Eric Biggers <ebigg...@kernel.org> writes:
> On Sat, May 30, 2020 at 02:17:02AM -0400, Gabriel Krisman Bertazi wrote:
>> > > > + /*
>> > + * If the dentry name is stored in-line, then it may be concurrently
>> > + * modified by a rename. If this happens, the VFS will eventually retry
>> > + * the lookup, so it doesn't matter what ->d_compare() returns.
>> > + * However, it's unsafe to call utf8_strncasecmp() with an unstable
>> > + * string. Therefore, we have to copy the name into a temporary buffer.
>> > + */
>> > + if (len <= DNAME_INLINE_LEN - 1) {
>> > + unsigned int i;
>> > +
>> > + for (i = 0; i < len; i++)
>> > + strbuf[i] = READ_ONCE(str[i]);
>> > + strbuf[len] = 0;
>> > + qstr.name = strbuf;
>> > + }
>> > +
>>
>> Could we avoid this if the casefolded version were cached in the dentry?
>> Then we could use utf8_strncasecmp_folded which would be safe. Would
>> this be acceptable for vfs?
>
> The VFS assumes that each dentry has one name, the one in d_name. That's what
> it passes to ->d_compare(), and that's what it updates in __d_move().
>
> So while ext4 and f2fs could put the casefolded name in ->d_fsdata,
> ->d_compare() wouldn't actually have access to it (unless we added d_fsdata
> as a
> parameter to ->d_compare()). Also, the casefolded name would get outdated
> when
> __d_move() changes d_name.
Thanks for the explanation. I see. I was thinking this cached name
could be invalidated if the name changes and all that jazz, but I didn't
understand this code well enough to give it a try. I hate the need to
do this copy here, though.
Anyway,
Reviewed-by: Gabriel Krisman Bertazi <kris...@collabora.com>
>
> We could instead make d_name always be the casefolded name. I'm not sure that
> would be possible, though. For one, I don't think ->lookup() is allowed to
> just
> change the dentry name. It would also make getcwd(), /proc/*/fd/, etc. always
> show casefolded names, which could be problematic. And probably other issues
> I
> can't think of off the top of my head.
>
> - Eric
--
Gabriel Krisman Bertazi
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