Subject: Re: [PATCH] t6112: avoid tilde expansion

From: SZEDER Gábor <[email-masked]>
Date: Mon, 20 Apr 2026 22:52:16 +0200

On Sat, Apr 18, 2026 at 06:26:00PM -0700, Elijah Newren wrote:
> On Sat, Apr 18, 2026 at 9:33 AM SZEDER Gábor <[email-masked]> wrote:
> >
> > e987df5fe6 (list-objects-filter: implement composite filters,
> > 2019-06-27) introduced a test to "t6112-rev-list-filters-objects.sh"
> > that checks the output of a Git command with the following commands:
> >
> >   grep ~$omitted_1 actual &&
> >   grep ~$omitted_2 actual &&
> >   grep ~$omitted_3 actual &&
> >
> > Since the leading tilde in the pattern is not quoted/escaped, it is
> > subject to tilde expansion.  So if the system has a user whose
> > username happens to be equal to the content of one of those
> > "$omitted_*" variables, then "grep" would look for the user's home
> > directory.  Luckily, those variables contain object hashes, so this is
> > not very likely.

On second thought, tilde expansion should happen before
parameter/variable expansion, so the above is wrong: we are looking
for a user named "$omitted_1" and not a user named like whatever
object hash the "$omitted_1" variable holds.

Still unlikely, but we should still avoid it.

> > Furthermore, Bash versions v5.0 and earlier seem to be buggy and don't
> > handle this particular tilde expansion very well, and either segfault
> > right away or, in case of v3.2, get stuck in an endless loop and
> > segfault upon receiving ctrl-c.
> 
> Interesting find on the bash segfault behavior.

Actually, I'm going to take back that statement about the Bash
segfault.

I tested older Bash versions with binaries that I compiled myself from
the sources at git://git.savannah.gnu.org/bash.git, and those
exhibited the segfault in v5.0 and below.  Bisect shows that v5.1 is
not just the first release but the first commit that doesn't segfault
on t6112.

However.

Bash v5.1 was released on 2020-12-06, about a year and a half after
those unintended tilde expansions were added.  I find it hard to
believe that noone stumbled upon this issue during that time...
Suspicious, I booted my old laptop running an outdated Debian
derivative with distro shipped Bash 5.0.17, and, lo and behold, t6112
passed just fine.

So I'm inclined to think that something is wrong here...  No idea
what, though.  I can reproduce the segfault with something as simple
as "./bash-v5.0 -c 'v=~a'", and the segfault doesn't even come from
Bash but from within getpwnam() called during tilde expansion.
Oh, well.

Will send a patch with updated log message some time later.