BUG: Using pointer that points to a destructed string's content

[Tamas Szakaly]

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> > The following line is from _notmuch_message_add_directory_terms in
> > lib/message.cc (line 652 in HEAD):
> >
> > direntry = (*i).c_str ();
> >
> > 'i' is a Xapian::TermIterator, whose operator* returns a std::string by value.
> > This means that c_str() is called on a temporary, which is destructed after the
> > full expression (essentially the particular line in this case), so 'direntry'
> > will point to a destructed std::string's data.
> > (See https://gcc.gnu.org/onlinedocs/gcc/Temporaries.html)
> 
> Does the following patch fix it for you? I have to double check that
> direntry wasn't needed for something, but the test suite passes ;).
> 
> diff --git a/lib/message.cc b/lib/message.cc
> index a7a13cc..24d0d5b 100644
> --- a/lib/message.cc
> +++ b/lib/message.cc
> @@ -649,10 +649,8 @@ _notmuch_message_add_directory_terms (void *ctx, notmuch_message_t *message)
>  	/* Indicate that there are filenames remaining. */
>  	status = NOTMUCH_STATUS_DUPLICATE_MESSAGE_ID;
>  
> -	direntry = (*i).c_str ();
> -	direntry += direntry_prefix_len;
> -
> -	directory_id = strtol (direntry, &colon, 10);
> +	directory_id = strtol (
> +	    (*i).c_str () + direntry_prefix_len, &colon, 10);
>  
>  	if (colon == NULL || *colon != ':')
>  	    INTERNAL_ERROR ("malformed direntry");
> 

Nope, it's still not correct: with this code "colon" will point into the
temporary string's data. I'm using this patch now:

- --- work/notmuch-0.18.1/lib/message.cc	2014-06-25 12:30:10.000000000 +0200
+++ /root/message.cc	2014-12-20 08:15:10.000000000 +0100
@@ -601,18 +601,19 @@
 	unsigned int directory_id;
 	const char *direntry, *directory;
 	char *colon;
+	const std::string& tmp = *i;
 
+	direntry = tmp.c_str ();
+	
 	/* Terminate loop at first term without desired prefix. */
- -	if (strncmp ((*i).c_str (), direntry_prefix, direntry_prefix_len))
+	if (strncmp (direntry, direntry_prefix, direntry_prefix_len))
 	    break;
 
 	/* Indicate that there are filenames remaining. */
 	status = NOTMUCH_STATUS_DUPLICATE_MESSAGE_ID;
 
- -	direntry = (*i).c_str ();
- -	direntry += direntry_prefix_len;
- -
- -	directory_id = strtol (direntry, &colon, 10);
+	directory_id = strtol (
+		direntry + direntry_prefix_len, &colon, 10);
 
 	if (colon == NULL || *colon != ':')
 	    INTERNAL_ERROR ("malformed direntry");

This way operator* and c_str will be called only once, and its also better than
my first suggestion (using strdup), since it requires no additional memory
allocation. 

Regards,
sghctoma
- -- 
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