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SubjectRe: [PATCH v5] fs: introduce is_dot_or_dotdot helper for cleanup
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On 12/11/2019 10:48 AM, Eric Biggers wrote:
> On Wed, Dec 11, 2019 at 10:20:01AM +0800, Tiezhu Yang wrote:
>> diff --git a/include/linux/namei.h b/include/linux/namei.h
>> index 7fe7b87..0fd9315 100644
>> --- a/include/linux/namei.h
>> +++ b/include/linux/namei.h
>> @@ -92,4 +92,14 @@ retry_estale(const long error, const unsigned int flags)
>> return error == -ESTALE && !(flags & LOOKUP_REVAL);
>> }
>>
>> +static inline bool is_dot_or_dotdot(const unsigned char *name, size_t len)
>> +{
>> + if (unlikely(name[0] == '.')) {
>> + if (len == 1 || (len == 2 && name[1] == '.'))
>> + return true;
>> + }
>> +
>> + return false;
>> +}
>> +
>> #endif /* _LINUX_NAMEI_H */
> I had suggested adding a len >= 1 check to handle the empty name case correctly.
> What I had in mind was
>
> static inline bool is_dot_or_dotdot(const unsigned char *name, size_t len)
> {
> if (len >= 1 && unlikely(name[0] == '.')) {
> if (len < 2 || (len == 2 && name[1] == '.'))
> return true;
> }
>
> return false;
> }
>
> As is, you're proposing that it always dereference the first byte even when
> len=0, which seems like a bad idea for a shared helper function. Did you check
> whether it's okay for all the existing callers? fscrypt_fname_disk_to_usr() is
> called from 6 places, did you check all of them?
>
> How about keeping the existing optimized code for the hot path in fs/namei.c
> (i.e. not using the helper function), while having the helper function do the
> extra check to handle len=0 correctly?

Hi Eric,

Thank you for reminding me. How about using the following helper for
all callers?

static inline bool is_dot_or_dotdot(const unsigned char *name, size_t len)
{
if (len == 1 && name[0] == '.')
return true;

if (len == 2 && name[0] == '.' && name[1] == '.')
return true;

return false;
}

Hi Matthew,

How do you think? I think the performance influence is very small
due to is_dot_or_dotdot() is a such short static inline function.

Thanks,

Tiezhu Yang

>
> - Eric

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