Re: [musl] Pending patches for MT-fork stuff

[Rich Felker <dalias@libc.org>]

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On Mon, Sep 28, 2020 at 07:26:15PM -0400, Rich Felker wrote:
> In investigating the MT-fork deadlock stuff and working on a patch
> that makes it attempt to trap rather than deadlocking, I found a
> problem with interaction of fork and aio. A fix for that, along with
> the patch to trap, and a minimal testcase for the aio bug, are
> attached.
> 
> There's also a problematic interaction of abort with fork that was
> just found in glibc that also exists in musl, that I still need to
> fix. I'll follow up later with a proposed solution for that.

And here's the proposed abort fix.

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From 000b86dffecf68c1f479e532dcf4e9a403062521 Mon Sep 17 00:00:00 2001
From: Rich Felker <dalias@aerifal.cx>
Date: Mon, 28 Sep 2020 19:30:19 -0400
Subject: [PATCH 4/5] move __abort_lock to its own file and drop pointless
 weak_alias trick

the dummy definition of __abort_lock in sigaction.c was performing
exactly the same role that putting the lock in its own source file
could and should have been used to achieve.

while we're moving it, give it a proper declaration.
---
 src/exit/abort.c            | 2 --
 src/exit/abort_lock.c       | 3 +++
 src/internal/pthread_impl.h | 2 ++
 src/signal/sigaction.c      | 6 ------
 4 files changed, 5 insertions(+), 8 deletions(-)
 create mode 100644 src/exit/abort_lock.c

diff --git a/src/exit/abort.c b/src/exit/abort.c
index e1980f10..f21f458e 100644
--- a/src/exit/abort.c
+++ b/src/exit/abort.c
@@ -6,8 +6,6 @@
 #include "lock.h"
 #include "ksigaction.h"
 
-hidden volatile int __abort_lock[1];
-
 _Noreturn void abort(void)
 {
 	raise(SIGABRT);
diff --git a/src/exit/abort_lock.c b/src/exit/abort_lock.c
new file mode 100644
index 00000000..3af72c7b
--- /dev/null
+++ b/src/exit/abort_lock.c
@@ -0,0 +1,3 @@
+#include "pthread_impl.h"
+
+volatile int __abort_lock[1];
diff --git a/src/internal/pthread_impl.h b/src/internal/pthread_impl.h
index 23ee566c..21d51be6 100644
--- a/src/internal/pthread_impl.h
+++ b/src/internal/pthread_impl.h
@@ -197,6 +197,8 @@ hidden void __tl_sync(pthread_t);
 
 extern hidden volatile int __thread_list_lock;
 
+extern hidden volatile int __abort_lock[1];
+
 extern hidden unsigned __default_stacksize;
 extern hidden unsigned __default_guardsize;
 
diff --git a/src/signal/sigaction.c b/src/signal/sigaction.c
index c109bea0..a4737404 100644
--- a/src/signal/sigaction.c
+++ b/src/signal/sigaction.c
@@ -7,12 +7,6 @@
 #include "lock.h"
 #include "ksigaction.h"
 
-static volatile int dummy_lock[1] = { 0 };
-
-extern hidden volatile int __abort_lock[1];
-
-weak_alias(dummy_lock, __abort_lock);
-
 static int unmask_done;
 static unsigned long handler_set[_NSIG/(8*sizeof(long))];
 
-- 
2.21.0


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From 7e661e4ff9f70b625e496b54729943bf3f7ecfac Mon Sep 17 00:00:00 2001
From: Rich Felker <dalias@aerifal.cx>
Date: Mon, 28 Sep 2020 19:32:34 -0400
Subject: [PATCH 5/5] fix race deadlock with abort in forked child of
 multithreaded parent

if the multithreaded parent forked while another thread was calling
sigaction for SIGABRT or calling abort, the child could inherit a lock
state in which future calls to abort will deadlock. this is
nonconforming since abort is AS-safe and permitted to be called in the
MT-forked child.

since there is no userspace data state protected by the lock (it's
only used to exclude sigaction for SIGABRT while abort is in
progress), just reset the lock word to 0 in the child rather than
holding the lock across the fork.
---
 src/process/fork.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/src/process/fork.c b/src/process/fork.c
index 538a0bcb..b5a5fa11 100644
--- a/src/process/fork.c
+++ b/src/process/fork.c
@@ -33,6 +33,7 @@ pid_t fork(void)
 		__thread_list_lock = 0;
 		libc.threads_minus_1 = 0;
 		if (libc.need_locks > 0) libc.need_locks = -2;
+		*__abort_lock = 0;
 	}
 	__aio_atfork(!ret);
 	__restore_sigs(&set);
-- 
2.21.0