linker/dlfcn.c

/*
 * Copyright (C) 2007 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
#include <dlfcn.h>
#include <pthread.h>
#include "linker.h"

/* This file hijacks the symbols stubbed out in libdl.so. */

#define DL_SUCCESS                    0
#define DL_ERR_CANNOT_FIND_LIBRARY    1
#define DL_ERR_INVALID_LIBRARY_HANDLE 2
#define DL_ERR_BAD_SYMBOL_NAME        3
#define DL_ERR_SYMBOL_NOT_FOUND       4
#define DL_ERR_SYMBOL_NOT_GLOBAL      5

static const char *dl_errors[] = {
    [DL_SUCCESS] = NULL,
    [DL_ERR_CANNOT_FIND_LIBRARY] = "Cannot find library",
    [DL_ERR_INVALID_LIBRARY_HANDLE] = "Invalid library handle",
    [DL_ERR_BAD_SYMBOL_NAME] = "Invalid symbol name",
    [DL_ERR_SYMBOL_NOT_FOUND] = "Symbol not found",
    [DL_ERR_SYMBOL_NOT_GLOBAL] = "Symbol is not global",
};

static int dl_last_err = DL_SUCCESS;

#define likely(expr)   __builtin_expect (expr, 1)
#define unlikely(expr) __builtin_expect (expr, 0)

static pthread_mutex_t dl_lock = PTHREAD_MUTEX_INITIALIZER;

void *dlopen(const char *filename, int flag) 
{
    soinfo *ret;

    pthread_mutex_lock(&dl_lock);
    ret = find_library(filename);
    if (unlikely(ret == NULL)) {
        dl_last_err = DL_ERR_CANNOT_FIND_LIBRARY;
    } else {
        ret->refcount++;
    }
    pthread_mutex_unlock(&dl_lock);
    return ret;
}

const char *dlerror(void)
{
    const char *err = dl_errors[dl_last_err];
    dl_last_err = DL_SUCCESS;
    return err;
}

void *dlsym(void *handle, const char *symbol)
{
    unsigned base;
    Elf32_Sym *sym;
    unsigned bind;

    pthread_mutex_lock(&dl_lock);
    
    if(unlikely(handle == 0)) { 
        dl_last_err = DL_ERR_INVALID_LIBRARY_HANDLE;
        goto err;
    }
    if(unlikely(symbol == 0)) {
        dl_last_err = DL_ERR_BAD_SYMBOL_NAME;
        goto err;
    }
    
    if(handle == RTLD_DEFAULT) {
        sym = lookup(symbol, &base);
    } else if(handle == RTLD_NEXT) {
        sym = lookup(symbol, &base);
    } else {
        sym = lookup_in_library((soinfo*) handle, symbol);
        base = ((soinfo*) handle)->base;
    }

    if(likely(sym != 0)) {
        bind = ELF32_ST_BIND(sym->st_info);
    
        if(likely((bind == STB_GLOBAL) && (sym->st_shndx != 0))) {
            unsigned ret = sym->st_value + base;
            pthread_mutex_unlock(&dl_lock);
            return (void*)ret;
        }

        dl_last_err = DL_ERR_SYMBOL_NOT_GLOBAL;
    }
    else dl_last_err = DL_ERR_SYMBOL_NOT_FOUND;

err:
    pthread_mutex_unlock(&dl_lock);
    return 0;
}

int dlclose(void *handle)
{
    pthread_mutex_lock(&dl_lock);
    (void)unload_library((soinfo*)handle);
    pthread_mutex_unlock(&dl_lock);
    return 0;
}

#if defined(ANDROID_ARM_LINKER)
//                     0000000 00011111 111112 22222222 233333333334444444444
//                     0123456 78901234 567890 12345678 901234567890123456789
#define ANDROID_LIBDL_STRTAB \
                      "dlopen\0dlclose\0dlsym\0dlerror\0dl_unwind_find_exidx\0"

#elif defined(ANDROID_X86_LINKER)
//                     0000000 00011111 111112 22222222 2333333333344444
//                     0123456 78901234 567890 12345678 9012345678901234
#define ANDROID_LIBDL_STRTAB \
                      "dlopen\0dlclose\0dlsym\0dlerror\0dl_iterate_phdr\0"

#else /* !defined(ANDROID_ARM_LINKER) && !defined(ANDROID_X86_LINKER) */
#error Unsupported architecture. Only ARM and x86 are presently supported.
#endif


static Elf32_Sym libdl_symtab[] = {
      // total length of libdl_info.strtab, including trailing 0
      // This is actually the the STH_UNDEF entry. Technically, it's
      // supposed to have st_name == 0, but instead, it points to an index
      // in the strtab with a \0 to make iterating through the symtab easier.
    { st_name: sizeof(ANDROID_LIBDL_STRTAB) - 1,
    },
    { st_name: 0,   // starting index of the name in libdl_info.strtab
      st_value: (Elf32_Addr) &dlopen,
      st_info: STB_GLOBAL << 4,
      st_shndx: 1,
    },
    { st_name: 7,
      st_value: (Elf32_Addr) &dlclose,
      st_info: STB_GLOBAL << 4,
      st_shndx: 1,
    },
    { st_name: 15,
      st_value: (Elf32_Addr) &dlsym,
      st_info: STB_GLOBAL << 4,
      st_shndx: 1,
    },
    { st_name: 21,
      st_value: (Elf32_Addr) &dlerror,
      st_info: STB_GLOBAL << 4,
      st_shndx: 1,
    },
#ifdef ANDROID_ARM_LINKER
    { st_name: 29,
      st_value: (Elf32_Addr) &dl_unwind_find_exidx,
      st_info: STB_GLOBAL << 4,
      st_shndx: 1,
    },
#elif defined(ANDROID_X86_LINKER)
    { st_name: 29,
      st_value: (Elf32_Addr) &dl_iterate_phdr,
      st_info: STB_GLOBAL << 4,
      st_shndx: 1,
    },
#endif
};

/* Fake out a hash table with a single bucket.
 * A search of the hash table will look through
 * libdl_symtab starting with index [1], then
 * use libdl_chains to find the next index to
 * look at.  libdl_chains should be set up to
 * walk through every element in libdl_symtab,
 * and then end with 0 (sentinel value).
 *
 * I.e., libdl_chains should look like
 * { 0, 2, 3, ... N, 0 } where N is the number
 * of actual symbols, or nelems(libdl_symtab)-1
 * (since the first element of libdl_symtab is not
 * a real symbol).
 *
 * (see _elf_lookup())
 *
 * Note that adding any new symbols here requires
 * stubbing them out in libdl.
 */
static unsigned libdl_buckets[1] = { 1 };
static unsigned libdl_chains[6] = { 0, 2, 3, 4, 5, 0 };

soinfo libdl_info = {
    name: "libdl.so",
    flags: FLAG_LINKED,

    strtab: ANDROID_LIBDL_STRTAB,
    symtab: libdl_symtab,

    nbucket: 1,
    nchain: 6,
    bucket: libdl_buckets,
    chain: libdl_chains,
};