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Date: Fri, 7 Oct 2022 21:36:16 -0400
From: Rich Felker <dalias@libc.org>
To: musl@lists.openwall.com
Message-ID: <20221008013615.GE29905@brightrain.aerifal.cx>
References: <033092a85a52d9f8e8d73a235e2381ae@ispras.ru>
 <20221006182151.GV29905@brightrain.aerifal.cx>
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User-Agent: Mutt/1.5.21 (2010-09-15)
Subject: Re: [musl] MT fork and key_lock in pthread_key_create.c


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On Thu, Oct 06, 2022 at 02:21:51PM -0400, Rich Felker wrote:
> On Thu, Oct 06, 2022 at 09:37:50AM +0300, Alexey Izbyshev wrote:
> > Hi,
> > 
> > I noticed that fork() doesn't take key_lock that is used to protect
> > the global table of thread-specific keys. I couldn't find mentions
> > of this lock in the MT fork discussion in the mailing list archive.
> > Was this lock overlooked?
> 
> I think what happened was that we made the main list of locks to
> review and take care of via grep for LOCK, and then manually added
> known instances of locks using other locking primitives. This one must
> have been missed.
> 
> Having special-case lock types like this is kinda a pain, but as long
> as there aren't too many I guess it's not a big deal.

Proposed patch attached.

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diff --git a/src/internal/fork_impl.h b/src/internal/fork_impl.h
index ae3a79e5..354e733b 100644
--- a/src/internal/fork_impl.h
+++ b/src/internal/fork_impl.h
@@ -16,3 +16,4 @@ extern hidden volatile int *const __vmlock_lockptr;
 
 hidden void __malloc_atfork(int);
 hidden void __ldso_atfork(int);
+hidden void __pthread_key_atfork(int);
diff --git a/src/process/fork.c b/src/process/fork.c
index 80e804b1..56f19313 100644
--- a/src/process/fork.c
+++ b/src/process/fork.c
@@ -37,6 +37,7 @@ static void dummy(int x) { }
 weak_alias(dummy, __fork_handler);
 weak_alias(dummy, __malloc_atfork);
 weak_alias(dummy, __aio_atfork);
+weak_alias(dummy, __pthread_key_atfork);
 weak_alias(dummy, __ldso_atfork);
 
 static void dummy_0(void) { }
@@ -51,6 +52,7 @@ pid_t fork(void)
 	int need_locks = libc.need_locks > 0;
 	if (need_locks) {
 		__ldso_atfork(-1);
+		__pthread_key_atfork(-1);
 		__aio_atfork(-1);
 		__inhibit_ptc();
 		for (int i=0; i<sizeof atfork_locks/sizeof *atfork_locks; i++)
@@ -78,6 +80,7 @@ pid_t fork(void)
 				else **atfork_locks[i] = 0;
 		__release_ptc();
 		if (ret) __aio_atfork(0);
+		__pthread_key_atfork(!ret);
 		__ldso_atfork(!ret);
 	}
 	__restore_sigs(&set);
diff --git a/src/thread/pthread_key_create.c b/src/thread/pthread_key_create.c
index d1120941..39770c7a 100644
--- a/src/thread/pthread_key_create.c
+++ b/src/thread/pthread_key_create.c
@@ -1,4 +1,5 @@
 #include "pthread_impl.h"
+#include "fork_impl.h"
 
 volatile size_t __pthread_tsd_size = sizeof(void *) * PTHREAD_KEYS_MAX;
 void *__pthread_tsd_main[PTHREAD_KEYS_MAX] = { 0 };
@@ -20,6 +21,13 @@ static void dummy_0(void)
 weak_alias(dummy_0, __tl_lock);
 weak_alias(dummy_0, __tl_unlock);
 
+void __pthread_key_atfork(int who)
+{
+	if (who<0) __pthread_rwlock_rdlock(&key_lock);
+	else if (!who) __pthread_rwlock_unlock(&key_lock);
+	else key_lock = (pthread_rwlock_t)PTHREAD_RWLOCK_INITIALIZER;
+}
+
 int __pthread_key_create(pthread_key_t *k, void (*dtor)(void *))
 {
 	pthread_t self = __pthread_self();

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