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 <linux/auxvec.h>
23 #include <linux/elf.h>
24
25 #include "parse_vdso.h"
26
27 /* And here's the code. */
28 #ifndef ELF_BITS
29 # if __SIZEOF_LONG__ >= 8
30 # define ELF_BITS 64
31 # else
32 # define ELF_BITS 32
33 # endif
34 #endif
35
36 #define ELF_BITS_XFORM2(bits, x) Elf##bits##_##x
37 #define ELF_BITS_XFORM(bits, x) ELF_BITS_XFORM2(bits, x)
38 #define ELF(x) ELF_BITS_XFORM(ELF_BITS, x)
39
40 #ifdef __s390x__
41 #define ELF_HASH_ENTRY ELF(Xword)
42 #else
43 #define ELF_HASH_ENTRY ELF(Word)
44 #endif
45
46 static struct vdso_info
47 {
48 bool valid;
49
50 /* Load information */
51 uintptr_t load_addr;
52 uintptr_t load_offset; /* load_addr - recorded vaddr */
53
54 /* Symbol table */
55 ELF(Sym) *symtab;
56 const char *symstrings;
57 ELF(Word) *gnu_hash, *gnu_bucket;
58 ELF_HASH_ENTRY *bucket, *chain;
59 ELF_HASH_ENTRY nbucket, nchain;
60
61 /* Version table */
62 ELF(Versym) *versym;
63 ELF(Verdef) *verdef;
64 } vdso_info;
65
66 /*
67 * Straight from the ELF specification...and then tweaked slightly, in order to
68 * avoid a few clang warnings.
69 */
elf_hash(const char * name)70 static unsigned long elf_hash(const char *name)
71 {
72 unsigned long h = 0, g;
73 const unsigned char *uch_name = (const unsigned char *)name;
74
75 while (*uch_name)
76 {
77 h = (h << 4) + *uch_name++;
78 g = h & 0xf0000000;
79 if (g)
80 h ^= g >> 24;
81 h &= ~g;
82 }
83 return h;
84 }
85
gnu_hash(const char * name)86 static uint32_t gnu_hash(const char *name)
87 {
88 const unsigned char *s = (void *)name;
89 uint32_t h = 5381;
90
91 for (; *s; s++)
92 h += h * 32 + *s;
93 return h;
94 }
95
vdso_init_from_sysinfo_ehdr(uintptr_t base)96 void vdso_init_from_sysinfo_ehdr(uintptr_t base)
97 {
98 size_t i;
99 bool found_vaddr = false;
100
101 vdso_info.valid = false;
102
103 vdso_info.load_addr = base;
104
105 ELF(Ehdr) *hdr = (ELF(Ehdr)*)base;
106 if (hdr->e_ident[EI_CLASS] !=
107 (ELF_BITS == 32 ? ELFCLASS32 : ELFCLASS64)) {
108 return; /* Wrong ELF class -- check ELF_BITS */
109 }
110
111 ELF(Phdr) *pt = (ELF(Phdr)*)(vdso_info.load_addr + hdr->e_phoff);
112 ELF(Dyn) *dyn = 0;
113
114 /*
115 * We need two things from the segment table: the load offset
116 * and the dynamic table.
117 */
118 for (i = 0; i < hdr->e_phnum; i++)
119 {
120 if (pt[i].p_type == PT_LOAD && !found_vaddr) {
121 found_vaddr = true;
122 vdso_info.load_offset = base
123 + (uintptr_t)pt[i].p_offset
124 - (uintptr_t)pt[i].p_vaddr;
125 } else if (pt[i].p_type == PT_DYNAMIC) {
126 dyn = (ELF(Dyn)*)(base + pt[i].p_offset);
127 }
128 }
129
130 if (!found_vaddr || !dyn)
131 return; /* Failed */
132
133 /*
134 * Fish out the useful bits of the dynamic table.
135 */
136 ELF_HASH_ENTRY *hash = 0;
137 vdso_info.symstrings = 0;
138 vdso_info.gnu_hash = 0;
139 vdso_info.symtab = 0;
140 vdso_info.versym = 0;
141 vdso_info.verdef = 0;
142 for (i = 0; dyn[i].d_tag != DT_NULL; i++) {
143 switch (dyn[i].d_tag) {
144 case DT_STRTAB:
145 vdso_info.symstrings = (const char *)
146 ((uintptr_t)dyn[i].d_un.d_ptr
147 + vdso_info.load_offset);
148 break;
149 case DT_SYMTAB:
150 vdso_info.symtab = (ELF(Sym) *)
151 ((uintptr_t)dyn[i].d_un.d_ptr
152 + vdso_info.load_offset);
153 break;
154 case DT_HASH:
155 hash = (ELF_HASH_ENTRY *)
156 ((uintptr_t)dyn[i].d_un.d_ptr
157 + vdso_info.load_offset);
158 break;
159 case DT_GNU_HASH:
160 vdso_info.gnu_hash =
161 (ELF(Word) *)((uintptr_t)dyn[i].d_un.d_ptr +
162 vdso_info.load_offset);
163 break;
164 case DT_VERSYM:
165 vdso_info.versym = (ELF(Versym) *)
166 ((uintptr_t)dyn[i].d_un.d_ptr
167 + vdso_info.load_offset);
168 break;
169 case DT_VERDEF:
170 vdso_info.verdef = (ELF(Verdef) *)
171 ((uintptr_t)dyn[i].d_un.d_ptr
172 + vdso_info.load_offset);
173 break;
174 }
175 }
176 if (!vdso_info.symstrings || !vdso_info.symtab ||
177 (!hash && !vdso_info.gnu_hash))
178 return; /* Failed */
179
180 if (!vdso_info.verdef)
181 vdso_info.versym = 0;
182
183 /* Parse the hash table header. */
184 if (vdso_info.gnu_hash) {
185 vdso_info.nbucket = vdso_info.gnu_hash[0];
186 /* The bucket array is located after the header (4 uint32) and the bloom
187 * filter (size_t array of gnu_hash[2] elements).
188 */
189 vdso_info.gnu_bucket = vdso_info.gnu_hash + 4 +
190 sizeof(size_t) / 4 * vdso_info.gnu_hash[2];
191 } else {
192 vdso_info.nbucket = hash[0];
193 vdso_info.nchain = hash[1];
194 vdso_info.bucket = &hash[2];
195 vdso_info.chain = &hash[vdso_info.nbucket + 2];
196 }
197
198 /* That's all we need. */
199 vdso_info.valid = true;
200 }
201
vdso_match_version(ELF (Versym)ver,const char * name,ELF (Word)hash)202 static bool vdso_match_version(ELF(Versym) ver,
203 const char *name, ELF(Word) hash)
204 {
205 /*
206 * This is a helper function to check if the version indexed by
207 * ver matches name (which hashes to hash).
208 *
209 * The version definition table is a mess, and I don't know how
210 * to do this in better than linear time without allocating memory
211 * to build an index. I also don't know why the table has
212 * variable size entries in the first place.
213 *
214 * For added fun, I can't find a comprehensible specification of how
215 * to parse all the weird flags in the table.
216 *
217 * So I just parse the whole table every time.
218 */
219
220 /* First step: find the version definition */
221 ver &= 0x7fff; /* Apparently bit 15 means "hidden" */
222 ELF(Verdef) *def = vdso_info.verdef;
223 while(true) {
224 if ((def->vd_flags & VER_FLG_BASE) == 0
225 && (def->vd_ndx & 0x7fff) == ver)
226 break;
227
228 if (def->vd_next == 0)
229 return false; /* No definition. */
230
231 def = (ELF(Verdef) *)((char *)def + def->vd_next);
232 }
233
234 /* Now figure out whether it matches. */
235 ELF(Verdaux) *aux = (ELF(Verdaux)*)((char *)def + def->vd_aux);
236 return def->vd_hash == hash
237 && !strcmp(name, vdso_info.symstrings + aux->vda_name);
238 }
239
check_sym(ELF (Sym)* sym,ELF (Word)i,const char * name,const char * version,unsigned long ver_hash)240 static bool check_sym(ELF(Sym) *sym, ELF(Word) i, const char *name,
241 const char *version, unsigned long ver_hash)
242 {
243 /* Check for a defined global or weak function w/ right name. */
244 if (ELF64_ST_TYPE(sym->st_info) != STT_FUNC)
245 return false;
246 if (ELF64_ST_BIND(sym->st_info) != STB_GLOBAL &&
247 ELF64_ST_BIND(sym->st_info) != STB_WEAK)
248 return false;
249 if (strcmp(name, vdso_info.symstrings + sym->st_name))
250 return false;
251
252 /* Check symbol version. */
253 if (vdso_info.versym &&
254 !vdso_match_version(vdso_info.versym[i], version, ver_hash))
255 return false;
256
257 return true;
258 }
259
vdso_sym(const char * version,const char * name)260 void *vdso_sym(const char *version, const char *name)
261 {
262 unsigned long ver_hash;
263 if (!vdso_info.valid)
264 return 0;
265
266 ver_hash = elf_hash(version);
267 ELF(Word) i;
268
269 if (vdso_info.gnu_hash) {
270 uint32_t h1 = gnu_hash(name), h2, *hashval;
271
272 i = vdso_info.gnu_bucket[h1 % vdso_info.nbucket];
273 if (i == 0)
274 return 0;
275 h1 |= 1;
276 hashval = vdso_info.gnu_bucket + vdso_info.nbucket +
277 (i - vdso_info.gnu_hash[1]);
278 for (;; i++) {
279 ELF(Sym) *sym = &vdso_info.symtab[i];
280 h2 = *hashval++;
281 if (h1 == (h2 | 1) &&
282 check_sym(sym, i, name, version, ver_hash))
283 return (void *)(vdso_info.load_offset +
284 sym->st_value);
285 if (h2 & 1)
286 break;
287 }
288 } else {
289 i = vdso_info.bucket[elf_hash(name) % vdso_info.nbucket];
290 for (; i; i = vdso_info.chain[i]) {
291 ELF(Sym) *sym = &vdso_info.symtab[i];
292 if (sym->st_shndx != SHN_UNDEF &&
293 check_sym(sym, i, name, version, ver_hash))
294 return (void *)(vdso_info.load_offset +
295 sym->st_value);
296 }
297 }
298
299 return 0;
300 }
301