[v4] grep: fail if call could output and name is null

Commit Message

Emily Shaffer May 23, 2019, 8:23 p.m. UTC

grep_source(), which performs much of the work for Git's grep library,
allows passing an arbitrary struct grep_source which represents the text
which grep_source() should search to match a pattern in the provided
struct grep_opt. In most callers, the grep_source::name field is set to
an appropriate prefix to print before a colon when a result matches:

	README:Git is an Open Source project covered by the GNU General

One caller, grep_buffer(), leaves the grep_source::name field set to
NULL because there isn't enough context to determine an appropriate name
for this kind of output line. In practice, this has been fine: the only
caller of grep_buffer() is "git log --grep", and that caller sets
grep_opt::status_only, which disables output and only checks whether a
match exists. But this is brittle: a future caller can call
grep_buffer() without grep_opt::status_only set, and as soon as it hits
a match, grep_source() will try to print the match and segfault:

	(null):Git is an Open Source project covered by the GNU General

For example, a future caller might want to print all matching lines from
commits which match a regex.

Futureproof by diagnosing early a use of the API that could trigger that
condition, before we know whether the pattern matches:

	BUG: grep.c:1783: grep call which could print a name requires
		grep_source.name be non-NULL
	Aborted

This way, the caller's author gets an indication of how to fix the issue
- by providing grep_source::name or setting grep_opt::status_only - and
they are warned of the potential for a segfault unconditionally, rather
than only if there is a match.

Noticed while adding such a call to a tutorial on revision walks.

Signed-off-by: Emily Shaffer <[e-mail]>
Reviewed-by: Jonathan Nieder <[e-mail]>
---
Since v3, only the commit message has changed. Reworked based on
Jonathan Nieder's suggestions (with some modifications for readability).

 - Emily

 grep.c | 4 ++++
 1 file changed, 4 insertions(+)

Comments

Jonathan Nieder May 23, 2019, 8:34 p.m. UTC | #1

Emily Shaffer wrote:

> Signed-off-by: Emily Shaffer <[e-mail]>
> Reviewed-by: Jonathan Nieder <[e-mail]>
> ---
> Since v3, only the commit message has changed. Reworked based on
> Jonathan Nieder's suggestions (with some modifications for readability).
>
>  grep.c | 4 ++++
>  1 file changed, 4 insertions(+)

This is indeed
Reviewed-by: Jonathan Nieder <[e-mail]>

Thanks for your patient work.

Junio C Hamano May 28, 2019, 5:57 p.m. UTC | #2

Jonathan Nieder <[e-mail]> writes:

> Emily Shaffer wrote:
>
>> Signed-off-by: Emily Shaffer <[e-mail]>
>> Reviewed-by: Jonathan Nieder <[e-mail]>
>> ---
>> Since v3, only the commit message has changed. Reworked based on
>> Jonathan Nieder's suggestions (with some modifications for readability).
>>
>>  grep.c | 4 ++++
>>  1 file changed, 4 insertions(+)
>
> This is indeed
> Reviewed-by: Jonathan Nieder <[e-mail]>
>
> Thanks for your patient work.

Thanks, both.  Will queue.

Patch

diff --git a/grep.c b/grep.c
index 0d50598acd..f7c3a5803e 100644
--- a/grep.c
+++ b/grep.c
@@ -1780,6 +1780,10 @@  static int grep_source_1(struct grep_opt *opt, struct grep_source *gs, int colle
 	enum grep_context ctx = GREP_CONTEXT_HEAD;
 	xdemitconf_t xecfg;
 
+	if (!opt->status_only && gs->name == NULL)
+		BUG("grep call which could print a name requires "
+		    "grep_source.name be non-NULL");
+
 	if (!opt->output)
 		opt->output = std_output;