Subject: Re: [PATCH 2/2] xdiff/xprepare: fix a memory leak

From: Ramsay Jones <[email-masked]>
Date: Sat, 5 Mar 2016 01:19:18 +0000

On 04/03/16 23:50, Junio C Hamano wrote:
> Ramsay Jones <[email-masked]> writes:
> 
>> The xdl_prepare_env() function may initialise an xdlclassifier_t
>> data structure via xdl_init_classifier(), which allocates memory
>> to several fields, for example 'rchash', 'rcrecs' and 'ncha'.
>> If this function later exits due to the failure of xdl_optimize_ctxs(),
>> then this xdlclassifier_t structure, and the memory allocated to it,
>> is not cleaned up.
>>
>> In order to fix the memory leak, insert a call to xdl_free_classifier()
>> before returning.
>>
>> Signed-off-by: Ramsay Jones <[email-masked]>
>> ---
>>  xdiff/xprepare.c | 1 +
>>  1 file changed, 1 insertion(+)
>>
>> diff --git a/xdiff/xprepare.c b/xdiff/xprepare.c
>> index 5ffcf99..13b55ab 100644
>> --- a/xdiff/xprepare.c
>> +++ b/xdiff/xprepare.c
>> @@ -301,6 +301,7 @@ int xdl_prepare_env(mmfile_t *mf1, mmfile_t *mf2, xpparam_t const *xpp,
>>  
>>  		xdl_free_ctx(&xe->xdf2);
>>  		xdl_free_ctx(&xe->xdf1);
>> +		xdl_free_classifier(&cf);
>>  		return -1;
>>  	}
> 
> This looks obviously correct from the pattern of prepare's and
> free's in the code that this part follows.  This potential leak has
> been that way since 3443546f (Use a *real* built-in diff generator,
> 2006-03-24), i.e. the very beginning.
> 
> I find it somewhat strange that the call to xdl_free_classifier() at
> the end of this function is made conditional to XDF_HISTOGRAM_DIFF,
> though.  I can half-buy the argument "that is because we do not call
> init-classifier for XDF_HISTOGRAM_DIFF", but in the error path we
> call free-classifier unconditionally, so the code clearly knows that
> it is safe to call free-classifier on a classifier that is cleared
> with the initial memset(&cf, 0, sizeof cf).

Indeed, this is actually why I noticed that XDF_DIFF_ALG() wasn't used.
Rather than doing patch #1, I did consider making this call unconditional.
I can't remember why I didn't. (Hmm, perhaps I just chickened out! ;-))

ATB,
Ramsay Jones