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Date: Thu, 30 Aug 2018 07:30:28 -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 3:56 PM, Rich Felker <dalias@...c.org> wrote:
> On Wed, Aug 29, 2018 at 06:42:47PM -0400, Rich Felker 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.
>
> I think the patch is also incorrect as written. It removes the code
> for handling timeval values which would be out of range if naively
> converted to timespec (either<0 or usec>999999, and overflows). I'm
> not sure how SYS_select handles updating the remaining time in the
> overflow cases, so if you're trying to duplicate it, researching that
> is probably called for. It may be right to just normalize it like
> you're doing when converting back, but this will effectively report
> the wrong amount of elapsed time if tv_sec+tv_usec/1000000 overflows.
>
> Rich
Linux returns -EINVAL for not normalized timespec and timeval inputs.
Regards,
Peter Hurley
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