Re: [PATCH 11/15] find multi-byte comment chars in unterminated buffers

> Am 07.03.24 um 12:08 schrieb Jeff King:
>> On Thu, Mar 07, 2024 at 04:26:38AM -0500, Jeff King wrote:
>>
>>> IMHO this is the trickiest commit of the whole series, as it would be
>>> easy to get the length computations subtly wrong.
>>
>> And sure enough...
>>
>>> diff --git a/trailer.c b/trailer.c
>>> index fe18faf6c5..f59c90b4b5 100644
>>> --- a/trailer.c
>>> +++ b/trailer.c
>>> @@ -882,7 +882,7 @@ static size_t find_trailer_block_start(const char *buf, size_t len)
>>>
>>>  	/* The first paragraph is the title and cannot be trailers */
>>>  	for (s = buf; s < buf + len; s = next_line(s)) {
>>> -		if (s[0] == comment_line_char)
>>> +		if (starts_with_mem(s, buf + len - s, comment_line_str))
>>>  			continue;
>>>  		if (is_blank_line(s))
>>>  			break;
>>> @@ -902,7 +902,7 @@ static size_t find_trailer_block_start(const char *buf, size_t len)
>>>  		const char **p;
>>>  		ssize_t separator_pos;
>>>
>>> -		if (bol[0] == comment_line_char) {
>>> +		if (starts_with_mem(bol, buf + end_of_title - bol, comment_line_str)) {
>>>  			non_trailer_lines += possible_continuation_lines;
>>>  			possible_continuation_lines = 0;
>>>  			continue;
>>
>> This second hunk needs:
>>
>> diff --git a/trailer.c b/trailer.c
>> index f59c90b4b5..fdb0b8137e 100644
>> --- a/trailer.c
>> +++ b/trailer.c
>> @@ -902,7 +902,7 @@ static size_t find_trailer_block_start(const char *buf, size_t len)
>>  		const char **p;
>>  		ssize_t separator_pos;
>>
>> -		if (starts_with_mem(bol, buf + end_of_title - bol, comment_line_str)) {
>> +		if (starts_with_mem(bol, buf + len - bol, comment_line_str)) {
>>  			non_trailer_lines += possible_continuation_lines;
>>  			possible_continuation_lines = 0;
>>  			continue;
>>
>> I was trying to bound the size based on the loop, which is:
>>
>>           for (l = last_line(buf, len);
>>                l >= end_of_title;
>>                l = last_line(buf, l)) {
>>                   const char *bol = buf + l;
>>
>> but I misread "end_of_title" as an upper bound, not a lower one. Which
>> makes sense because we're iterating backwards over the lines. So I
>> suppose we could bound it by the previous "bol" value. But in practice,
>> your prefix won't cross such a boundary anyway, as it won't have a
>> newline in it (maybe that's something we should enforce? I guess you
>> could set core.commentChar to '\n' even before my series, which would be
>> slightly insane).
>>
>> So just bounding ourselves to "buf + len" seems reasonable, as that
>> makes sure we don't step outside the buffer passed into the function.