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UPSTREAM: signal: unconditionally leave the frozen state in ptrace_stop()

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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: 76f969e894 ("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>
Signed-off-by: Tejun Heo <tj@kernel.org>

Change-Id: If644b15ead36ce13f0c2c3dd57eebe3658e3edf7
(cherry picked from commit 05b2892637)
Bug: 154548692
Signed-off-by: Marco Ballesio <balejs@google.com>
This commit is contained in:
Roman Gushchin
2019-05-16 10:38:21 -07:00
committed by Marco Ballesio
parent 5318f3163c
commit f7840a0a07
1 changed files with 1 additions and 0 deletions
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1
kernel/signal.c
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@@ -2134,6 +2134,7 @@ static void ptrace_stop(int exit_code, int why, int clear_code, siginfo_t *info)
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.