Re: [PATCH 2/2] coredump: Allow coredumps to pipes to work with io_uring

[Olivier Langlois]

On Wed, 2022-07-20 at 11:51 -0500, Eric W. Biederman wrote:
> 
> 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@xxxxxxxxxxxx>
> 
Hi Eric,

What is stopping the task calling do_coredump() to be interrupted and
call task_work_add() from the interrupt context?

This is precisely what I was experiencing last summer when I did work
on this issue.

My understanding of how async I/O works with io_uring is that the task
is added to a wait queue without being put to sleep and when the
io_uring callback is called from the interrupt context, task_work_add()
is called so that the next time io_uring syscall is invoked, pending
work is processed to complete the I/O.

So if:

1. io_uring request is initiated AND the task is in a wait queue
2. do_coredump() is called before the I/O is completed

IMHO, this is how you end up having task_work_add() called while the
coredump is generated.

So far, the only way that I have found making sure that this was not
happening was to cancel every pending io_uring requests before writing
the coredump by calling io_uring_task_cancel():

--- a/fs/coredump.c
+++ b/fs/coredump.c
@@ -43,7 +43,7 @@
 #include <linux/timekeeping.h>
 #include <linux/sysctl.h>
 #include <linux/elf.h>
-
+#include <linux/io_uring.h>
 #include <linux/uaccess.h>
 #include <asm/mmu_context.h>
 #include <asm/tlb.h>
@@ -561,6 +561,8 @@
 		need_suid_safe = true;
 	}
 
+	io_uring_task_cancel();
+
 	retval = coredump_wait(siginfo->si_signo, &core_state);
 	if (retval < 0)
 		goto fail_creds;


Greetings,