[PATCH RESEND] signal: unconditionally leave the frozen state in ptrace_stop()

[PATCH RESEND] signal: unconditionally leave the frozen state in ptrace_stop()

[Roman Gushchin]

Alex Xu reported a regression in strace, caused by the introduction of
the cgroup v2 freezer. The regression can be reproduced by stracing
the following simple program:

  #include <unistd.h>

  int main() {
      write(1, "a", 1);
      return 0;
  }

An attempt to run strace ./a.out leads to the infinite loop:
  [ pre-main omitted ]
  write(1, "a", 1)                        = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
  write(1, "a", 1)                        = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
  write(1, "a", 1)                        = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
  write(1, "a", 1)                        = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
  write(1, "a", 1)                        = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
  write(1, "a", 1)                        = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
  [ repeats forever ]

The problem occurs because the traced task leaves ptrace_stop()
(and the signal handling loop) with the frozen bit set. So let's
call cgroup_leave_frozen(true) unconditionally after sleeping
in ptrace_stop().

With this patch applied, strace works as expected:
  [ pre-main omitted ]
  write(1, "a", 1)                        = 1
  exit_group(0)                           = ?
  +++ exited with 0 +++

Reported-by: Alex Xu <alex_y_xu@yahoo.ca>
Fixes: 76f969e8948d ("cgroup: cgroup v2 freezer")
Signed-off-by: Roman Gushchin <guro@fb.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Cc: Tejun Heo <tj@kernel.org>
---
 kernel/signal.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/signal.c b/kernel/signal.c
index 8607b11ff936..565ba14d89d5 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2112,6 +2112,7 @@ static void ptrace_stop(int exit_code, int why, int clear_code, kernel_siginfo_t
 		preempt_enable_no_resched();
 		cgroup_enter_frozen();
 		freezable_schedule();
+		cgroup_leave_frozen(true);
 	} else {
 		/*
 		 * By the time we got the lock, our tracer went away.
-- 
2.20.1

Re: [PATCH RESEND] signal: unconditionally leave the frozen state in ptrace_stop()

[Tejun Heo]

On Thu, May 16, 2019 at 10:38:21AM -0700, Roman Gushchin wrote:
> Alex Xu reported a regression in strace, caused by the introduction of
> the cgroup v2 freezer. The regression can be reproduced by stracing
> the following simple program:
> 
>   #include <unistd.h>
> 
>   int main() {
>       write(1, "a", 1);
>       return 0;
>   }
> 
> An attempt to run strace ./a.out leads to the infinite loop:
>   [ pre-main omitted ]
>   write(1, "a", 1)                        = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
>   write(1, "a", 1)                        = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
>   write(1, "a", 1)                        = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
>   write(1, "a", 1)                        = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
>   write(1, "a", 1)                        = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
>   write(1, "a", 1)                        = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
>   [ repeats forever ]
> 
> The problem occurs because the traced task leaves ptrace_stop()
> (and the signal handling loop) with the frozen bit set. So let's
> call cgroup_leave_frozen(true) unconditionally after sleeping
> in ptrace_stop().
> 
> With this patch applied, strace works as expected:
>   [ pre-main omitted ]
>   write(1, "a", 1)                        = 1
>   exit_group(0)                           = ?
>   +++ exited with 0 +++
> 
> Reported-by: Alex Xu <alex_y_xu@yahoo.ca>
> Fixes: 76f969e8948d ("cgroup: cgroup v2 freezer")
> Signed-off-by: Roman Gushchin <guro@fb.com>
> Acked-by: Oleg Nesterov <oleg@redhat.com>
> Cc: Tejun Heo <tj@kernel.org>

Applied to cgroup/for-5.2-fixes.

Thanks.

-- 
tejun