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Fix acquisition of nested write compat rtld locks.

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Obtaining compat rtld lock in write mode sets process signal mask to
block all signals.  Previous mask is stored in the global variable
oldsigmask.  If a lock is write-locked while another lock is already
write-locked, oldsigmask is overwritten by the total mask and on the
last unlock, all signals except traps appear to be blocked.

Fix this by counting the write-lock nested level, and only storing to
oldsigmask/restoring from it at the outermost level.

Masking signals disables involuntary preemption for libc_r, and there
could be no voluntary context switches in the locked code
(dl_iterate_phdr(3) keeps a lock around user callback, but it was
added long after libc_r was renounced).  Due to this, remembering the
level in the global variable after the lock is obtained should be
safe, because no two libc_r threads can acquire different write locks
in parallel.

PR:	215826
Reported by:	kami
Tested by:	yamagi@yamagi.org (previous version)
To be reviewed by:	kan
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks
This commit is contained in:
Konstantin Belousov 2017-01-10 19:26:55 +00:00
parent 0a50238935
commit b88a8d3d1d
1 changed files with 28 additions and 23 deletions
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51
libexec/rtld-elf/rtld_lock.c
View File

@ -64,7 +64,7 @@ typedef struct Struct_Lock {
} Lock;
static sigset_t fullsigmask, oldsigmask;
static int thread_flag;
static int thread_flag, wnested;
static void *
def_lock_create(void)
@ -117,29 +117,34 @@ def_rlock_acquire(void *lock)
static void
def_wlock_acquire(void *lock)
{
Lock *l = (Lock *)lock;
sigset_t tmp_oldsigmask;
Lock *l;
sigset_t tmp_oldsigmask;
for ( ; ; ) {
sigprocmask(SIG_BLOCK, &fullsigmask, &tmp_oldsigmask);
if (atomic_cmpset_acq_int(&l->lock, 0, WAFLAG))
break;
sigprocmask(SIG_SETMASK, &tmp_oldsigmask, NULL);
}
oldsigmask = tmp_oldsigmask;
l = (Lock *)lock;
for (;;) {
sigprocmask(SIG_BLOCK, &fullsigmask, &tmp_oldsigmask);
if (atomic_cmpset_acq_int(&l->lock, 0, WAFLAG))
break;
sigprocmask(SIG_SETMASK, &tmp_oldsigmask, NULL);
}
if (atomic_fetchadd_int(&wnested, 1) == 0)
oldsigmask = tmp_oldsigmask;
}
static void
def_lock_release(void *lock)
{
Lock *l = (Lock *)lock;
Lock *l;
if ((l->lock & WAFLAG) == 0)
atomic_add_rel_int(&l->lock, -RC_INCR);
else {
atomic_add_rel_int(&l->lock, -WAFLAG);
sigprocmask(SIG_SETMASK, &oldsigmask, NULL);
}
l = (Lock *)lock;
if ((l->lock & WAFLAG) == 0)
atomic_add_rel_int(&l->lock, -RC_INCR);
else {
assert(wnested > 0);
atomic_add_rel_int(&l->lock, -WAFLAG);
if (atomic_fetchadd_int(&wnested, -1) == 1)
sigprocmask(SIG_SETMASK, &oldsigmask, NULL);
}
}
static int
@ -373,12 +378,12 @@ _rtld_atfork_pre(int *locks)
return;
/*
* Warning: this does not work with the rtld compat locks
* above, since the thread signal mask is corrupted (set to
* all signals blocked) if two locks are taken in write mode.
* The caller of the _rtld_atfork_pre() must provide the
* working implementation of the locks, and libthr locks are
* fine.
* Warning: this did not worked well with the rtld compat
* locks above, when the thread signal mask was corrupted (set
* to all signals blocked) if two locks were taken
* simultaneously in the write mode. The caller of the
* _rtld_atfork_pre() must provide the working implementation
* of the locks anyway, and libthr locks are fine.
*/
wlock_acquire(rtld_phdr_lock, &ls[0]);
wlock_acquire(rtld_bind_lock, &ls[1]);