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Make dlerror(3) thread-safe.

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I gave up trying to use the usual thread-local buffer idiom; calls to
calloc(3) and free(3) from any of the "dl" functions -- which live in
the dynamic linker -- end up resolving to the dynamic linker's stubs.
I tried to work around that, but was just making things more complicated.
This alternative costs us a well-known TLS slot (instead of the
dynamically-allocated TLS slot we'd have used otherwise, so no difference
there), plus an extra buffer inside every pthread_internal_t.

Bug: 5404023
Change-Id: Ie9614edd05b6d1eeaf7bf9172792d616c6361767
This commit is contained in:
Elliott Hughes
2012-10-16 15:54:46 -07:00
parent a9944cfe9e
commit 5419b94747
10 changed files with 124 additions and 94 deletions
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97
linker/dlfcn.cpp
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@@ -14,39 +14,52 @@
* limitations under the License.
*/
#include "linker.h"
#include <dlfcn.h>
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <bionic/pthread_internal.h>
#include <private/bionic_tls.h>
#include <private/ScopedPthreadMutexLocker.h>
#include "linker.h"
#include "linker_format.h"
#include <private/ThreadLocalBuffer.h>
/* This file hijacks the symbols stubbed out in libdl.so. */
static char dl_err_buf[1024];
static const char* dl_err_str;
#define likely(expr) __builtin_expect (expr, 1)
#define unlikely(expr) __builtin_expect (expr, 0)
static pthread_mutex_t gDlMutex = PTHREAD_RECURSIVE_MUTEX_INITIALIZER;
static void set_dlerror(const char* msg, const char* detail) {
if (detail != NULL) {
format_buffer(dl_err_buf, sizeof(dl_err_buf), "%s: %s", msg, detail);
} else {
format_buffer(dl_err_buf, sizeof(dl_err_buf), "%s", msg);
}
dl_err_str = (const char*) &dl_err_buf[0];
static const char* __bionic_set_dlerror(char* new_value) {
void* tls = const_cast<void*>(__get_tls());
char** dlerror_slot = &reinterpret_cast<char**>(tls)[TLS_SLOT_DLERROR];
const char* old_value = *dlerror_slot;
*dlerror_slot = new_value;
return old_value;
}
void *dlopen(const char* filename, int flag) {
static void __bionic_format_dlerror(const char* msg, const char* detail) {
char* buffer = __get_thread()->dlerror_buffer;
strlcpy(buffer, msg, __BIONIC_DLERROR_BUFFER_SIZE);
if (detail != NULL) {
strlcat(buffer, ": ", __BIONIC_DLERROR_BUFFER_SIZE);
strlcat(buffer, detail, __BIONIC_DLERROR_BUFFER_SIZE);
}
__bionic_set_dlerror(buffer);
}
const char* dlerror() {
const char* old_value = __bionic_set_dlerror(NULL);
return old_value;
}
void* dlopen(const char* filename, int flag) {
ScopedPthreadMutexLocker locker(&gDlMutex);
soinfo* result = find_library(filename);
if (result == NULL) {
set_dlerror("dlopen failed", linker_get_error());
__bionic_format_dlerror("dlopen failed", linker_get_error());
return NULL;
}
soinfo_call_constructors(result);
@@ -54,21 +67,15 @@ void *dlopen(const char* filename, int flag) {
return result;
}
const char* dlerror() {
const char* old_value = dl_err_str;
dl_err_str = NULL;
return (const char*) old_value;
}
void* dlsym(void* handle, const char* symbol) {
ScopedPthreadMutexLocker locker(&gDlMutex);
if (unlikely(handle == 0)) {
set_dlerror("dlsym library handle is null", NULL);
if (handle == NULL) {
__bionic_format_dlerror("dlsym library handle is null", NULL);
return NULL;
}
if (unlikely(symbol == 0)) {
set_dlerror("dlsym symbol name is null", NULL);
if (symbol == NULL) {
__bionic_format_dlerror("dlsym symbol name is null", NULL);
return NULL;
}
@@ -89,18 +96,18 @@ void* dlsym(void* handle, const char* symbol) {
sym = soinfo_lookup(found, symbol);
}
if (likely(sym != 0)) {
if (sym != NULL) {
unsigned bind = ELF32_ST_BIND(sym->st_info);
if (likely((bind == STB_GLOBAL) && (sym->st_shndx != 0))) {
if (bind == STB_GLOBAL && sym->st_shndx != 0) {
unsigned ret = sym->st_value + found->load_bias;
return (void*) ret;
}
set_dlerror("symbol found but not global", symbol);
__bionic_format_dlerror("symbol found but not global", symbol);
return NULL;
} else {
set_dlerror("undefined symbol", symbol);
__bionic_format_dlerror("undefined symbol", symbol);
return NULL;
}
}
@@ -159,7 +166,7 @@ int dlclose(void* handle) {
/* st_other */ 0, \
shndx }
static Elf32_Sym libdl_symtab[] = {
static Elf32_Sym gLibDlSymtab[] = {
// Total length of libdl_info.strtab, including trailing 0.
// This is actually the STH_UNDEF entry. Technically, it's
// supposed to have st_name == 0, but instead, it points to an index
@@ -179,24 +186,24 @@ static Elf32_Sym libdl_symtab[] = {
// 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,
// gLibDlSymtab starting with index [1], then
// use gLibDlChains to find the next index to
// look at. gLibDlChains should be set up to
// walk through every element in gLibDlSymtab,
// and then end with 0 (sentinel value).
//
// That is, libdl_chains should look like
// That is, gLibDlChains 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
// of actual symbols, or nelems(gLibDlSymtab)-1
// (since the first element of gLibDlSymtab is not
// a real symbol).
//
// (see soinfo_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[7] = { 0, 2, 3, 4, 5, 6, 0 };
static unsigned gLibDlBuckets[1] = { 1 };
static unsigned gLibDlChains[7] = { 0, 2, 3, 4, 5, 6, 0 };
// This is used by the dynamic linker. Every process gets these symbols for free.
soinfo libdl_info = {
@@ -210,12 +217,12 @@ soinfo libdl_info = {
flags: FLAG_LINKED,
strtab: ANDROID_LIBDL_STRTAB,
symtab: libdl_symtab,
symtab: gLibDlSymtab,
nbucket: 1,
nchain: 7,
bucket: libdl_buckets,
chain: libdl_chains,
bucket: gLibDlBuckets,
chain: gLibDlChains,
plt_got: 0, plt_rel: 0, plt_rel_count: 0, rel: 0, rel_count: 0,
preinit_array: 0, preinit_array_count: 0, init_array: 0, init_array_count: 0,