On Mon, Aug 14, 2023 at 10:50:13AM -0400, Gabriel Krisman Bertazi wrote:
> Eric Biggers <ebiggers@xxxxxxxxxx> writes:
>
> > On Fri, Aug 11, 2023 at 08:41:42PM -0400, Gabriel Krisman Bertazi wrote:
> >> + /*
> >> + * Filesystems will call into d_revalidate without setting
> >> + * LOOKUP_ flags even for file creation (see lookup_one*
> >> + * variants). Reject negative dentries in this case, since we
> >> + * can't know for sure it won't be used for creation.
> >> + */
> >> + if (!flags)
> >> + return 0;
> >> +
> >> + /*
> >> + * If the lookup is for creation, then a negative dentry can
> >> + * only be reused if it's a case-sensitive match, not just a
> >> + * case-insensitive one. This is needed to make the new file be
> >> + * created with the name the user specified, preserving case.
> >> + */
> >> + if (flags & (LOOKUP_CREATE | LOOKUP_RENAME_TARGET)) {
> >> + /*
> >> + * ->d_name won't change from under us in the creation
> >> + * path only, since d_revalidate during creation and
> >> + * renames is always called with the parent inode
> >> + * locked. It isn't the case for all lookup callpaths,
> >> + * so ->d_name must not be touched outside
> >> + * (LOOKUP_CREATE|LOOKUP_RENAME_TARGET) context.
> >> + */
> >> + if (dentry->d_name.len != name->len ||
> >> + memcmp(dentry->d_name.name, name->name, name->len))
> >> + return 0;
> >> + }
> >
> > This is still really confusing to me. Can you consider the below? The code is
> > the same except for the reordering, but the explanation is reworked to be much
> > clearer (IMO). Anything I am misunderstanding?
> >
> > /*
> > * If the lookup is for creation, then a negative dentry can only be
> > * reused if it's a case-sensitive match, not just a case-insensitive
> > * one. This is needed to make the new file be created with the name
> > * the user specified, preserving case.
> > *
> > * LOOKUP_CREATE or LOOKUP_RENAME_TARGET cover most creations. In these
> > * cases, ->d_name is stable and can be compared to 'name' without
> > * taking ->d_lock because the caller holds dir->i_rwsem for write.
> > * (This is because the directory lock blocks the dentry from being
> > * concurrently instantiated, and negative dentries are never moved.)
> > *
> > * All other creations actually use flags==0. These come from the edge
> > * case of filesystems calling functions like lookup_one() that do a
> > * lookup without setting the lookup flags at all. Such lookups might
> > * or might not be for creation, and if not don't guarantee stable
> > * ->d_name. Therefore, invalidate all negative dentries when flags==0.
> > */
> > if (flags & (LOOKUP_CREATE | LOOKUP_RENAME_TARGET)) {
> > if (dentry->d_name.len != name->len ||
> > memcmp(dentry->d_name.name, name->name, name->len))
> > return 0;
> > }
> > if (!flags)
> > return 0;
>
> I don't see it as particularly better or less confusing than the
> original. but I also don't mind taking it into the next iteration.
>
Your commit message is still much longer and covers some quite different details
which seem irrelevant to me. So if you don't see my explanation as being much
different, I think we're still not on the same page. I hope that I'm not
misunderstanding anything, in which I believe that what I wrote above is a good
explanation and your commit message should be substantially simplified.
Remember, longer != better. Keep things as simple as possible.
- Eric
