Re: [PATCH] ext4: avoid utf8_strncasecmp() with unstable name

[Eric Biggers <ebiggers@kernel.org>]

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  Also, the casefolded name would get outdated when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.

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