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Date: Thu, 30 Aug 2018 07:31:10 -0700
From: Peter Hurley <peter@...aki.com>
To: musl@...ts.openwall.com
Subject: Re: [PATCH] Preserve select() behavior on arm64
On Wed, Aug 29, 2018 at 3:42 PM, Rich Felker <dalias@...c.org> wrote:
> On Fri, May 25, 2018 at 01:52:40PM -0700, Peter Hurley wrote:
>> On Linux, all the select-related syscalls update the timeout value to
>> indicate how much elapsed time the syscall consumed, and many programs
>> expect this behavior when running on Linux.
>>
>> Newer archs like arm64 have deprecated the select syscall because the
>> pselect syscall is a superset implementation of the select syscall.
>>
>> A complication of implementing select() with the pselect syscall is
>> that the timeouts are specified in different units; select() accepts
>> a struct timeval ptr whereas the pselect syscall takes a struct
>> timespec ptr. These are trivial convertible; struct timeval is
>> specified in seconds + microseconds and struct timespec is in
>> seconds + nanoseconds.
>>
>> For kernel configurations without select syscall available, update the
>> caller's struct timeval argument after the pselect syscall.
>> ---
>> src/select/select.c | 17 +++++++++--------
>> 1 file changed, 9 insertions(+), 8 deletions(-)
>>
>> diff --git a/src/select/select.c b/src/select/select.c
>> index 7b5f6dcf7a53..45d4cb7a3d0a 100644
>> --- a/src/select/select.c
>> +++ b/src/select/select.c
>> @@ -12,15 +12,16 @@ int select(int n, fd_set *restrict rfds, fd_set *restrict wfds, fd_set *restrict
>> #else
>> syscall_arg_t data[2] = { 0, _NSIG/8 };
>> struct timespec ts;
>> + int result;
>> if (tv) {
>> - if (tv->tv_sec < 0 || tv->tv_usec < 0)
>> - return __syscall_ret(-EINVAL);
>> - time_t extra_secs = tv->tv_usec / 1000000;
>> - ts.tv_nsec = tv->tv_usec % 1000000 * 1000;
>> - const time_t max_time = (1ULL<<8*sizeof(time_t)-1)-1;
>> - ts.tv_sec = extra_secs > max_time - tv->tv_sec ?
>> - max_time : tv->tv_sec + extra_secs;
>> + ts->tv_sec = tv->tv_sec;
>> + ts->tv_nsec = tv->usec * 1000;
>> }
>> - return syscall_cp(SYS_pselect6, n, rfds, wfds, efds, tv ? &ts : 0, data);
>> + result = syscall_cp(SYS_pselect6, n, rfds, wfds, efds, tv ? &ts : 0, data);
>> + if (tv) {
>> + tv->tv_sec = ts->tv_sec;
>> + tv->tv_usec = ts->tv_nsec / 1000;
>> + }
>> + return result;
>> #endif
>> }
>> --
>> 2.14.2
>
> Sorry for not replying to this sooner. I was unsure what to say at
> first and then it just slipped my mind to go back to it.
>
> Normally we don't aim to duplicate behavior that is explicitly
> documented as non-portable, because programs depending on it should be
> fixed (they're already broken on many non-linux systems).
>
> Where it varies by arch because of implementation details like the
> above (whether there's a SYS_select or SYS_pselect has to be used)
> maybe the principle of consistent behavior across archs has something
> to say here.
>
> I'd almost rather do it the opposite (modern/pselect-like) direction,
> making a copy of the struct to pass to SYS_select so the caller's copy
> doesn't get clobbered, but maybe you like that even less..
>
> Thoughts from anyone else?
>
> Rich
Thanks for the reply.
With respect to non-portability, this is behavior that is specifically
allowed for select() in POSIX.
My goal with the patch is principle-of-least-surprise between arches
on musl, and libc on Linux.
Regards,
Peter Hurley
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