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?

The above mentioned test does the following (simplified):

        fd = open("/dev/null", O_RDONLY);
        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:


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);
@@ -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.
 .Fa nfds
-was less than 0.
+was smaller than the smallest descriptor specified in the sets.
 .Xr accept 2 ,

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