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