fileargs_init(3) doesn't work without CAPABILITIES (was: Re: tail(1) broken in 13-stable)

[Peter Jeremy]

On 2021-May-06 19:07:23 -0400, monochrome <monochrome at twcny.rr.com> wrote:
...
>On 5/6/21 7:49 AM, Peter Jeremy via freebsd-stable wrote:
...
>> server% tail /COPYRIGHT <&-
>> Assertion failed: (procfd > STDERR_FILENO), function service_clean, file /usr/src/lib/libcasper/libcasper/service.c, line 394.
>> tail: unable to init casper: Socket is not connected

>I get a different error on a 13.0-RELEASE machine I converted from 12 to 
>current about a year ago (bash and sh):
>
>$ tail /COPYRIGHT <&-
>tail: can't limit stdio rights: Bad file descriptor

I've done some more testing across a number of systems and narrowed the
difference in behaviour down to the presence of the CAPABILITIES option in
the kernel (it looks like I never added it to my kernel config on that
system):

If CAPABILITIES is present then the cap_rights_limit(2) call for the closed
FD fails, generating the "can't limit stdio rights" error.  (Whether this
behaviour is reasonable is a different issue - it was introduced in r348708,
based on https://reviews.freebsd.org/D20393 and the issue of closed file
descriptors doesn't seem to have been considered).

If CAPABILITIES is not present then the cap_rights_limit() failure is
(correctly) ignored but the subsequent fileargs_init(3) call gets upset at
opening a FD <= 2.  This behaviour seems wrong - if CAPABILITIES aren't
present in the kernel then the userland behaviour should be the same as if
WITHOUT_CASPER is specified.

IMO, this is a bug in fileargs_init(3).

-- 
Peter Jeremy
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