.symstrings + sym->st_name))
249 return false;
250
251 /* Check symbol version. */
252 if (vdso_info.versym &&
253 !vdso_match_version(vdso_info.versym[i], version, ver_hash))
254 return false;
255
256 return true;
257}
258
259void *vdso_sym(const char *version, const char *name)
260{
261 unsigned long ver_hash;
262 if (!vdso_info.valid)
263 return 0;
264
265 ver_hash = elf_hash(version);
266 ELF(Word) i;
267
268 if (vdso_info.gnu_hash) {
269 uint32_t h1 = gnu_hash(name), h2, *hashval;
270
271 i = vdso_info.gnu_bucket[h1 % vdso_info.nbucket];
272 if (i == 0)
273 return 0;
274 h1 |= 1;
275 hashval = vdso_info.gnu_bucket + vdso_info.nbucket +
276 (i - vdso_info.gnu_hash[1]);
277 for (;; i++) {
278 ELF(Sym) *sym = &vdso_info.symtab[i];
279 h2 = *hashval++;
280 if (h1 == (h2 | 1) &&
281 check_sym(sym, i, name, version, ver_hash))
282 return (void *)(vdso_info.load_offset +
283 sym->st_value);
284 if (h2 & 1)
285 break;
286 }
287 } else {
288 i = vdso_info.bucket[elf_hash(name) % vdso_info.nbucket];
289 for (; i; i = vdso_info.chain[i]) {
290 ELF(Sym) *sym = &vdso_info.symtab[i];
291 if (sym->st_shndx != SHN_UNDEF &&
292 check_sym(sym, i, name, version, ver_hash))
293 return (void *)(vdso_info.load_offset +
294 sym->st_value);
295 }
296 }
297
298 return 0;
299}