From: Michal Hocko <mhocko@xxxxxxxx> commit 735f2770a770156100f534646158cb58cb8b2939 upstream. Commit fec1d0115240 ("[PATCH] Disable CLONE_CHILD_CLEARTID for abnormal exit") has caused a subtle regression in nscd which uses CLONE_CHILD_CLEARTID to clear the nscd_certainly_running flag in the shared databases, so that the clients are notified when nscd is restarted. Now, when nscd uses a non-persistent database, clients that have it mapped keep thinking the database is being updated by nscd, when in fact nscd has created a new (anonymous) one (for non-persistent databases it uses an unlinked file as backend). The original proposal for the CLONE_CHILD_CLEARTID change claimed (https://lkml.org/lkml/2006/10/25/233): : The NPTL library uses the CLONE_CHILD_CLEARTID flag on clone() syscalls : on behalf of pthread_create() library calls. This feature is used to : request that the kernel clear the thread-id in user space (at an address : provided in the syscall) when the thread disassociates itself from the : address space, which is done in mm_release(). : : Unfortunately, when a multi-threaded process incurs a core dump (such as : from a SIGSEGV), the core-dumping thread sends SIGKILL signals to all of : the other threads, which then proceed to clear their user-space tids : before synchronizing in exit_mm() with the start of core dumping. This : misrepresents the state of process's address space at the time of the : SIGSEGV and makes it more difficult for someone to debug NPTL and glibc : problems (misleading him/her to conclude that the threads had gone away : before the fault). : : The fix below is to simply avoid the CLONE_CHILD_CLEARTID action if a : core dump has been initiated. The resulting patch from Roland (https://lkml.org/lkml/2006/10/26/269) seems to have a larger scope than the original patch asked for. It seems that limitting the scope of the check to core dumping should work for SIGSEGV issue describe above. [Changelog partly based on Andreas' description] Fixes: fec1d0115240 ("[PATCH] Disable CLONE_CHILD_CLEARTID for abnormal exit") Link: http://lkml.kernel.org/r/1471968749-26173-1-git-send-email-mhocko@xxxxxxxxxx Signed-off-by: Michal Hocko <mhocko@xxxxxxxx> Tested-by: William Preston <wpreston@xxxxxxxx> Acked-by: Oleg Nesterov <oleg@xxxxxxxxxx> Cc: Roland McGrath <roland@xxxxxxxxxxxxx> Cc: Andreas Schwab <schwab@xxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Signed-off-by: Willy Tarreau <w@xxxxxx> --- kernel/fork.c | 10 ++++------ 1 file changed, 4 insertions(+), 6 deletions(-) diff --git a/kernel/fork.c b/kernel/fork.c index 2358bd4..612e78d 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -775,14 +775,12 @@ void mm_release(struct task_struct *tsk, struct mm_struct *mm) deactivate_mm(tsk, mm); /* - * If we're exiting normally, clear a user-space tid field if - * requested. We leave this alone when dying by signal, to leave - * the value intact in a core dump, and to save the unnecessary - * trouble, say, a killed vfork parent shouldn't touch this mm. - * Userland only wants this done for a sys_exit. + * Signal userspace if we're not exiting with a core dump + * because we want to leave the value intact for debugging + * purposes. */ if (tsk->clear_child_tid) { - if (!(tsk->flags & PF_SIGNALED) && + if (!(tsk->signal->flags & SIGNAL_GROUP_COREDUMP) && atomic_read(&mm->mm_users) > 1) { /* * We don't check the error code - if userspace has -- 2.8.0.rc2.1.gbe9624a -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html