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synchronize shared library dtor exec against concurrent loads/ctors 

previously, going way back, there was simply no synchronization here.
a call to exit concurrent with ctor execution from dlopen could cause
a dtor to execute concurrently with its corresponding ctor, or could
cause dtors for newly-constructed libraries to be skipped.

introduce a shutting_down state that blocks further ctor execution,
producing the quiescence the dtor execution loop needs to ensure any
kind of consistency, and that blocks further calls to dlopen so that a
call into dlopen from a dtor cannot deadlock.

better approaches to some of this may be possible, but the changes
here at least make things safe.
This commit is contained in:
Rich Felker 2019-03-01 22:47:29 -05:00
parent 188759bbee
commit 8e43b5613e
1 changed files with 17 additions and 1 deletions
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18
ldso/dynlink.c
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@ -124,6 +124,7 @@ static int runtime;
static int ldd_mode;
static int ldso_fail;
static int noload;
static int shutting_down;
static jmp_buf *rtld_fail;
static pthread_rwlock_t lock;
static struct debug debug;
@ -1350,7 +1351,18 @@ void __libc_exit_fini()
{
struct dso *p;
size_t dyn[DYN_CNT];
int self = __pthread_self()->tid;
/* Take both locks before setting shutting_down, so that
* either lock is sufficient to read its value. The lock
* order matches that in dlopen to avoid deadlock. */
pthread_rwlock_wrlock(&lock);
pthread_mutex_lock(&init_fini_lock);
shutting_down = 1;
pthread_rwlock_unlock(&lock);
for (p=fini_head; p; p=p->fini_next) {
while (p->ctor_visitor && p->ctor_visitor!=self)
pthread_cond_wait(&ctor_cond, &init_fini_lock);
if (!p->constructed) continue;
decode_vec(p->dynv, dyn, DYN_CNT);
if (dyn[0] & (1<<DT_FINI_ARRAY)) {
@ -1431,7 +1443,7 @@ static void do_init_fini(struct dso **queue)
pthread_mutex_lock(&init_fini_lock);
for (i=0; (p=queue[i]); i++) {
while (p->ctor_visitor && p->ctor_visitor!=self)
while ((p->ctor_visitor && p->ctor_visitor!=self) || shutting_down)
pthread_cond_wait(&ctor_cond, &init_fini_lock);
if (p->ctor_visitor || p->constructed)
continue;
@ -1937,6 +1949,10 @@ void *dlopen(const char *file, int mode)
__inhibit_ptc();
p = 0;
if (shutting_down) {
error("Cannot dlopen while program is exiting.");
goto end;
}
orig_tls_tail = tls_tail;
orig_tls_cnt = tls_cnt;
orig_tls_offset = tls_offset;