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