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.
--
Gabriel Krisman Bertazi
