Re: [PATCH 3/3] lib/uuid: avoid double traversal in __uuid_parse()

[Josh Law]

12 Mar 2026 20:48:59 Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>:

> On Thu, 12 Mar 2026 18:41:13 +0000 Josh Law <hlcj1234567@xxxxxxxxx> wrote:
>
>> __uuid_parse() calls uuid_is_valid() to walk all 36 characters for
>> format validation, then walks the string a second time to parse the
>> hex bytes.  Combine both passes into one: validate each hex digit
>> inline via hex_to_bin() return value and check the four dash positions
>> after the loop.
>>
>> uuid_is_valid() remains exported unchanged for callers that only need
>> validation without parsing.
>>
>> --- a/lib/uuid.c
>> +++ b/lib/uuid.c
>> @@ -108,16 +108,20 @@ static int __uuid_parse(const char *uuid, __u8 b[16], const u8 ei[16])
>>     static const u8 si[16] = {0,2,4,6,9,11,14,16,19,21,24,26,28,30,32,34};
>>     unsigned int i;
>>
>> -   if (!uuid_is_valid(uuid))
>> -       return -EINVAL;
>> -
>>     for (i = 0; i < 16; i++) {
>>         int hi = hex_to_bin(uuid[si[i] + 0]);
>>         int lo = hex_to_bin(uuid[si[i] + 1]);
>>
>> +       if (hi < 0 || lo < 0)
>> +           return -EINVAL;
>> +
>>         b[ei[i]] = (hi << 4) | lo;
>>     }
>>
>> +   if (uuid[8] != '-' || uuid[13] != '-' ||
>> +       uuid[18] != '-' || uuid[23] != '-')
>> +       return -EINVAL;
>> +
>>     return 0;
>
> This rather messifies the code, and for what?  Is this in any way a hot
> path?


ehhhh. If I had to say, yes it is a hot path, it runs quite a lot

perf increases ever so slightly, so that's your call Andrew.