linux-kernel - [PATCH 2/2] coredump: Allow coredumps to pipes to work with io

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Date:   Wed, 20 Jul 2022 11:51:27 -0500
From:   "Eric W. Biederman" <ebiederm@...ssion.com>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     Olivier Langlois <olivier@...llion01.com>,
        Pavel Begunkov <asml.silence@...il.com>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        io-uring@...r.kernel.org, Alexander Viro <viro@...iv.linux.org.uk>,
        Oleg Nesterov <oleg@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH 2/2] coredump: Allow coredumps to pipes to work with io_uring

Now that io_uring like everything else stops for coredumps in
get_signal the code can once again allow any interruptible
condition after coredump_wait to interrupt the coredump.

Clear TIF_NOTIFY_SIGNAL after coredump_wait, to guarantee that
anything that sets TIF_NOTIFY_SIGNAL before coredump_wait completed
won't cause the coredumps to interrupted.

With all of the other threads in the process stopped io_uring doesn't
call task_work_add on the thread running do_coredump.  Combined with
the clearing of TIF_NOTIFY_SIGNAL this allows processes that use
io_uring to coredump through pipes.

Restore dump_interrupted to be a simple call to signal_pending
effectively reverting commit 06af8679449d ("coredump: Limit what can
interrupt coredumps").  At this point only SIGKILL delivered to the
coredumping thread should be able to cause signal_pending to return
true.

A nice followup would be to find a reliable race free way to modify
task_work_add and probably set_notify_signal to skip setting
TIF_NOTIFY_SIGNAL once it is clear a task will no longer process
signals and other interruptible conditions.  That would allow
TIF_NOTIFY_SIGNAL to be cleared where TIF_SIGPENDING is cleared in
coredump_zap_process.

To be as certain as possible that this works, I tested this with
commit 1d5f5ea7cb7d ("io-wq: remove worker to owner tw dependency")
reverted.  Which means that not only is TIF_NOTIFY_SIGNAL prevented
from stopping coredumps to pipes, the sequence of stopping threads to
participate in the coredump avoids deadlocks that were possible
previously.

Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
---
 fs/coredump.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/fs/coredump.c b/fs/coredump.c
index 67dda77c500f..c06594f56cbb 100644
--- a/fs/coredump.c
+++ b/fs/coredump.c
@@ -476,7 +476,7 @@ static bool dump_interrupted(void)
 	 * but then we need to teach dump_write() to restart and clear
 	 * TIF_SIGPENDING.
 	 */
-	return fatal_signal_pending(current) || freezing(current);
+	return signal_pending(current);
 }

 static void wait_for_dump_helpers(struct file *file)
@@ -589,6 +589,9 @@ void do_coredump(const kernel_siginfo_t *siginfo)

 	old_cred = override_creds(cred);

+	/* Don't break out of interruptible sleeps */
+	clear_notify_signal();
+
 	ispipe = format_corename(&cn, &cprm, &argv, &argc);

 	if (ispipe) {
-- 
2.35.3