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>
This commit is contained in:
Andy Lutomirski
H. Peter Anvin
parent
574c44fa8f
commit
98eedc3a9d
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On some architectures, when the kernel loads any userspace program it
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maps an ELF DSO into that program's address space. This DSO is called
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the vDSO and it often contains useful and highly-optimized alternatives
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to real syscalls.
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These functions are called just like ordinary C function according to
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your platform's ABI. Call them from a sensible context. (For example,
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if you set CS on x86 to something strange, the vDSO functions are
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within their rights to crash.) In addition, if you pass a bad
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pointer to a vDSO function, you might get SIGSEGV instead of -EFAULT.
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To find the DSO, parse the auxiliary vector passed to the program's
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entry point. The AT_SYSINFO_EHDR entry will point to the vDSO.
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The vDSO uses symbol versioning; whenever you request a symbol from the
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vDSO, specify the version you are expecting.
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Programs that dynamically link to glibc will use the vDSO automatically.
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Otherwise, you can use the reference parser in Documentation/vDSO/parse_vdso.c.
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Unless otherwise noted, the set of symbols with any given version and the
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ABI of those symbols is considered stable. It may vary across architectures,
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though.
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(As of this writing, this ABI documentation as been confirmed for x86_64.
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The maintainers of the other vDSO-using architectures should confirm
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that it is correct for their architecture.)
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/*
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* parse_vdso.c: Linux reference vDSO parser
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* Written by Andrew Lutomirski, 2011.
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*
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* This code is meant to be linked in to various programs that run on Linux.
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* As such, it is available with as few restrictions as possible. This file
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* is licensed under the Creative Commons Zero License, version 1.0,
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* available at http://creativecommons.org/publicdomain/zero/1.0/legalcode
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*
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* The vDSO is a regular ELF DSO that the kernel maps into user space when
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* it starts a program. It works equally well in statically and dynamically
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* linked binaries.
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*
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* This code is tested on x86_64. In principle it should work on any 64-bit
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* architecture that has a vDSO.
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*/
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#include <stdbool.h>
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#include <stdint.h>
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#include <string.h>
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#include <elf.h>
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/*
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* To use this vDSO parser, first call one of the vdso_init_* functions.
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* If you've already parsed auxv, then pass the value of AT_SYSINFO_EHDR
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* to vdso_init_from_sysinfo_ehdr. Otherwise pass auxv to vdso_init_from_auxv.
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* Then call vdso_sym for each symbol you want. For example, to look up
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* gettimeofday on x86_64, use:
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*
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* <some pointer> = vdso_sym("LINUX_2.6", "gettimeofday");
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* or
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* <some pointer> = vdso_sym("LINUX_2.6", "__vdso_gettimeofday");
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*
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* vdso_sym will return 0 if the symbol doesn't exist or if the init function
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* failed or was not called. vdso_sym is a little slow, so its return value
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* should be cached.
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*
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* vdso_sym is threadsafe; the init functions are not.
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*
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* These are the prototypes:
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*/
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extern void vdso_init_from_auxv(void *auxv);
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extern void vdso_init_from_sysinfo_ehdr(uintptr_t base);
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extern void *vdso_sym(const char *version, const char *name);
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/* And here's the code. */
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#ifndef __x86_64__
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# error Not yet ported to non-x86_64 architectures
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#endif
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static struct vdso_info
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{
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bool valid;
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/* Load information */
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uintptr_t load_addr;
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uintptr_t load_offset; /* load_addr - recorded vaddr */
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/* Symbol table */
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Elf64_Sym *symtab;
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const char *symstrings;
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Elf64_Word *bucket, *chain;
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Elf64_Word nbucket, nchain;
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/* Version table */
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Elf64_Versym *versym;
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Elf64_Verdef *verdef;
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} vdso_info;
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/* Straight from the ELF specification. */
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static unsigned long elf_hash(const unsigned char *name)
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{
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unsigned long h = 0, g;
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while (*name)
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{
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h = (h << 4) + *name++;
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if (g = h & 0xf0000000)
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h ^= g >> 24;
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h &= ~g;
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}
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return h;
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}
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void vdso_init_from_sysinfo_ehdr(uintptr_t base)
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{
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size_t i;
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bool found_vaddr = false;
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vdso_info.valid = false;
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vdso_info.load_addr = base;
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Elf64_Ehdr *hdr = (Elf64_Ehdr*)base;
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Elf64_Phdr *pt = (Elf64_Phdr*)(vdso_info.load_addr + hdr->e_phoff);
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Elf64_Dyn *dyn = 0;
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/*
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* We need two things from the segment table: the load offset
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* and the dynamic table.
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*/
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for (i = 0; i < hdr->e_phnum; i++)
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{
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if (pt[i].p_type == PT_LOAD && !found_vaddr) {
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found_vaddr = true;
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vdso_info.load_offset = base
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+ (uintptr_t)pt[i].p_offset
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- (uintptr_t)pt[i].p_vaddr;
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} else if (pt[i].p_type == PT_DYNAMIC) {
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dyn = (Elf64_Dyn*)(base + pt[i].p_offset);
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}
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}
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if (!found_vaddr || !dyn)
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return; /* Failed */
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/*
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* Fish out the useful bits of the dynamic table.
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*/
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Elf64_Word *hash = 0;
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vdso_info.symstrings = 0;
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vdso_info.symtab = 0;
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vdso_info.versym = 0;
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vdso_info.verdef = 0;
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for (i = 0; dyn[i].d_tag != DT_NULL; i++) {
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switch (dyn[i].d_tag) {
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case DT_STRTAB:
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vdso_info.symstrings = (const char *)
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((uintptr_t)dyn[i].d_un.d_ptr
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+ vdso_info.load_offset);
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break;
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case DT_SYMTAB:
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vdso_info.symtab = (Elf64_Sym *)
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((uintptr_t)dyn[i].d_un.d_ptr
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+ vdso_info.load_offset);
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break;
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case DT_HASH:
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hash = (Elf64_Word *)
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((uintptr_t)dyn[i].d_un.d_ptr
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+ vdso_info.load_offset);
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break;
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case DT_VERSYM:
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vdso_info.versym = (Elf64_Versym *)
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((uintptr_t)dyn[i].d_un.d_ptr
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+ vdso_info.load_offset);
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break;
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case DT_VERDEF:
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vdso_info.verdef = (Elf64_Verdef *)
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((uintptr_t)dyn[i].d_un.d_ptr
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+ vdso_info.load_offset);
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break;
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}
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}
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if (!vdso_info.symstrings || !vdso_info.symtab || !hash)
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return; /* Failed */
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if (!vdso_info.verdef)
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vdso_info.versym = 0;
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/* Parse the hash table header. */
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vdso_info.nbucket = hash[0];
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vdso_info.nchain = hash[1];
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vdso_info.bucket = &hash[2];
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vdso_info.chain = &hash[vdso_info.nbucket + 2];
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/* That's all we need. */
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vdso_info.valid = true;
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}
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static bool vdso_match_version(Elf64_Versym ver,
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const char *name, Elf64_Word hash)
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{
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/*
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* This is a helper function to check if the version indexed by
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* ver matches name (which hashes to hash).
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*
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* The version definition table is a mess, and I don't know how
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* to do this in better than linear time without allocating memory
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* to build an index. I also don't know why the table has
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* variable size entries in the first place.
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*
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* For added fun, I can't find a comprehensible specification of how
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* to parse all the weird flags in the table.
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*
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* So I just parse the whole table every time.
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*/
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/* First step: find the version definition */
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ver &= 0x7fff; /* Apparently bit 15 means "hidden" */
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Elf64_Verdef *def = vdso_info.verdef;
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while(true) {
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if ((def->vd_flags & VER_FLG_BASE) == 0
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&& (def->vd_ndx & 0x7fff) == ver)
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break;
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if (def->vd_next == 0)
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return false; /* No definition. */
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def = (Elf64_Verdef *)((char *)def + def->vd_next);
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}
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/* Now figure out whether it matches. */
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Elf64_Verdaux *aux = (Elf64_Verdaux*)((char *)def + def->vd_aux);
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return def->vd_hash == hash
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&& !strcmp(name, vdso_info.symstrings + aux->vda_name);
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}
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void *vdso_sym(const char *version, const char *name)
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{
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unsigned long ver_hash;
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if (!vdso_info.valid)
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return 0;
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ver_hash = elf_hash(version);
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Elf64_Word chain = vdso_info.bucket[elf_hash(name) % vdso_info.nbucket];
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for (; chain != STN_UNDEF; chain = vdso_info.chain[chain]) {
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Elf64_Sym *sym = &vdso_info.symtab[chain];
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/* Check for a defined global or weak function w/ right name. */
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if (ELF64_ST_TYPE(sym->st_info) != STT_FUNC)
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continue;
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if (ELF64_ST_BIND(sym->st_info) != STB_GLOBAL &&
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ELF64_ST_BIND(sym->st_info) != STB_WEAK)
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continue;
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if (sym->st_shndx == SHN_UNDEF)
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continue;
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if (strcmp(name, vdso_info.symstrings + sym->st_name))
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continue;
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/* Check symbol version. */
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if (vdso_info.versym
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&& !vdso_match_version(vdso_info.versym[chain],
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version, ver_hash))
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continue;
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return (void *)(vdso_info.load_offset + sym->st_value);
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}
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return 0;
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}
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void vdso_init_from_auxv(void *auxv)
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{
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Elf64_auxv_t *elf_auxv = auxv;
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for (int i = 0; elf_auxv[i].a_type != AT_NULL; i++)
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{
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if (elf_auxv[i].a_type == AT_SYSINFO_EHDR) {
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vdso_init_from_sysinfo_ehdr(elf_auxv[i].a_un.a_val);
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return;
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}
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}
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vdso_info.valid = false;
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}
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@ -0,0 +1,111 @@
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/*
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* vdso_test.c: Sample code to test parse_vdso.c on x86_64
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* Copyright (c) 2011 Andy Lutomirski
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* Subject to the GNU General Public License, version 2
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*
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* You can amuse yourself by compiling with:
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* gcc -std=gnu99 -nostdlib
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* -Os -fno-asynchronous-unwind-tables -flto
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* vdso_test.c parse_vdso.c -o vdso_test
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* to generate a small binary with no dependencies at all.
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*/
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#include <sys/syscall.h>
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#include <sys/time.h>
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#include <unistd.h>
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#include <stdint.h>
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extern void *vdso_sym(const char *version, const char *name);
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extern void vdso_init_from_sysinfo_ehdr(uintptr_t base);
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extern void vdso_init_from_auxv(void *auxv);
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/* We need a libc functions... */
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int strcmp(const char *a, const char *b)
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{
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/* This implementation is buggy: it never returns -1. */
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while (*a || *b) {
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if (*a != *b)
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return 1;
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if (*a == 0 || *b == 0)
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return 1;
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a++;
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b++;
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}
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return 0;
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}
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/* ...and two syscalls. This is x86_64-specific. */
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static inline long linux_write(int fd, const void *data, size_t len)
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{
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long ret;
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asm volatile ("syscall" : "=a" (ret) : "a" (__NR_write),
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"D" (fd), "S" (data), "d" (len) :
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"cc", "memory", "rcx",
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"r8", "r9", "r10", "r11" );
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return ret;
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}
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static inline void linux_exit(int code)
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{
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asm volatile ("syscall" : : "a" (__NR_exit), "D" (code));
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}
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void to_base10(char *lastdig, uint64_t n)
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{
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while (n) {
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*lastdig = (n % 10) + '0';
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n /= 10;
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lastdig--;
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}
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}
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__attribute__((externally_visible)) void c_main(void **stack)
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{
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/* Parse the stack */
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long argc = (long)*stack;
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stack += argc + 2;
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/* Now we're pointing at the environment. Skip it. */
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while(*stack)
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stack++;
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stack++;
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/* Now we're pointing at auxv. Initialize the vDSO parser. */
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vdso_init_from_auxv((void *)stack);
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/* Find gettimeofday. */
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typedef long (*gtod_t)(struct timeval *tv, struct timezone *tz);
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gtod_t gtod = (gtod_t)vdso_sym("LINUX_2.6", "__vdso_gettimeofday");
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if (!gtod)
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linux_exit(1);
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struct timeval tv;
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long ret = gtod(&tv, 0);
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if (ret == 0) {
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char buf[] = "The time is .000000\n";
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to_base10(buf + 31, tv.tv_sec);
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to_base10(buf + 38, tv.tv_usec);
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linux_write(1, buf, sizeof(buf) - 1);
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} else {
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linux_exit(ret);
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}
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linux_exit(0);
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}
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/*
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* This is the real entry point. It passes the initial stack into
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* the C entry point.
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*/
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asm (
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".text\n"
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".global _start\n"
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".type _start,@function\n"
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"_start:\n\t"
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"mov %rsp,%rdi\n\t"
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"jmp c_main"
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);
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Reference in New Issue