On Wed, Apr 01, 2020 at 01:08:10PM +0200, Martin Pieuchot wrote:
> While working on moving pselect(2) outside of the KERNEL_LOCK() I found
> this surprising {p,}select(2) behavior exposed by the libc/sys/t_select.c
> regression test. Should it be considered as a bug or feature?
I think this is actually a feature. The standard does not mention that you
have to listen on something and it would allow to use select() as a
sleep() replacement. I would expect that pselect() should behave the same
way.
poll() does the same thing (you are allowed to pass a 0 poll fds).
> The above mentioned test does the following (simplified):
>
> fd = open("/dev/null", O_RDONLY);
> FD_ZERO(&rset);
> FD_SET(fd, &rset);
> pselect(1, &rset, NULL, NULL, NULL, &set);
>
> Note that in most environments, open(2) will return a value higher than
> 1 which means the value written by FD_SET() will never be checked by the
> current implementation of {p,}select(2). In this case, even if it isn't
> waiting for any condition, the current implementation will block.
>
> To put it differently, the code above is an obfuscated way to write:
>
> sigsuspend(&set);
>
> If we agree that this is not what the user wants to do, I'm suggesting
> the diff below which makes {p,}select(2) return EINVAL if no bit has
> been found in the given set up to `nfds'.
>
> Independently from our decision, I'd suggest to rename the variable
> name `nfds' in the syscall documentation because it can lead to this
> kind of confusion.
>
> Finally if we accept this change the regression test will have to be
> adapted because polling "/dev/null" for reading always return true so
> pselect(2) wont block waiting for a signal. Any suggestion?
>
> Index: sys/kern/sys_generic.c
> ===================================================================
> RCS file: /cvs/src/sys/kern/sys_generic.c,v
> retrieving revision 1.131
> diff -u -p -r1.131 sys_generic.c
> --- sys/kern/sys_generic.c 20 Mar 2020 04:11:05 -0000 1.131
> +++ sys/kern/sys_generic.c 1 Apr 2020 10:27:51 -0000
> @@ -703,7 +703,7 @@ selscan(struct proc *p, fd_set *ibits, f
> {
> caddr_t cibits = (caddr_t)ibits, cobits = (caddr_t)obits;
> struct filedesc *fdp = p->p_fd;
> - int msk, i, j, fd;
> + int msk, i, j, fd, foundfds = 0;
> fd_mask bits;
> struct file *fp;
> int n = 0;
> @@ -719,6 +719,7 @@ selscan(struct proc *p, fd_set *ibits, f
> bits &= ~(1 << j);
> if ((fp = fd_getfile(fdp, fd)) == NULL)
> return (EBADF);
> + foundfds++;
> if ((*fp->f_ops->fo_poll)(fp, flag[msk], p)) {
> FD_SET(fd, pobits);
> n++;
> @@ -727,6 +728,8 @@ selscan(struct proc *p, fd_set *ibits, f
> }
> }
> }
> + if (foundfds == 0)
> + return (EINVAL);
> *retval = n;
> return (0);
> }
> Index: lib/libc/sys/select.2
> ===================================================================
> RCS file: /cvs/src/lib/libc/sys/select.2,v
> retrieving revision 1.42
> diff -u -p -r1.42 select.2
> --- lib/libc/sys/select.2 17 Sep 2016 01:01:42 -0000 1.42
> +++ lib/libc/sys/select.2 1 Apr 2020 10:35:29 -0000
> @@ -188,7 +188,7 @@ The specified time limit is invalid.
> One of its components is negative or too large.
> .It Bq Er EINVAL
> .Fa nfds
> -was less than 0.
> +was smaller than the smallest descriptor specified in the sets.
> .El
> .Sh SEE ALSO
> .Xr accept 2 ,
>
--
:wq Claudio
