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From: Jinoh Kang <>
To: Junio C Hamano <>,
Cc: Johannes Schindelin <>
Subject: [PATCH v2] diff: handle NULL filespecs in run_external_diff
Date: Fri, 6 Nov 2020 17:02:19 +0000	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

[forwarding from to]

On 11/4/20 10:30 PM, Junio C Hamano wrote:
> Jinoh Kang <> writes:
>> `one` and `two` can be NULL when comparing an unmerged pair.
>> There is a conditional above that already tests for this.
>> Fixes: 3aef54e8b8 ("diff: munmap() file contents before running external diff")
>> Signed-off-by: Jinoh Kang <>
>> ---
>>  diff.c              |  6 ++++--
>>  t/ | 23 +++++++++++++++++++++++
>>  2 files changed, 27 insertions(+), 2 deletions(-)
>> diff --git a/diff.c b/diff.c
>> index d24f47df99..ae1ec2d6c8 100644
>> --- a/diff.c
>> +++ b/diff.c
>> @@ -4267,8 +4267,10 @@ static void run_external_diff(const char *pgm,
>>  	strvec_pushf(&env, "GIT_DIFF_PATH_COUNTER=%d", ++o->diff_path_counter);
>>  	strvec_pushf(&env, "GIT_DIFF_PATH_TOTAL=%d", q->nr);
>> -	diff_free_filespec_data(one);
>> -	diff_free_filespec_data(two);
>> +	if (one)
>> +		diff_free_filespec_data(one);
>> +	if (two)
>> +		diff_free_filespec_data(two);
>>  	if (run_command_v_opt_cd_env(argv.v, RUN_USING_SHELL, NULL, env.v))
>>  		die(_("external diff died, stopping at %s"), name);
> Have you considered allowing diff_free_filespec_data() to take NULL
> and return safely without doing anything?

Yes, I have. In fact, this kind of behavior is exactly what I would
expect from a function that "frees" something.  However, I was not
entirely sure if this applies here, for several reasons that follow:

> That models after free() and other "we are done with the resource
> and it is time to clean it up" functions,

This corresponds to `free_filespec`. In this particular case,
I strongly agree that it makes perfect sense to do nothing when
passed a NULL pointer.

However, I humbly opine that the free() semantics do not apply to
`diff_free_filespec_data`; rather, I prefer to see it as a function
that simply transitions a diff_filespec from one state to another.

For this reason, I'd consider the act of passing NULL to
diff_free_filespec_data as a bug in the first place.  Further,
if it does not entail a security issue, why not just crash *right
now* rather than (possibly) causing more obscure bugs later?

I would put the blame on its name, since "data" feels too generic
and makes the function sound like freeing the filespec _itself_.
diff_filespec carries a lot of other things besides just `data`
and `cnt_data`.

Please feel free to correct me if I'm wrong; after all, I am not
exactly one of the long-time maintainers.

> fixes this particular bug, and possibly simplifies existing
> callers that check the NULL-ness before calling it.

I was unable to find any callsites that explicitly check for
NULL-ness _immediately_ before calling diff_free_filespec_data.
Excluding 3aef54e8b8, `GCC -fanalyze` suggests to me that
all the callers have already dereferenced the pointer by the time
the call is made; therefore, a NULL pointer dereference would have
already happened before the flow could even get to
the diff_free_filespec_data function.

The NULL checks are usually placed in the beginning of caller
functions rather than close to the exit path (which then calls

>> diff --git a/t/ b/t/
>> index 524f30f7dc..8cc1c9117c 100755
>> --- a/t/
>> +++ b/t/
>> @@ -728,6 +728,29 @@ test_expect_success 'add -N and difftool -d' '
>>  	git difftool --dir-diff --extcmd ls
>>  '
>> +test_expect_success 'difftool --cached with unmerged files' '
>> +	test_when_finished git reset --hard &&
>> +	echo base > file &&
> Style.  "echo base >file &&" (no SP between redirect operator and
> its target).

My bad. Thanks!

> I do not think this is "security" matter.  I do appreciate you
> erring on the side of being cautious and sending the patch first
> here, but please take it to the regular mailing list.

Yes, no sensible web-based git browser would prepare a working tree,
start a conflicting merge *AND* run an external diff on it...

> Thanks.

Jinoh Kang

       reply	other threads:[~2020-11-06 17:02 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <>
     [not found] ` <>
2020-11-06 17:02   ` Jinoh Kang [this message]
2020-11-06 17:14     ` [PATCH v3] diff: make diff_free_filespec_data accept NULL Jinoh Kang
2020-11-10 12:08       ` Johannes Schindelin
2020-11-10 13:16         ` Jinoh Kang
2020-11-10 14:21         ` Jinoh Kang
2020-11-10 17:02         ` Junio C Hamano
2020-11-10 14:06       ` [PATCH v4] " Jinoh Kang
2020-11-10 15:38         ` Johannes Schindelin
2020-11-11 12:30           ` Jinoh Kang
2020-11-11 16:28             ` Johannes Schindelin
2020-11-10 19:41         ` Junio C Hamano
2020-11-11 12:15         ` [PATCH v5] " Jinoh Kang
2020-11-11 16:27           ` Johannes Schindelin
2020-11-11 19:18             ` Junio C Hamano
2020-11-06 19:18     ` [PATCH v2] diff: handle NULL filespecs in run_external_diff Junio C Hamano

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