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author | Andy Lutomirski <luto@mit.edu> | 2011-07-13 13:24:16 (GMT) |
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committer | H. Peter Anvin <hpa@linux.intel.com> | 2011-07-15 00:57:09 (GMT) |
commit | 98eedc3a9dbf90cecb91093d2a7fa083942b7d13 (patch) | |
tree | 0ed9320faed2d62caea337b978d5216a7fea55a8 /Documentation/vDSO/parse_vdso.c | |
parent | 574c44fa8fa6262ffd5939789ef51a6e98ed62d7 (diff) | |
download | linux-98eedc3a9dbf90cecb91093d2a7fa083942b7d13.tar.xz |
Document the vDSO and add a reference parser
It turns out that parsing the vDSO is nontrivial if you don't already
have an ELF dynamic loader around. So document it in Documentation/ABI
and add a reference CC0-licenced parser.
This code is dedicated to Go issue 1933:
http://code.google.com/p/go/issues/detail?id=1933
Signed-off-by: Andy Lutomirski <luto@mit.edu>
Link: http://lkml.kernel.org/r/a315a9514cd71bcf29436cc31e35aada21a5ff21.1310563276.git.luto@mit.edu
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Diffstat (limited to 'Documentation/vDSO/parse_vdso.c')
-rw-r--r-- | Documentation/vDSO/parse_vdso.c | 256 |
1 files changed, 256 insertions, 0 deletions
diff --git a/Documentation/vDSO/parse_vdso.c b/Documentation/vDSO/parse_vdso.c new file mode 100644 index 0000000..8587020 --- /dev/null +++ b/Documentation/vDSO/parse_vdso.c @@ -0,0 +1,256 @@ +/* + * parse_vdso.c: Linux reference vDSO parser + * Written by Andrew Lutomirski, 2011. + * + * 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_64. In principle it should work on any 64-bit + * architecture that has a vDSO. + */ + +#include <stdbool.h> +#include <stdint.h> +#include <string.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 __x86_64__ +# error Not yet ported to non-x86_64 architectures +#endif + +static struct vdso_info +{ + bool valid; + + /* Load information */ + uintptr_t load_addr; + uintptr_t load_offset; /* load_addr - recorded vaddr */ + + /* Symbol table */ + Elf64_Sym *symtab; + const char *symstrings; + Elf64_Word *bucket, *chain; + Elf64_Word nbucket, nchain; + + /* Version table */ + Elf64_Versym *versym; + Elf64_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; + + Elf64_Ehdr *hdr = (Elf64_Ehdr*)base; + Elf64_Phdr *pt = (Elf64_Phdr*)(vdso_info.load_addr + hdr->e_phoff); + Elf64_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 = (Elf64_Dyn*)(base + pt[i].p_offset); + } + } + + if (!found_vaddr || !dyn) + return; /* Failed */ + + /* + * Fish out the useful bits of the dynamic table. + */ + Elf64_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 = (Elf64_Sym *) + ((uintptr_t)dyn[i].d_un.d_ptr + + vdso_info.load_offset); + break; + case DT_HASH: + hash = (Elf64_Word *) + ((uintptr_t)dyn[i].d_un.d_ptr + + vdso_info.load_offset); + break; + case DT_VERSYM: + vdso_info.versym = (Elf64_Versym *) + ((uintptr_t)dyn[i].d_un.d_ptr + + vdso_info.load_offset); + break; + case DT_VERDEF: + vdso_info.verdef = (Elf64_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(Elf64_Versym ver, + const char *name, Elf64_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" */ + Elf64_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 = (Elf64_Verdef *)((char *)def + def->vd_next); + } + + /* Now figure out whether it matches. */ + Elf64_Verdaux *aux = (Elf64_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); + Elf64_Word chain = vdso_info.bucket[elf_hash(name) % vdso_info.nbucket]; + + for (; chain != STN_UNDEF; chain = vdso_info.chain[chain]) { + Elf64_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) +{ + Elf64_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; +} |