Re: [PATCH v2] fix clock_nanosleep error case

[Szabolcs Nagy <nsz@port70.net>]

* Daniel Sabogal <dsabogalcc@gmail.com> [2016-09-18 15:01:47 -0400]:
> On Sat, Sep 17, 2016 at 11:38 PM, Rich Felker <dalias@libc.org> wrote:
> > On Sat, Sep 17, 2016 at 12:05:45PM -0400, Daniel Sabogal wrote:
> >> posix requires that EINVAL be returned if the first parameter specifies
> >> the cpu-time clock of the calling thread (CLOCK_THREAD_CPUTIME_ID).
> >> linux returns ENOTSUP instead so we handle this.
> >> ---
> >> Applied Szabolcs' suggestion for remapping the return value.
> >> clock_nanosleep is required to be a cancellation point.
> >> ---
> >>  src/time/clock_nanosleep.c | 4 +++-
> >>  1 file changed, 3 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/src/time/clock_nanosleep.c b/src/time/clock_nanosleep.c
> >> index ec87b9e..9e4d9f1 100644
> >> --- a/src/time/clock_nanosleep.c
> >> +++ b/src/time/clock_nanosleep.c
> >> @@ -1,8 +1,10 @@
> >>  #include <time.h>
> >> +#include <errno.h>
> >>  #include "syscall.h"
> >>  #include "libc.h"
> >>
> >>  int clock_nanosleep(clockid_t clk, int flags, const struct timespec *req, struct timespec *rem)
> >>  {
> >> -     return -__syscall_cp(SYS_clock_nanosleep, clk, flags, req, rem);
> >> +     int r = -__syscall_cp(SYS_clock_nanosleep, clk, flags, req, rem);
> >> +     return clk == CLOCK_THREAD_CPUTIME_ID ? EINVAL : r;
> >>  }
> >
> > See:
> >
> > On Sat, Sep 17, 2016 at 04:57:09PM +0200, Szabolcs Nagy wrote:
> >> you elide a cancellation point here.
> >>
> >> i think you should check and remap the return value instead.
> >
> > "Remap the return value" would be more like:
> >
> >         return r==ENOTSUP ? EINVAL : r;
> 
> I wasn't sure about remapping all return values of ENOTSUP to EINVAL.
> There are other clocks (extensions) where linux and glibc return ENOTSUP.

ah. i assumed ENOTSUP is always wrong, then i guess the patch is ok.

> I looked through Debian Code Search, but didn't really find anything that
> actually uses or depends on such behavior for those extensions.
> 
> I suppose this might be fine.
> 
> > I don't know if it makes a big difference, but in principle it's
> > better to base conditions on a return value than an argument, since
> > basing them on an argument requires the value of that argument to be
> > preserved across the call and can thereby require spilling registers,
> > etc. I don't think that actually happens on any of the Linux syscall
> > ABIs but I'm not sure.
> 
> OK.