coredump: only SIGKILL should interrupt the coredumping task
There are 2 well known and ancient problems with coredump/signals, and a
lot of related bug reports:
- do_coredump() clears TIF_SIGPENDING but of course this can't help
if, say, SIGCHLD comes after that.
In this case the coredump can fail unexpectedly. See for example
wait_for_dump_helper()->signal_pending() check but there are other
reasons.
- At the same time, dumping a huge core on the slow media can take a
lot of time/resources and there is no way to kill the coredumping
task reliably. In particular this is not oom_kill-friendly.
This patch tries to fix the 1st problem, and makes the preparation for the
next changes.
We add the new SIGNAL_GROUP_COREDUMP flag set by zap_threads() to indicate
that this process dumps the core. prepare_signal() checks this flag and
nacks any signal except SIGKILL.
Note that this check tries to be conservative, in the long term we should
probably treat the SIGNAL_GROUP_EXIT case equally but this needs more
discussion. See marc.info/?l=linux-kernel&m=120508897917439
Notes:
- recalc_sigpending() doesn't check SIGNAL_GROUP_COREDUMP.
The patch assumes that dump_write/etc paths should never
call it, but we can change it as well.
- There is another source of TIF_SIGPENDING, freezer. This
will be addressed separately.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Tested-by: Mandeep Singh Baines <msb@chromium.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Neil Horman <nhorman@redhat.com>
Cc: "Rafael J. Wysocki" <rjw@sisk.pl>
Cc: Roland McGrath <roland@hack.frob.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/fs/coredump.c b/fs/coredump.c
index d52f6bd..f91cfd8 100644
--- a/fs/coredump.c
+++ b/fs/coredump.c
@@ -280,8 +280,8 @@
return nr;
}
-static inline int zap_threads(struct task_struct *tsk, struct mm_struct *mm,
- struct core_state *core_state, int exit_code)
+static int zap_threads(struct task_struct *tsk, struct mm_struct *mm,
+ struct core_state *core_state, int exit_code)
{
struct task_struct *g, *p;
unsigned long flags;
@@ -291,6 +291,9 @@
if (!signal_group_exit(tsk->signal)) {
mm->core_state = core_state;
nr = zap_process(tsk, exit_code);
+ /* ignore all signals except SIGKILL, see prepare_signal() */
+ tsk->signal->flags |= SIGNAL_GROUP_COREDUMP;
+ clear_tsk_thread_flag(tsk, TIF_SIGPENDING);
}
spin_unlock_irq(&tsk->sighand->siglock);
if (unlikely(nr < 0))
@@ -514,12 +517,6 @@
old_cred = override_creds(cred);
- /*
- * Clear any false indication of pending signals that might
- * be seen by the filesystem code called to write the core file.
- */
- clear_thread_flag(TIF_SIGPENDING);
-
ispipe = format_corename(&cn, &cprm);
if (ispipe) {
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 54ddcb8..a22baf8 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -626,6 +626,7 @@
#define SIGNAL_STOP_STOPPED 0x00000001 /* job control stop in effect */
#define SIGNAL_STOP_CONTINUED 0x00000002 /* SIGCONT since WCONTINUED reap */
#define SIGNAL_GROUP_EXIT 0x00000004 /* group exit in progress */
+#define SIGNAL_GROUP_COREDUMP 0x00000008 /* coredump in progress */
/*
* Pending notifications to parent.
*/
diff --git a/kernel/signal.c b/kernel/signal.c
index 27ece01..cede589 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -854,12 +854,14 @@
* Returns true if the signal should be actually delivered, otherwise
* it should be dropped.
*/
-static int prepare_signal(int sig, struct task_struct *p, bool force)
+static bool prepare_signal(int sig, struct task_struct *p, bool force)
{
struct signal_struct *signal = p->signal;
struct task_struct *t;
- if (unlikely(signal->flags & SIGNAL_GROUP_EXIT)) {
+ if (signal->flags & (SIGNAL_GROUP_EXIT | SIGNAL_GROUP_COREDUMP)) {
+ if (signal->flags & SIGNAL_GROUP_COREDUMP)
+ return sig == SIGKILL;
/*
* The process is in the middle of dying, nothing to do.
*/