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Date: Thu, 2 Aug 2018 14:53:26 -0400
From: Jonny Prouty <jonathanprouty@...il.com>
To: Rich Felker <dalias@...c.org>
Cc: musl@...ts.openwall.com
Subject: Re: Unexpected behaviour writing to /dev/full

Or it could be that it is just extremely commonplace to NOT check whether
you were able to successfully write to stdout before exiting. In which case
you are absolutely correct, this would be a bug in busybox (and similar
bugs in many other programs). I just tested both of the following commands
on systems using musl, uclibc-ng, and glibc, all returned 0:

# bm's vim
# vim --help &> /dev/full; echo $?
0

# gnu tar
# tar --help &> /dev/full; echo $?
0

I'm leaning towards this is just the app didn't check whether their write
to stdout was successful. Thanks for your help Rich.


P.S. Please excuse my brevity, I am corresponding from my phone.

On Thu, Aug 2, 2018, 2:32 PM Jonny Prouty <jonathanprouty@...il.com> wrote:

> I am indeed using busybox ls, but I can get similar behavior when running
> things that aren't shell builtins.  For instance in Alpine if you run:
>
> # /bin/rc-status > /dev/full ; echo $?
> 0
>
> or
>
> #/sbin/update-conf --help &> /dev/full ; echo $?
> 0
>
> Some programs do behave as I'd expect, however
>
> # /sbin/apk --help> /dev/full ; echo $?
> 1
>
> Something seems amiss.
>
> P.S. Please excuse my brevity, I am corresponding from my phone.
>
> On Aug 2, 2018 2:05 PM, "Rich Felker" <dalias@...c.org> wrote:
>
> On Thu, Aug 02, 2018 at 02:03:06PM -0400, Rich Felker wrote:
> > On Thu, Aug 02, 2018 at 01:51:21PM -0400, Jonny Prouty wrote:
> > > Hello all,
> > >
> > > I have a question regarding the interaction of atexit() (I believe its
> > > atexit, anyways) and exit statuses. First the issue I stumbled across,
> so
> > > you'll see where I'm coming from:
> > >
> > > # ls "$HOME" > /dev/full; echo $?
> > > 0
> > > # echo "$HOME" > /dev/full; echo $?
> > > 1
> > >
> > > I expected neither command to return 0 since ultimately an ENOSPC
> should be
> > > returned when writing to /dev/full. Indeed, failure statuses are
> returned
> > > for 'ls' and 'echo' derived from binaries built against glibc. I tried
> to
> > > walk the musl code and it looks like the exit codes are being set (or
> not
> > > set) by atexit(). In the case of 'ls', it seems that it was able to
> > > successfully get a directory listing, but the final fflush() of the
> output
> > > buffer fails with ENOSPC, but that is lost because it happened as a
> result
> > > of some function that was registered with atexit. I *think*. This
> > > interpretation is also borne of a desire to be able to ascribe this to
> the
> > > undefined re-entrant exit behaviour described in "Re-entrancy of exit"
> at
> > > https://wiki.musl-libc.org/functional-differences-from-glibc.html.
> 'echo'
> > > would be failing (as expected) because presumably stdout is flushed
> before
> > > it exits.
> > >
> > > Regardless, the fact that writing to /dev/full can return success seems
> > > wrong. Any thoughts are much appreciated. Please CC me on any
> responses.
> > > Thanks!
> > >
> > > Jonny
> > >
> > > P.S.
> > > musl behaves similarly to uClibc and uClibc-ng in my testing.
> >
> > What versions of ls and echo are you using? Busybox? GNU coreutils?
> > (Note that echo is probably a shell builtin in your shell; you need to
> > execute /bin/echo or similar to get the real echo program.)
> >
> > There is nothing musl can do directly to change the exit status;
> > failure when writing/flushing a file is an error returned to the
> > application, which determines its own exit status, not something that
> > automatically changes the return value of main/argument passed to
> > exit().
> >
> > Do you mean the program is trying to detect flush failure and set an
> > exit status from an atexit handler it installed? That's what gnulib
> > stuff does, if I recall, and it may be how the GNU coreutils ls and
> > other programs handle termination status. It's not a very good way to
> > do it, but it should work if they're doing things right. If they're
> > doing something that's unspecified or undefined from the atexit
> > handler, such as calling exit again (reentrantly), that's an
> > application bug.
> >
> > If you tell us which versions of the utilities you're using, I can
> > look into it a bit more.
>
> For what it's worth, I just tested and got the behavior you saw with
> busybox ls. With GNU coreutils ls linked against musl, I get a nonzero
> exit status. So if you're using a busybox-based distro like Alpine and
> didn't install GNU coreutils in place of it, this very well might just
> be busybox failing to report the error, which should probably be
> reported as a bug in busybox.
>
>
> Rich
>
>
>

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