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