From: Ramsay Jones <ramsay@ramsayjones•plus.com>
To: Jeff King <peff@peff•net>
Cc: git@vger•kernel.org
Subject: Re: [PATCH 0/4] plugging some mmap() leaks
Date: Fri, 6 Mar 2026 17:49:04 +0000 [thread overview]
Message-ID: <aa83861a-cc0f-4fc2-9599-182dac8b4e9a@ramsayjones.plus.com> (raw)
In-Reply-To: <20260306162106.GA3483423@coredump.intra.peff.net>
On 06/03/2026 4:21 pm, Jeff King wrote:
> On Fri, Mar 06, 2026 at 04:37:49AM +0000, Ramsay Jones wrote:
>
>> Many moons ago, when the cygwin build routinely set NO_MMAP I had an
>> valgrind build of git fail with a 'double free' caused by a call to
>> git_munmap() for a pointer that had already been git_munmap-ed!
>>
>> In addition, the failure was not reproducible (or at least I could not
>> find such a test). This was at a time when the testsuite took 4+ hours
>> to run for a regular build, let alone a valgrind build. So, to try and
>> pin down the failure, I created a debug version of the mmap compat
>> functions, which I ran with for several weeks, without failing ... :(
>>
>> It just so happens that about this time I was also testing running the
>> cygwin build without NO_MMAP set. This was a success, so I dropped
>> the NO_MMAP investigation, never having found the cause of the failure!
>
> Interesting. I guess a double-free via munmap() is probably a
> harmless-ish noop, rather than a heap corruption. I could believe we
> have such a bug somewhere, and it may even be racy (e.g., if it requires
> reprepare_packed_git(), or maybe even has to do with stat freshness when
> diff.c tries to reuse working tree files).
>
> We've been testing ASan builds with NO_MMAP for a few months now, so
> it's possible that might help flush it out. Though if you ran into it in
> 2012, it's possible it has since been unknowingly fixed. ;)
Yep, it was before 2012 and I suspect it has been 'fixed' (but could, of
course, still be lingering ...). ;)
>> Subject: [PATCH] mmap.c: log mmap() blocks to avoid double-delete bug
>> [...]
>> In order to guard the implementation from such a calling sequence,
>> we keep a list of mmap-block descriptors, which we then consult to
>> determine the validity of the input pointer to munmap(). This then
>> allows 'git_munmap()' to return -1 on error, as required, with
>> errno set to EINVAL.
>
> Gross. :)
Heh, agreed! :) There were several reasons I didn't submit it in all these
years.
> This is a clever workaround, but I think we should consider it a bug if
> we are calling munmap() twice and fix that.
Agreed. (I just wanted to bring this to your attention).
ATB,
Ramsay Jones
next prev parent reply other threads:[~2026-03-06 17:49 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-03-05 20:51 memory leak when cloning a repository Jacob Keller
2026-03-05 22:02 ` Jeff King
2026-03-05 23:03 ` [PATCH 0/4] plugging some mmap() leaks Jeff King
2026-03-05 23:08 ` [PATCH 1/4] check_connected(): delay opening new_pack Jeff King
2026-03-05 23:18 ` Jacob Keller
2026-03-05 23:09 ` [PATCH 2/4] check_connected(): fix leak of pack-index mmap Jeff King
2026-03-05 23:20 ` Jacob Keller
2026-03-05 23:12 ` [PATCH 3/4] pack-revindex: avoid double-loading .rev files Jeff King
2026-03-05 23:13 ` [PATCH 4/4] Makefile: turn on NO_MMAP when building with LSan Jeff King
2026-03-06 9:17 ` Jacob Keller
2026-03-06 16:25 ` [PATCH 5/4] meson: " Jeff King
2026-03-06 18:00 ` Ramsay Jones
2026-03-07 1:14 ` [PATCH 4/4] Makefile: " Junio C Hamano
2026-03-07 2:24 ` [PATCH 3.5/4] object-file: fix mmap() leak in odb_source_loose_read_object_stream() Jeff King
2026-03-07 5:35 ` Junio C Hamano
2026-03-10 12:23 ` Patrick Steinhardt
2026-03-06 4:37 ` [PATCH 0/4] plugging some mmap() leaks Ramsay Jones
2026-03-06 16:21 ` Jeff King
2026-03-06 17:49 ` Ramsay Jones [this message]
2026-03-06 18:37 ` Junio C Hamano
2026-03-06 18:55 ` Ramsay Jones
2026-03-06 22:05 ` Junio C Hamano
2026-03-06 23:25 ` Ramsay Jones
2026-03-07 1:15 ` Junio C Hamano
2026-03-05 23:16 ` memory leak when cloning a repository Jacob Keller
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