[PATCH] nptl: Use SA_RESTART for SIGCANCEL handler
Adhemerval Zanella
adhemerval.zanella@linaro.org
Tue Jun 22 18:30:20 GMT 2021
On 18/06/2021 08:38, Florian Weimer wrote:
> * Adhemerval Zanella via Libc-alpha:
>
>> The usage of signals to implementation pthread cancellation is an
>> implementation detail and should not be visible through cancellation
>> entrypoints.
>>
>> However now that pthread_cancel always send the SIGCANCEL, some
>> entrypoint might be interruptable and return EINTR to the caller
>> (for instance on sem_wait).
>>
>> Using SA_RESTART hides this, since the cancellation handler should
>> either act uppon cancellation (if asynchronous cancellation is enable)
>> or ignore the cancellation internal signal.
>
> I think this still needs a NEWS entry because there have been kernel
> bugs in this area (e.g. in CIFS).
Ok, I have added the following on "Deprecated and removed features, and
other changes affecting compatibility"
* The pthread cancellation handler is now setup with SA_RESTART. It should
not be visible to application since the cancellation handler should either
act uppon cancellation (if asynchronous cancellation is enabled) or
ignore the cancellation internal signal.
>
>> Checked on x86_64-linux-gnu and i686-linux-gnu.
>> ---
>> nptl/pthread_cancel.c | 6 +++++-
>> 1 file changed, 5 insertions(+), 1 deletion(-)
>>
>> diff --git a/nptl/pthread_cancel.c b/nptl/pthread_cancel.c
>> index 0698cd2046..cc25ff21f3 100644
>> --- a/nptl/pthread_cancel.c
>> +++ b/nptl/pthread_cancel.c
>> @@ -72,7 +72,11 @@ __pthread_cancel (pthread_t th)
>> {
>> struct sigaction sa;
>> sa.sa_sigaction = sigcancel_handler;
>> - sa.sa_flags = SA_SIGINFO;
>> + /* The signal handle should be non-interruptible to avoid the risk of
>> + spurious EINTR caused by SIGCANCEL sent to process or if
>> + pthread_cancel() is called while cancellation is disabled in the
>> + target thread. */
>> + sa.sa_flags = SA_SIGINFO | SA_RESTART;
>> __sigemptyset (&sa.sa_mask);
>> __libc_sigaction (SIGCANCEL, &sa, NULL);
>> atomic_store_relaxed (&init_sigcancel, 1);
>
> I really don't feel comfortable reviewing this. However I think it is
> still consistent with the (buggy) SYSCALL_CANCEL implementation:
>
> int sc_cancel_oldtype = LIBC_CANCEL_ASYNC (); \
> sc_ret = INLINE_SYSCALL_CALL (__VA_ARGS__); \
> LIBC_CANCEL_RESET (sc_cancel_oldtype); \
>
> We temporary enable async cancellation, in which case we unwind through
> the signal handler if canceled. We do not rely on a EINTR error return
> from the system call and a cancellation check outside of the signal
> handler. So adding SA_RESTART should really be okay.
Yes, we still cancel the thread even for partial results (BZ#12683).
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