[PATCH 5/5] coredump: ignore non-fatal signals when core dumping to a pipe

[Mandeep Singh Baines <msb@chromium.org>]

From: Ben Chan <benchan@chromium.org>

Make wait_for_dump_helpers() not abort piping the core dump data when the
crashing process has received a non-fatal signal.  The abort still occurs
in the case of SIGKILL.

Testing:

localhost ~ # echo "|/usr/bin/sleep 1d" > /proc/sys/kernel/core_pattern
localhost ~ # sleep 1d &
[1] 2514
localhost ~ # kill -ABRT $! # Cause coredump
localhost ~ # kill -USR1 $! # Send non-fatal signal
localhost ~ # top -p $! -n1 -b # Verify that we aren't dead or busy waiting
top - 16:45:34 up 2 min,  0 users,  load average: 0.71, 0.42, 0.17
Tasks:   1 total,   0 running,   1 sleeping,   0 stopped,   0 zombie
Cpu(s): 26.0%us,  8.5%sy,  0.0%ni, 65.1%id,  0.2%wa,  0.0%hi,  0.1%si,  0.0%st
Mem:    991516k total,   418556k used,   572960k free,     5948k buffers
Swap:        0k total,        0k used,        0k free,   289928k cached

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
 2514 root      20   0  1868  392  336 S    0  0.0   0:00.00 sleep

localhost ~ # echo mem > /sys/power/state # Suspend
localhost ~ # top -p $! -n1 -b # Verify that we aren't dead or busy waiting
top - 16:46:46 up 3 min,  0 users,  load average: 1.68, 0.69, 0.28
Tasks:   1 total,   0 running,   1 sleeping,   0 stopped,   0 zombie
Cpu(s): 24.1%us,  7.7%sy,  0.0%ni, 67.9%id,  0.2%wa,  0.0%hi,  0.1%si,  0.0%st
Mem:    991516k total,   419956k used,   571560k free,     5996k buffers
Swap:        0k total,        0k used,        0k free,   290208k cached

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
 2514 root      20   0  1868  392  336 S    0  0.0   0:00.00 sleep

localhost ~ # kill -KILL $!
[1]+  Aborted                 (core dumped) sleep 1d

Addresses http://crosbug.com/21559

Changes since v1:
* Mandeep Singh Baines
  * To prevent blocking suspend, add try_to_freeze().
Changes since v2:
* LKML: <20130215150117.GB30829@redhat.com> Oleg Nestorov
  * Block non-fatal signals to avoid poll_wait busy waiting.
* LKML: <20130215152538.9a61a44e.akpm@linux-foundation.org> Andrew Morton
  * Added comment re: try_to_freeze and clarified commit message.
Changes since v3:
* Mandeep Singh Baines
  * Clear signal pending caused by fake signal from freeze_task().
  * Document how the patch was tested.
Changes since v4:
* Mandeep Singh Baines
  * Moved clearing of fake signal to __refrigerator() (separate patch).
  * SIGKILL will remain in shared_pending since SIGNAL_GROUP_EXIT is set,
    so fatal_signal_pending() will return false. Add a sigkill_pending()
    helper that does the right thing.

Signed-off-by: Ben Chan <benchan@chromium.org>
Signed-off-by: Mandeep Singh Baines <msb@chromium.org>
CC: Oleg Nesterov <oleg@redhat.com>
CC: Tejun Heo <tj@kernel.org>
CC: Andrew Morton <akpm@linux-foundation.org>
CC: Rafael J. Wysocki <rjw@sisk.pl>
CC: Ingo Molnar <mingo@redhat.com>
---
 fs/coredump.c | 25 ++++++++++++++++++++++++-
 1 file changed, 24 insertions(+), 1 deletion(-)

diff --git a/fs/coredump.c b/fs/coredump.c
index 1774932..3eb799d 100644
--- a/fs/coredump.c
+++ b/fs/coredump.c
@@ -32,6 +32,7 @@
 #include <linux/pipe_fs_i.h>
 #include <linux/oom.h>
 #include <linux/compat.h>
+#include <linux/freezer.h>
 
 #include <asm/uaccess.h>
 #include <asm/mmu_context.h>
@@ -407,9 +408,21 @@ static void coredump_finish(struct mm_struct *mm)
 	mm->core_state = NULL;
 }
 
+static int sigkill_pending(struct task_struct *tsk)
+{
+	return	signal_pending(tsk) &&
+		(sigismember(&tsk->pending.signal, SIGKILL) ||
+		 sigismember(&tsk->signal->shared_pending.signal, SIGKILL));
+}
+
 static void wait_for_dump_helpers(struct file *file)
 {
 	struct pipe_inode_info *pipe;
+	sigset_t blocked, previous;
+
+	/* Block all but fatal signals. */
+	siginitsetinv(&blocked, sigmask(SIGKILL));
+	sigprocmask(SIG_BLOCK, &blocked, &previous);
 
 	pipe = file->f_path.dentry->d_inode->i_pipe;
 
@@ -417,16 +430,26 @@ static void wait_for_dump_helpers(struct file *file)
 	pipe->readers++;
 	pipe->writers--;
 
-	while ((pipe->readers > 1) && (!signal_pending(current))) {
+	while ((pipe->readers > 1) && (!sigkill_pending(current))) {
 		wake_up_interruptible_sync(&pipe->wait);
 		kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN);
 		pipe_wait(pipe);
+
+		/*
+		 * Non-fatal signals are blocked. So we need to try
+		 * to freeze in order to not block suspend.
+		 */
+		pipe_unlock(pipe);
+		try_to_freeze();
+		pipe_lock(pipe);
 	}
 
 	pipe->readers--;
 	pipe->writers++;
 	pipe_unlock(pipe);
 
+	/* Restore signals. */
+	sigprocmask(SIG_SETMASK, &previous, NULL);
 }
 
 /*
-- 
1.7.12.4