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(Word) *gnu_hash; 57 ELF_HASH_ENTRY *bucket, *chain; 58 ELF_HASH_ENTRY nbucket, nchain; 59 60 /* Version table */ 61 ELF(Versym) *versym; 62 ELF(Verdef) *verdef; 63 } vdso_info; 64 65 /* 66 * Straight from the ELF specification...and then tweaked slightly, in order to 67 * avoid a few clang warnings. 68 */ 69 static unsigned long elf_hash(const char *name) 70 { 71 unsigned long h = 0, g; 72 const unsigned char *uch_name = (const unsigned char *)name; 73 74 while (*uch_name) 75 { 76 h = (h << 4) + *uch_name++; 77 g = h & 0xf0000000; 78 if (g) 79 h ^= g >> 24; 80 h &= ~g; 81 } 82 return h; 83 } 84 85 static uint32_t gnu_hash(const char *name) 86 { 87 const unsigned char *s = (void *)name; 88 uint32_t h = 5381; 89 90 for (; *s; s++) 91 h += h * 32 + *s; 92 return h; 93 } 94 95 void vdso_init_from_sysinfo_ehdr(uintptr_t base) 96 { 97 size_t i; 98 bool found_vaddr = false; 99 100 vdso_info.valid = false; 101 102 vdso_info.load_addr = base; 103 104 ELF(Ehdr) *hdr = (ELF(Ehdr)*)base; 105 if (hdr->e_ident[EI_CLASS] != 106 (ELF_BITS == 32 ? ELFCLASS32 : ELFCLASS64)) { 107 return; /* Wrong ELF class -- check ELF_BITS */ 108 } 109 110 ELF(Phdr) *pt = (ELF(Phdr)*)(vdso_info.load_addr + hdr->e_phoff); 111 ELF(Dyn) *dyn = 0; 112 113 /* 114 * We need two things from the segment table: the load offset 115 * and the dynamic table. 116 */ 117 for (i = 0; i < hdr->e_phnum; i++) 118 { 119 if (pt[i].p_type == PT_LOAD && !found_vaddr) { 120 found_vaddr = true; 121 vdso_info.load_offset = base 122 + (uintptr_t)pt[i].p_offset 123 - (uintptr_t)pt[i].p_vaddr; 124 } else if (pt[i].p_type == PT_DYNAMIC) { 125 dyn = (ELF(Dyn)*)(base + pt[i].p_offset); 126 } 127 } 128 129 if (!found_vaddr || !dyn) 130 return; /* Failed */ 131 132 /* 133 * Fish out the useful bits of the dynamic table. 134 */ 135 ELF_HASH_ENTRY *hash = 0; 136 vdso_info.symstrings = 0; 137 vdso_info.gnu_hash = 0; 138 vdso_info.symtab = 0; 139 vdso_info.versym = 0; 140 vdso_info.verdef = 0; 141 for (i = 0; dyn[i].d_tag != DT_NULL; i++) { 142 switch (dyn[i].d_tag) { 143 case DT_STRTAB: 144 vdso_info.symstrings = (const char *) 145 ((uintptr_t)dyn[i].d_un.d_ptr 146 + vdso_info.load_offset); 147 break; 148 case DT_SYMTAB: 149 vdso_info.symtab = (ELF(Sym) *) 150 ((uintptr_t)dyn[i].d_un.d_ptr 151 + vdso_info.load_offset); 152 break; 153 case DT_HASH: 154 hash = (ELF_HASH_ENTRY *) 155 ((uintptr_t)dyn[i].d_un.d_ptr 156 + vdso_info.load_offset); 157 break; 158 case DT_GNU_HASH: 159 vdso_info.gnu_hash = 160 (ELF(Word) *)((uintptr_t)dyn[i].d_un.d_ptr + 161 vdso_info.load_offset); 162 break; 163 case DT_VERSYM: 164 vdso_info.versym = (ELF(Versym) *) 165 ((uintptr_t)dyn[i].d_un.d_ptr 166 + vdso_info.load_offset); 167 break; 168 case DT_VERDEF: 169 vdso_info.verdef = (ELF(Verdef) *) 170 ((uintptr_t)dyn[i].d_un.d_ptr 171 + vdso_info.load_offset); 172 break; 173 } 174 } 175 if (!vdso_info.symstrings || !vdso_info.symtab || 176 (!hash && !vdso_info.gnu_hash)) 177 return; /* Failed */ 178 179 if (!vdso_info.verdef) 180 vdso_info.versym = 0; 181 182 /* Parse the hash table header. */ 183 if (vdso_info.gnu_hash) { 184 vdso_info.nbucket = vdso_info.gnu_hash[0]; 185 /* The bucket array is located after the header (4 uint32) and the bloom 186 * filter (size_t array of gnu_hash[2] elements). 187 */ 188 vdso_info.bucket = vdso_info.gnu_hash + 4 + 189 sizeof(size_t) / 4 * vdso_info.gnu_hash[2]; 190 } else { 191 vdso_info.nbucket = hash[0]; 192 vdso_info.nchain = hash[1]; 193 vdso_info.bucket = &hash[2]; 194 vdso_info.chain = &hash[vdso_info.nbucket + 2]; 195 } 196 197 /* That's all we need. */ 198 vdso_info.valid = true; 199 } 200 201 static bool vdso_match_version(ELF(Versym) ver, 202 const char *name, ELF(Word) hash) 203 { 204 /* 205 * This is a helper function to check if the version indexed by 206 * ver matches name (which hashes to hash). 207 * 208 * The version definition table is a mess, and I don't know how 209 * to do this in better than linear time without allocating memory 210 * to build an index. I also don't know why the table has 211 * variable size entries in the first place. 212 * 213 * For added fun, I can't find a comprehensible specification of how 214 * to parse all the weird flags in the table. 215 * 216 * So I just parse the whole table every time. 217 */ 218 219 /* First step: find the version definition */ 220 ver &= 0x7fff; /* Apparently bit 15 means "hidden" */ 221 ELF(Verdef) *def = vdso_info.verdef; 222 while(true) { 223 if ((def->vd_flags & VER_FLG_BASE) == 0 224 && (def->vd_ndx & 0x7fff) == ver) 225 break; 226 227 if (def->vd_next == 0) 228 return false; /* No definition. */ 229 230 def = (ELF(Verdef) *)((char *)def + def->vd_next); 231 } 232 233 /* Now figure out whether it matches. */ 234 ELF(Verdaux) *aux = (ELF(Verdaux)*)((char *)def + def->vd_aux); 235 return def->vd_hash == hash 236 && !strcmp(name, vdso_info.symstrings + aux->vda_name); 237 } 238 239 static bool check_sym(ELF(Sym) *sym, ELF(Word) i, const char *name, 240 const char *version, unsigned long ver_hash) 241 { 242 /* Check for a defined global or weak function w/ right name. */ 243 if (ELF64_ST_TYPE(sym->st_info) != STT_FUNC) 244 return false; 245 if (ELF64_ST_BIND(sym->st_info) != STB_GLOBAL && 246 ELF64_ST_BIND(sym->st_info) != STB_WEAK) 247 return false; 248 if (strcmp(name, vdso_info.symstrings + sym->st_name)) 249 return false; 250 251 /* Check symbol version. */ 252 if (vdso_info.versym && 253 !vdso_match_version(vdso_info.versym[i], version, ver_hash)) 254 return false; 255 256 return true; 257 } 258 259 void *vdso_sym(const char *version, const char *name) 260 { 261 unsigned long ver_hash; 262 if (!vdso_info.valid) 263 return 0; 264 265 ver_hash = elf_hash(version); 266 ELF(Word) i; 267 268 if (vdso_info.gnu_hash) { 269 uint32_t h1 = gnu_hash(name), h2, *hashval; 270 271 i = vdso_info.bucket[h1 % vdso_info.nbucket]; 272 if (i == 0) 273 return 0; 274 h1 |= 1; 275 hashval = vdso_info.bucket + vdso_info.nbucket + 276 (i - vdso_info.gnu_hash[1]); 277 for (;; i++) { 278 ELF(Sym) *sym = &vdso_info.symtab[i]; 279 h2 = *hashval++; 280 if (h1 == (h2 | 1) && 281 check_sym(sym, i, name, version, ver_hash)) 282 return (void *)(vdso_info.load_offset + 283 sym->st_value); 284 if (h2 & 1) 285 break; 286 } 287 } else { 288 i = vdso_info.bucket[elf_hash(name) % vdso_info.nbucket]; 289 for (; i; i = vdso_info.chain[i]) { 290 ELF(Sym) *sym = &vdso_info.symtab[i]; 291 if (sym->st_shndx != SHN_UNDEF && 292 check_sym(sym, i, name, version, ver_hash)) 293 return (void *)(vdso_info.load_offset + 294 sym->st_value); 295 } 296 } 297 298 return 0; 299 } 300 301 void vdso_init_from_auxv(void *auxv) 302 { 303 ELF(auxv_t) *elf_auxv = auxv; 304 for (int i = 0; elf_auxv[i].a_type != AT_NULL; i++) 305 { 306 if (elf_auxv[i].a_type == AT_SYSINFO_EHDR) { 307 vdso_init_from_sysinfo_ehdr(elf_auxv[i].a_un.a_val); 308 return; 309 } 310 } 311 312 vdso_info.valid = false; 313 } 314