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 /* 25 * To use this vDSO parser, first call one of the vdso_init_* functions. 26 * If you've already parsed auxv, then pass the value of AT_SYSINFO_EHDR 27 * to vdso_init_from_sysinfo_ehdr. Otherwise pass auxv to vdso_init_from_auxv. 28 * Then call vdso_sym for each symbol you want. For example, to look up 29 * gettimeofday on x86_64, use: 30 * 31 * <some pointer> = vdso_sym("LINUX_2.6", "gettimeofday"); 32 * or 33 * <some pointer> = vdso_sym("LINUX_2.6", "__vdso_gettimeofday"); 34 * 35 * vdso_sym will return 0 if the symbol doesn't exist or if the init function 36 * failed or was not called. vdso_sym is a little slow, so its return value 37 * should be cached. 38 * 39 * vdso_sym is threadsafe; the init functions are not. 40 * 41 * These are the prototypes: 42 */ 43 extern void vdso_init_from_auxv(void *auxv); 44 extern void vdso_init_from_sysinfo_ehdr(uintptr_t base); 45 extern void *vdso_sym(const char *version, const char *name); 46 47 48 /* And here's the code. */ 49 #ifndef ELF_BITS 50 # if ULONG_MAX > 0xffffffffUL 51 # define ELF_BITS 64 52 # else 53 # define ELF_BITS 32 54 # endif 55 #endif 56 57 #define ELF_BITS_XFORM2(bits, x) Elf##bits##_##x 58 #define ELF_BITS_XFORM(bits, x) ELF_BITS_XFORM2(bits, x) 59 #define ELF(x) ELF_BITS_XFORM(ELF_BITS, x) 60 61 static struct vdso_info 62 { 63 bool valid; 64 65 /* Load information */ 66 uintptr_t load_addr; 67 uintptr_t load_offset; /* load_addr - recorded vaddr */ 68 69 /* Symbol table */ 70 ELF(Sym) *symtab; 71 const char *symstrings; 72 ELF(Word) *bucket, *chain; 73 ELF(Word) nbucket, nchain; 74 75 /* Version table */ 76 ELF(Versym) *versym; 77 ELF(Verdef) *verdef; 78 } vdso_info; 79 80 /* Straight from the ELF specification. */ 81 static unsigned long elf_hash(const unsigned char *name) 82 { 83 unsigned long h = 0, g; 84 while (*name) 85 { 86 h = (h << 4) + *name++; 87 if (g = h & 0xf0000000) 88 h ^= g >> 24; 89 h &= ~g; 90 } 91 return h; 92 } 93 94 void vdso_init_from_sysinfo_ehdr(uintptr_t base) 95 { 96 size_t i; 97 bool found_vaddr = false; 98 99 vdso_info.valid = false; 100 101 vdso_info.load_addr = base; 102 103 ELF(Ehdr) *hdr = (ELF(Ehdr)*)base; 104 if (hdr->e_ident[EI_CLASS] != 105 (ELF_BITS == 32 ? ELFCLASS32 : ELFCLASS64)) { 106 return; /* Wrong ELF class -- check ELF_BITS */ 107 } 108 109 ELF(Phdr) *pt = (ELF(Phdr)*)(vdso_info.load_addr + hdr->e_phoff); 110 ELF(Dyn) *dyn = 0; 111 112 /* 113 * We need two things from the segment table: the load offset 114 * and the dynamic table. 115 */ 116 for (i = 0; i < hdr->e_phnum; i++) 117 { 118 if (pt[i].p_type == PT_LOAD && !found_vaddr) { 119 found_vaddr = true; 120 vdso_info.load_offset = base 121 + (uintptr_t)pt[i].p_offset 122 - (uintptr_t)pt[i].p_vaddr; 123 } else if (pt[i].p_type == PT_DYNAMIC) { 124 dyn = (ELF(Dyn)*)(base + pt[i].p_offset); 125 } 126 } 127 128 if (!found_vaddr || !dyn) 129 return; /* Failed */ 130 131 /* 132 * Fish out the useful bits of the dynamic table. 133 */ 134 ELF(Word) *hash = 0; 135 vdso_info.symstrings = 0; 136 vdso_info.symtab = 0; 137 vdso_info.versym = 0; 138 vdso_info.verdef = 0; 139 for (i = 0; dyn[i].d_tag != DT_NULL; i++) { 140 switch (dyn[i].d_tag) { 141 case DT_STRTAB: 142 vdso_info.symstrings = (const char *) 143 ((uintptr_t)dyn[i].d_un.d_ptr 144 + vdso_info.load_offset); 145 break; 146 case DT_SYMTAB: 147 vdso_info.symtab = (ELF(Sym) *) 148 ((uintptr_t)dyn[i].d_un.d_ptr 149 + vdso_info.load_offset); 150 break; 151 case DT_HASH: 152 hash = (ELF(Word) *) 153 ((uintptr_t)dyn[i].d_un.d_ptr 154 + vdso_info.load_offset); 155 break; 156 case DT_VERSYM: 157 vdso_info.versym = (ELF(Versym) *) 158 ((uintptr_t)dyn[i].d_un.d_ptr 159 + vdso_info.load_offset); 160 break; 161 case DT_VERDEF: 162 vdso_info.verdef = (ELF(Verdef) *) 163 ((uintptr_t)dyn[i].d_un.d_ptr 164 + vdso_info.load_offset); 165 break; 166 } 167 } 168 if (!vdso_info.symstrings || !vdso_info.symtab || !hash) 169 return; /* Failed */ 170 171 if (!vdso_info.verdef) 172 vdso_info.versym = 0; 173 174 /* Parse the hash table header. */ 175 vdso_info.nbucket = hash[0]; 176 vdso_info.nchain = hash[1]; 177 vdso_info.bucket = &hash[2]; 178 vdso_info.chain = &hash[vdso_info.nbucket + 2]; 179 180 /* That's all we need. */ 181 vdso_info.valid = true; 182 } 183 184 static bool vdso_match_version(ELF(Versym) ver, 185 const char *name, ELF(Word) hash) 186 { 187 /* 188 * This is a helper function to check if the version indexed by 189 * ver matches name (which hashes to hash). 190 * 191 * The version definition table is a mess, and I don't know how 192 * to do this in better than linear time without allocating memory 193 * to build an index. I also don't know why the table has 194 * variable size entries in the first place. 195 * 196 * For added fun, I can't find a comprehensible specification of how 197 * to parse all the weird flags in the table. 198 * 199 * So I just parse the whole table every time. 200 */ 201 202 /* First step: find the version definition */ 203 ver &= 0x7fff; /* Apparently bit 15 means "hidden" */ 204 ELF(Verdef) *def = vdso_info.verdef; 205 while(true) { 206 if ((def->vd_flags & VER_FLG_BASE) == 0 207 && (def->vd_ndx & 0x7fff) == ver) 208 break; 209 210 if (def->vd_next == 0) 211 return false; /* No definition. */ 212 213 def = (ELF(Verdef) *)((char *)def + def->vd_next); 214 } 215 216 /* Now figure out whether it matches. */ 217 ELF(Verdaux) *aux = (ELF(Verdaux)*)((char *)def + def->vd_aux); 218 return def->vd_hash == hash 219 && !strcmp(name, vdso_info.symstrings + aux->vda_name); 220 } 221 222 void *vdso_sym(const char *version, const char *name) 223 { 224 unsigned long ver_hash; 225 if (!vdso_info.valid) 226 return 0; 227 228 ver_hash = elf_hash(version); 229 ELF(Word) chain = vdso_info.bucket[elf_hash(name) % vdso_info.nbucket]; 230 231 for (; chain != STN_UNDEF; chain = vdso_info.chain[chain]) { 232 ELF(Sym) *sym = &vdso_info.symtab[chain]; 233 234 /* Check for a defined global or weak function w/ right name. */ 235 if (ELF64_ST_TYPE(sym->st_info) != STT_FUNC) 236 continue; 237 if (ELF64_ST_BIND(sym->st_info) != STB_GLOBAL && 238 ELF64_ST_BIND(sym->st_info) != STB_WEAK) 239 continue; 240 if (sym->st_shndx == SHN_UNDEF) 241 continue; 242 if (strcmp(name, vdso_info.symstrings + sym->st_name)) 243 continue; 244 245 /* Check symbol version. */ 246 if (vdso_info.versym 247 && !vdso_match_version(vdso_info.versym[chain], 248 version, ver_hash)) 249 continue; 250 251 return (void *)(vdso_info.load_offset + sym->st_value); 252 } 253 254 return 0; 255 } 256 257 void vdso_init_from_auxv(void *auxv) 258 { 259 ELF(auxv_t) *elf_auxv = auxv; 260 for (int i = 0; elf_auxv[i].a_type != AT_NULL; i++) 261 { 262 if (elf_auxv[i].a_type == AT_SYSINFO_EHDR) { 263 vdso_init_from_sysinfo_ehdr(elf_auxv[i].a_un.a_val); 264 return; 265 } 266 } 267 268 vdso_info.valid = false; 269 } 270
