Re: [PATCH 2/3] diff: use size_t for the size passed to count_lines()

[Phillip Wood <phillip.wood123@gmail.com>]

On 04/11/2026 13:58, Phillip Wood wrote:
> On 01/11/2026 15:36, Elia Pinto wrote:
>> count_lines() takes the size as an "int", whereas callers have "unsigned
>> long" or "size_t". It is used for the diffstat of complete rewrites
>> ("git diff -B --stat") and in emit_rewrite_diff().
>>
>> With a file of 2GiB or more, the "int" becomes negative (or truncated
>> above 4GiB), leading to an incorrect line count. This only affects the
>> diffstat output, with no memory access issues.
>>
>> This was verified using a 16-bit simulation, where "git diff -B --stat"
>> reported 0 changed lines instead of the actual count.
>>
>  > Make the size parameter a size_t. The line counter remains "int".
> 
> That means the counter is subject to overflow as a file can contain as 
> many lines as its size.
> 
> I don't know what the simulation entailed, but this patch changes the 
> return type without changing either the type of the returned value which 
> can now overflow, or the types where the return values is stored by 
> callers.

Sorry I seem to have misread the patch when I wrote that - the return 
type is not changed. We're still left with the possibility of the count 
overflowing now that "size" is a size_t though I think.

Phillip

> I'm concerned that the preparation of these patches and the others 
> you've sent has relied too heavily on a flawed mechanical process, and 
> have not enough on manual review reading the code and reasoning about it.
> 
> Thanks
> 
> Phillip
> 
> 
>> Triggering this requires processing a file of 2GiB or more, so no test
>> is added.
>>
>> Signed-off-by: Elia Pinto <gitter.spiros@gmail.com>
>> ---
>>   diff.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/diff.c b/diff.c
>> index b4ac17f8ef..ed7a931c41 100644
>> --- a/diff.c
>> +++ b/diff.c
>> @@ -655,7 +655,7 @@ struct line_range_callback {
>>       int ret;            /* latched error from orig_line_fn */
>>   };
>> -static int count_lines(const char *data, int size)
>> +static int count_lines(const char *data, size_t size)
>>   {
>>       int count, ch, completely_empty = 1, nl_just_seen = 0;
>>       count = 0;
>