Re: [PATCH v7 2/8] fs: Add standard casefolding support

Eric Biggers
2020-02-12T06:34:44Z

On Mon, Feb 10, 2020 at 11:42:07PM +0000, Al Viro wrote:
> On Mon, Feb 10, 2020 at 03:11:13PM -0800, Daniel Rosenberg wrote:
> > On Fri, Feb 7, 2020 at 6:12 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
> > >
> > > On Fri, Feb 07, 2020 at 05:35:46PM -0800, Daniel Rosenberg wrote:
> > >
> > >
> > > Again, is that safe in case when the contents of the string str points to
> > > keeps changing under you?
> >
> > I'm not sure what you mean. I thought it was safe to use the str and
> > len passed into d_compare. Even if it gets changed under RCU
> > conditions I thought there was some code to ensure that the name/len
> > pair passed in is consistent, and any other inconsistencies would get
> > caught by d_seq later. Are there unsafe code paths that can follow?
>
> If you ever fetch the same byte twice, you might see different values.
> You need a fairly careful use of READ_ONCE() or equivalents to make
> sure that you don't get screwed over by that.
>
> Sure, ->d_seq mismatch will throw the result out, but you need to make
> sure you won't oops/step on uninitialized memory/etc. in process.
>
> It's not impossible to get right, but it's not trivial and you need all
> code working with that much more careful than normal for string handling.

It looks like this is a real problem, not just a "theoretical" data race.
For example, see:

utf8ncursor():
/* The first byte of s may not be an utf8 continuation. */
if (len > 0 && (*s & 0xC0) == 0x80)
return -1;

and then utf8byte():
} else if ((*u8c->s & 0xC0) == 0x80) {
/* This is a continuation of the current character. */
if (!u8c->p)
u8c->len--;
return (unsigned char)*u8c->s++;

The first byte of the string is checked in two different functions, so it's very
likely to be loaded twice. In between, it could change from a non-continuation
byte to a continuation byte. That would cause the string length to be
decremented from 0 to UINT_MAX. Then utf8_strncasecmp() would run beyond the
bounds of the string until something happened to mismatch.

That's just an example that I found right away; there are probably more.

IMO, this needs to be fixed before anyone can actually use the ext4 and f2fs
casefolding stuff.

I don't know the best solution. One option is to fix fs/unicode/ to handle
concurrently modified strings. Another could be to see what it would take to
serialize lookups and renames for casefolded directories...

- Eric