parse_vdso.c

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/*
 * 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;
}