/* * parse_vdso.c: Linux reference vDSO parser * Written by Andrew Lutomirski, 2011-2014. * * This code is meant to be linked in to various programs that run on Linux. * As such, it is available with as few restrictions as possible. This file * is licensed under the Creative Commons Zero License, version 1.0, * available at http://creativecommons.org/publicdomain/zero/1.0/legalcode * * The vDSO is a regular ELF DSO that the kernel maps into user space when * it starts a program. It works equally well in statically and dynamically * linked binaries. * * This code is tested on x86. In principle it should work on any * architecture that has a vDSO. */ #include <stdbool.h> #include <stdint.h> #include <string.h> #include <limits.h> #include <elf.h> /* * To use this vDSO parser, first call one of the vdso_init_* functions. * If you've already parsed auxv, then pass the value of AT_SYSINFO_EHDR * to vdso_init_from_sysinfo_ehdr. Otherwise pass auxv to vdso_init_from_auxv. * Then call vdso_sym for each symbol you want. For example, to look up * gettimeofday on x86_64, use: * * <some pointer> = vdso_sym("LINUX_2.6", "gettimeofday"); * or * <some pointer> = vdso_sym("LINUX_2.6", "__vdso_gettimeofday"); * * vdso_sym will return 0 if the symbol doesn't exist or if the init function * failed or was not called. vdso_sym is a little slow, so its return value * should be cached. * * vdso_sym is threadsafe; the init functions are not. * * These are the prototypes: */ extern void vdso_init_from_auxv(void *auxv); extern void vdso_init_from_sysinfo_ehdr(uintptr_t base); extern void *vdso_sym(const char *version, const char *name); /* And here's the code. */ #ifndef ELF_BITS # if ULONG_MAX > 0xffffffffUL # define ELF_BITS 64 # else # define ELF_BITS 32 # endif #endif #define ELF_BITS_XFORM2(bits, x) Elf##bits##_##x #define ELF_BITS_XFORM(bits, x) ELF_BITS_XFORM2(bits, x) #define ELF(x) ELF_BITS_XFORM(ELF_BITS, x) static struct vdso_info { bool valid; /* Load information */ uintptr_t load_addr; uintptr_t load_offset; /* load_addr - recorded vaddr */ /* Symbol table */ ELF(Sym) *symtab; const char *symstrings; ELF(Word) *bucket, *chain; ELF(Word) nbucket, nchain; /* Version table */ ELF(Versym) *versym; ELF(Verdef) *verdef; } vdso_info; /* Straight from the ELF specification. */ static unsigned long elf_hash(const unsigned char *name) { unsigned long h = 0, g; while (*name) { h = (h << 4) + *name++; if (g = h & 0xf0000000) h ^= g >> 24; h &= ~g; } return h; } void vdso_init_from_sysinfo_ehdr(uintptr_t base) { size_t i; bool found_vaddr = false; vdso_info.valid = false; vdso_info.load_addr = base; ELF(Ehdr) *hdr = (ELF(Ehdr)*)base; if (hdr->e_ident[EI_CLASS] != (ELF_BITS == 32 ? ELFCLASS32 : ELFCLASS64)) { return; /* Wrong ELF class -- check ELF_BITS */ } ELF(Phdr) *pt = (ELF(Phdr)*)(vdso_info.load_addr + hdr->e_phoff); ELF(Dyn) *dyn = 0; /* * We need two things from the segment table: the load offset * and the dynamic table. */ for (i = 0; i < hdr->e_phnum; i++) { if (pt[i].p_type == PT_LOAD && !found_vaddr) { found_vaddr = true; vdso_info.load_offset = base + (uintptr_t)pt[i].p_offset - (uintptr_t)pt[i].p_vaddr; } else if (pt[i].p_type == PT_DYNAMIC) { dyn = (ELF(Dyn)*)(base + pt[i].p_offset); } } if (!found_vaddr || !dyn) return; /* Failed */ /* * Fish out the useful bits of the dynamic table. */ ELF(Word) *hash = 0; vdso_info.symstrings = 0; vdso_info.symtab = 0; vdso_info.versym = 0; vdso_info.verdef = 0; for (i = 0; dyn[i].d_tag != DT_NULL; i++) { switch (dyn[i].d_tag) { case DT_STRTAB: vdso_info.symstrings = (const char *) ((uintptr_t)dyn[i].d_un.d_ptr + vdso_info.load_offset); break; case DT_SYMTAB: vdso_info.symtab = (ELF(Sym) *) ((uintptr_t)dyn[i].d_un.d_ptr + vdso_info.load_offset); break; case DT_HASH: hash = (ELF(Word) *) ((uintptr_t)dyn[i].d_un.d_ptr + vdso_info.load_offset); break; case DT_VERSYM: vdso_info.versym = (ELF(Versym) *) ((uintptr_t)dyn[i].d_un.d_ptr + vdso_info.load_offset); break; case DT_VERDEF: vdso_info.verdef = (ELF(Verdef) *) ((uintptr_t)dyn[i].d_un.d_ptr + vdso_info.load_offset); break; } } if (!vdso_info.symstrings || !vdso_info.symtab || !hash) return; /* Failed */ if (!vdso_info.verdef) vdso_info.versym = 0; /* Parse the hash table header. */ vdso_info.nbucket = hash[0]; vdso_info.nchain = hash[1]; vdso_info.bucket = &hash[2]; vdso_info.chain = &hash[vdso_info.nbucket + 2]; /* That's all we need. */ vdso_info.valid = true; } static bool vdso_match_version(ELF(Versym) ver, const char *name, ELF(Word) hash) { /* * This is a helper function to check if the version indexed by * ver matches name (which hashes to hash). * * The version definition table is a mess, and I don't know how * to do this in better than linear time without allocating memory * to build an index. I also don't know why the table has * variable size entries in the first place. * * For added fun, I can't find a comprehensible specification of how * to parse all the weird flags in the table. * * So I just parse the whole table every time. */ /* First step: find the version definition */ ver &= 0x7fff; /* Apparently bit 15 means "hidden" */ ELF(Verdef) *def = vdso_info.verdef; while(true) { if ((def->vd_flags & VER_FLG_BASE) == 0 && (def->vd_ndx & 0x7fff) == ver) break; if (def->vd_next == 0) return false; /* No definition. */ def = (ELF(Verdef) *)((char *)def + def->vd_next); } /* Now figure out whether it matches. */ ELF(Verdaux) *aux = (ELF(Verdaux)*)((char *)def + def->vd_aux); return def->vd_hash == hash && !strcmp(name, vdso_info.symstrings + aux->vda_name); } void *vdso_sym(const char *version, const char *name) { unsigned long ver_hash; if (!vdso_info.valid) return 0; ver_hash = elf_hash(version); ELF(Word) chain = vdso_info.bucket[elf_hash(name) % vdso_info.nbucket]; for (; chain != STN_UNDEF; chain = vdso_info.chain[chain]) { ELF(Sym) *sym = &vdso_info.symtab[chain]; /* Check for a defined global or weak function w/ right name. */ if (ELF64_ST_TYPE(sym->st_info) != STT_FUNC) continue; if (ELF64_ST_BIND(sym->st_info) != STB_GLOBAL && ELF64_ST_BIND(sym->st_info) != STB_WEAK) continue; if (sym->st_shndx == SHN_UNDEF) continue; if (strcmp(name, vdso_info.symstrings + sym->st_name)) continue; /* Check symbol version. */ if (vdso_info.versym && !vdso_match_version(vdso_info.versym[chain], version, ver_hash)) continue; return (void *)(vdso_info.load_offset + sym->st_value); } return 0; } void vdso_init_from_auxv(void *auxv) { ELF(auxv_t) *elf_auxv = auxv; for (int i = 0; elf_auxv[i].a_type != AT_NULL; i++) { if (elf_auxv[i].a_type == AT_SYSINFO_EHDR) { vdso_init_from_sysinfo_ehdr(elf_auxv[i].a_un.a_val); return; } } vdso_info.valid = false; }