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@xxxxxxxx>
Fixes: 76f969e8948d ("cgroup: cgroup v2 freezer")
Signed-off-by: Roman Gushchin <guro@xxxxxx>
Acked-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
---
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
