From: Ramsay Jones <ramsay@ramsayjones•plus.com>
To: Junio C Hamano <gitster@pobox•com>
Cc: Jeff King <peff@peff•net>, git@vger•kernel.org
Subject: Re: [PATCH 0/4] plugging some mmap() leaks
Date: Fri, 6 Mar 2026 23:25:03 +0000 [thread overview]
Message-ID: <c3ae9ff6-8577-48de-8473-9ec8d22ebc71@ramsayjones.plus.com> (raw)
In-Reply-To: <xmqqjyvoy5p2.fsf@gitster.g>
On 06/03/2026 10:05 pm, Junio C Hamano wrote:
> Ramsay Jones <ramsay@ramsayjones•plus.com> writes:
>
>> On 06/03/2026 6:37 pm, Junio C Hamano wrote:
>>> Ramsay Jones <ramsay@ramsayjones•plus.com> writes:
>>>
>>>> When compiling with the NO_MMAP build variable set, the built-in
>>>> 'git_mmap()' and 'git_munmap()' compatability routines use simple
>>>> memory allocation and file I/O to emulate the required behaviour.
>>>> The current implementation is vunerable to the "double-delete" bug
>>>> (where the pointer returned by malloc() is passed to free() two or
>>>> more times), should the mapped memory block address be passed to
>>>> munmap() multiple times.
>>>
>>> Sorry if I am missing something glaringly obvious, but quite
>>> honestly I am confused. Wouldn't it be a bug to call munmap() again
>>> on the same region of memory obtained from mmap() and then already
>>> unmapped by calling munmap()?
>>
>> Yes. The (second) call to munmap() with the (already unmapped) memory
>> region would return -1 with errno set to EINVAL.
>>
>> The emulation layer does not detect this situation and simply calls
>> free() on the given pointer. Hence the 'double-delete' bug.
>>
>>> Or can the emulation layer cause such a second free() even if the
>>> munmap() is done once and only once per memory region obtained from
>>> a single mmap()?
>>
>> No. If you only git_munmap() once for a given memory region, everything
>> is fine.
>
> Hmph. You make it sound as if we have some code that calls munmap()
> on something that we are not sure if we have unmapped just in case,
> trusting that it won't crash us and instead give us EINVAL, and that
> is very much deliberate? Unless we have such a code, bending over
> backwards to track what has already been unmapped and return -1 with
> EINVAL from munmap() for a second call is of dubious value, no? I
> still must be missing something...
Sorry to confuse you, I'm not trying to, honest! :)
First, forget that patch. It was not meant to be applied by anyone to
anything. It was just to (hopefully) support the explanation (to Jeff
specifically) of a bug which happened to me long ago.
In particular, valgrind demonstrated a bug, which has probably been fixed
in the intervening years, which directly implied that some code called
munmap() on a memory region which had already been unmapped.
Given that this was on cygwin and, at that time, was built with NO_MMAP,
this meant that free() was called twice on the same malloc()ed memory.
(Indeed this is what valgrind reported).
If you look at the (currently 31) calls to munmap(), only one seems to look
at the return (in refs/packed-backend.c:183). So, calling munmap() twice
on the same memory region will probably go unnoticed when NO_MMAP is not
set. I have no idea why munmap() was called twice on the same memory region,
since I didn't track down the code responsible.
It was just an FYI about a _potential_ lurking bug when using the mmap compat
routines.
Have I cleared that up, or confused you more. :)
ATB,
Ramsay Jones
next prev parent reply other threads:[~2026-03-06 23:25 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
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 [this message]
2026-03-07 1:15 ` Junio C Hamano
2026-03-05 23:16 ` memory leak when cloning a repository Jacob Keller
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=c3ae9ff6-8577-48de-8473-9ec8d22ebc71@ramsayjones.plus.com \
--to=ramsay@ramsayjones$(echo .)plus.com \
--cc=git@vger$(echo .)kernel.org \
--cc=gitster@pobox$(echo .)com \
--cc=peff@peff$(echo .)net \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox