Re: [PATCH v5] fs: introduce is_dot_or_dotdot helper for cleanup

[Tiezhu Yang]

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