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Re: [PATCH 12/15] use get_commit_buffer everywhere

From
Jeff King <peff@peff.net>
Date
Jun 10, 2014, 00:02 UTC
Message-ID
<20140610000223.GA20644@sigill.intra.peff.net>
In-Reply-To
<xmqqbnu1emfa.fsf@gitster.dls.corp.google.com>
On Mon, Jun 09, 2014 at 03:40:57PM -0700, Junio C Hamano wrote:
Show 30 quoted lines
> Jeff King <peff@peff.net> writes:
> 
> > diff --git a/notes-merge.c b/notes-merge.c
> > index 94a1a8a..7885ab2 100644
> > --- a/notes-merge.c
> > +++ b/notes-merge.c
> > @@ -671,7 +671,8 @@ int notes_merge_commit(struct notes_merge_options *o,
> >  	DIR *dir;
> >  	struct dirent *e;
> >  	struct strbuf path = STRBUF_INIT;
> > -	char *msg = strstr(partial_commit->buffer, "\n\n");
> > +	const char *buffer = get_commit_buffer(partial_commit);
> > +	const char *msg = strstr(buffer, "\n\n");
> 
> This tightening causes...
> 
> >  	struct strbuf sb_msg = STRBUF_INIT;
> >  	int baselen;
> >  
> > @@ -720,6 +721,7 @@ int notes_merge_commit(struct notes_merge_options *o,
> >  	}
> >  
> >  	strbuf_attach(&sb_msg, msg, strlen(msg), strlen(msg) + 1);
> 
> ...a new error here:
> 
> notes-merge.c:723:2: error: passing argument 2 of 'strbuf_attach'
> discards 'const' qualifier from pointer target type [-Werror]
> strbuf.h:19:13: note: expected 'void *' but argument is of type
> 'const char *'

That's weird. I compile with -Wall -Werror, and my gcc doesn't complain. Hmph.

I agree it's not right, though. I think the original is questionable, too. It takes a pointer into the middle of partial_commit->buffer and attaches it to a strbuf. That's wrong because:

  1. It's pointing into the middle of an allocated buffer, not the
     beginning.
  2. We do not own partial_commit->buffer in the first place.

So any call to strbuf_detach on the result would be disastrous. The compiler doesn't notice because of the const leak in strstr, and it doesn't cause a bug in practice because the only use of the strbuf is to pass it as a const to create_notes_commit.

I feel like the most elegant solution is for create_notes_commit to take a buf/len pair rather than a strbuf, but it unfortunately is just feeding that to commit_tree. Adjusting that code path would affect quite a few other spots.

The other obvious option is actually populating the strbuf, but it feels ugly to have to make a copy just to satisfy the function interface.

Maybe a cast and a warning comment are the least evil thing, as below? I dunno, it feels pretty wrong.

diff --git a/notes-merge.c b/notes-merge.c
index 94a1a8a..1f3b309 100644
--- a/notes-merge.c
+++ b/notes-merge.c
@@ -671,7 +671,7 @@ int notes_merge_commit(struct notes_merge_options *o,
 	DIR *dir;
 	struct dirent *e;
 	struct strbuf path = STRBUF_INIT;
-	char *msg = strstr(partial_commit->buffer, "\n\n");
+	const char *msg = strstr(partial_commit->buffer, "\n\n");
 	struct strbuf sb_msg = STRBUF_INIT;
 	int baselen;
 
@@ -719,7 +719,15 @@ int notes_merge_commit(struct notes_merge_options *o,
 		strbuf_setlen(&path, baselen);
 	}
 
-	strbuf_attach(&sb_msg, msg, strlen(msg), strlen(msg) + 1);
+	/*
+	 * This is a bit tricky. We should not be attaching msg, which
+	 * is not owned by us and is not even the start of a heap buffer, to a
+	 * strbuf. But the create_notes_commit interface really wants
+	 * a strbuf, even though it will only ever use it as a buf/len pair and
+	 * never modify it. So this is tentatively safe as long as nobody ever
+	 * modifies, detaches, or releases the strbuf.
+	 */
+	strbuf_attach(&sb_msg, (char *)msg, strlen(msg), strlen(msg) + 1);
 	create_notes_commit(partial_tree, partial_commit->parents, &sb_msg,
 			    result_sha1);
 	if (o->verbosity >= 4)

I'm still confused and disturbed that my gcc is not noticing this
obvious const violation. Hmm, shutting off ccache seems to make it work.
Doubly disturbing.

-Peff
Previous: Junio C HamanoNext: Jeff King
Message 24 of 60 in “store length of commit->buffer”
  1. 0/15 store length of commit->bufferJeff King, Jun 9, 2014
  2. 01/15 alloc: include any-object allocations in alloc_reportJeff King, Jun 9, 2014
  3. 02/15 commit: push commit_index update into alloc_commit_nodeJeff King, Jun 9, 2014
  4. Junio C HamanoJun 10, 2014
  5. 03/15 do not create "struct commit" with xcallocJeff King, Jun 9, 2014
  6. Junio C HamanoJun 10, 2014
  7. Jeff KingJun 10, 2014
  8. 04/15 logmsg_reencode: return const bufferJeff King, Jun 9, 2014
  9. Eric SunshineJun 10, 2014
  10. 05/15 sequencer: use logmsg_reencode in get_messageJeff King, Jun 9, 2014
  11. Junio C HamanoJun 10, 2014
  12. Jeff KingJun 10, 2014
  13. 06/15 provide a helper to free commit bufferJeff King, Jun 9, 2014
  14. 07/15 provide a helper to set the commit bufferJeff King, Jun 9, 2014
  15. 08/15 provide helpers to access the commit bufferJeff King, Jun 9, 2014
  16. Eric SunshineJun 10, 2014
  17. 09/15 use get_cached_commit_buffer where appropriateJeff King, Jun 9, 2014
  18. 10/15 use get_commit_buffer to avoid duplicate codeJeff King, Jun 9, 2014
  19. Eric SunshineJun 10, 2014
  20. Jeff KingJun 10, 2014
  21. 11/15 convert logmsg_reencode to get_commit_bufferJeff King, Jun 9, 2014
  22. 12/15 use get_commit_buffer everywhereJeff King, Jun 9, 2014
  23. Junio C HamanoJun 9, 2014
  24. Jeff KingJun 10, 2014
  25. Jeff KingJun 10, 2014
  26. Junio C HamanoJun 10, 2014
  27. Jeff KingJun 10, 2014
  28. 13/15 commit-slab: provide a static initializerJeff King, Jun 9, 2014
  29. 14/15 commit: convert commit->buffer to a slabJeff King, Jun 9, 2014
  30. 15/15 commit: record buffer length in cacheJeff King, Jun 9, 2014
  31. Christian CouderJun 10, 2014
  32. Jeff KingJun 10, 2014
  33. Jeff KingJun 10, 2014
  34. Christian CouderJun 10, 2014
  35. 0/17 store length of commit->bufferJeff King, Jun 10, 2014
  36. 01/17 commit_tree: take a pointer/len pair rather than a const strbufJeff King, Jun 10, 2014
  37. 02/17 replace dangerous uses of strbuf_attachJeff King, Jun 10, 2014
  38. 03/17 alloc: include any-object allocations in alloc_reportJeff King, Jun 10, 2014
  39. 04/17 commit: push commit_index update into alloc_commit_nodeJeff King, Jun 10, 2014
  40. 05/17 do not create "struct commit" with xcallocJeff King, Jun 10, 2014
  41. 06/17 logmsg_reencode: return const bufferJeff King, Jun 10, 2014
  42. 07/17 sequencer: use logmsg_reencode in get_messageJeff King, Jun 10, 2014
  43. 08/17 provide a helper to free commit bufferJeff King, Jun 10, 2014
  44. Junio C HamanoJun 12, 2014
  45. Jeff KingJun 12, 2014
  46. Jeff KingJun 12, 2014
  47. 09/17 provide a helper to set the commit bufferJeff King, Jun 10, 2014
  48. 10/17 provide helpers to access the commit bufferJeff King, Jun 10, 2014
  49. 11/17 use get_cached_commit_buffer where appropriateJeff King, Jun 10, 2014
  50. 12/17 use get_commit_buffer to avoid duplicate codeJeff King, Jun 10, 2014
  51. 13/17 convert logmsg_reencode to get_commit_bufferJeff King, Jun 10, 2014
  52. 14/17 use get_commit_buffer everywhereJeff King, Jun 10, 2014
  53. 15/17 commit-slab: provide a static initializerJeff King, Jun 10, 2014
  54. Junio C HamanoJun 12, 2014
  55. Jeff KingJun 12, 2014
  56. 16/17 commit: convert commit->buffer to a slabJeff King, Jun 10, 2014
  57. 17/17 commit: record buffer length in cacheJeff King, Jun 10, 2014
  58. Jeff KingJun 10, 2014
  59. Junio C HamanoJun 12, 2014
  60. reuse cached commit buffer when parsing signaturesJeff King, Jun 13, 2014

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