Re: [PATCH v2] fix clock_nanosleep error case

[Rich Felker <dalias@libc.org>]

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 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.

Rich