Re: [syzbot] KMSAN: uninit-value in longest_match

[Alexander Potapenko]

> > Can't we just pass __GFP_ZERO when allocating the workspace here:
> >
> > ...
> > This is what Chrome folks did in the same situation (
> > https://chromium.googlesource.com/chromium/src.git/+/962cbbe81708214ff8e14e2bc8a07271cb15f1b9
> > )
>
> Sure, the btrfs folks can do that if they want to avoid this warning for the
> btrfs zlib compression specifically.  This would not solve the issue for the
> other users of zlib compression in the kernel.

We probably can't fix zlib without sacrificing performance, so I think
the right thing to do is to actually modify the users of zlib
compression to pass only initialized data to zlib.
Doing so will make the calls to zlib_deflate() well-defined, whereas
marking longest_match() as buggy will just hide future error reports,
but won't fix the code in question.