capsicum_helpers: Squash errors from closed fds

[cem]

Summary

Squash EBADF from closed stdin, stdout, or stderr in caph_limit_stdio().
Any program used during special shell scripts may commonly be forked
from a parent process with closed standard stream. Do the common sense
thing for this common use.

Reported by: Iblis Lin <iblis AT hs.ntnu.edu.tw>

https://lists.freebsd.org/pipermail/freebsd-current/2016-November/063949.html

Test Plan

Tested with closed stdin:

$ truss basename $HOME <& -
...
cap_rights_limit(0x0,0x7fffffffe280)             ERR#9 'Bad file descriptor'
cap_rights_limit(0x1,0x7fffffffe280)             = 0 (0x0)
cap_ioctls_limit(0x1,0x7fffffffe270,0x2)         = 0 (0x0)
cap_fcntls_limit(0x1,0x8)                        = 0 (0x0)
cap_rights_limit(0x2,0x7fffffffe280)             = 0 (0x0)
cap_ioctls_limit(0x2,0x7fffffffe270,0x2)         = 0 (0x0)
cap_fcntls_limit(0x2,0x8)                        = 0 (0x0)
cap_enter()                                      = 0 (0x0)
fstat(1,{ mode=crw--w---- ,inode=124,size=0,blksize=4096 }) = 0 (0x0)
ioctl(1,TIOCGETA,0xffffe0c0)                     = 0 (0x0)
write(1,"cmeyer\n",7)                            = 7 (0x7)
...

[oshogbo]

We have special flag to the caph_limit_stream() to ignore EBADF.

[cem]

In D8657#179470, @oshogbo wrote:

We have special flag to the caph_limit_stream() to ignore EBADF.

Yes, but caph_limit_stdin and friends do not take a flag argument. They could be replaced with several caph_limit_stream() calls instead, I guess. The approaches are functionally equivalent; if you think the style would be better that way, it's fine with me.

[cem]

Use caph_limit_stream() to ignore EBADF rather than handling directly in
caph_limit_stdio().

[oshogbo]

lib/libcapsicum/capsicum_helpers.h

98 ↗

(On Diff #22582)

It's over 80.
Otherwise I agree with that change.