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