Re: [PATCH] fs: ratelimit __find_get_block_slow() failure message.

[Jan Kara <jack@suse.cz>]

On Wed 16-01-19 12:03:27, Dmitry Vyukov wrote:
> On Wed, Jan 16, 2019 at 11:43 AM Jan Kara <jack@suse.cz> wrote:
> >
> > On Wed 16-01-19 10:47:56, Dmitry Vyukov wrote:
> > > On Fri, Jan 11, 2019 at 1:46 PM Tetsuo Handa
> > > <penguin-kernel@i-love.sakura.ne.jp> wrote:
> > > >
> > > > On 2019/01/11 19:48, Dmitry Vyukov wrote:
> > > > >> How did you arrive to the conclusion that it is harmless?
> > > > >> There is only one relevant standard covering this, which is the C
> > > > >> language standard, and it is very clear on this -- this has Undefined
> > > > >> Behavior, that is the same as, for example, reading/writing random
> > > > >> pointers.
> > > > >>
> > > > >> Check out this on how any race that you might think is benign can be
> > > > >> badly miscompiled and lead to arbitrary program behavior:
> > > > >> https://software.intel.com/en-us/blogs/2013/01/06/benign-data-races-what-could-possibly-go-wrong
> > > > >
> > > > > Also there is no other practical definition of data race for automatic
> > > > > data race detectors than: two conflicting non-atomic concurrent
> > > > > accesses. Which this code is. Which means that if we continue writing
> > > > > such code we are not getting data race detection and don't detect
> > > > > thousands of races in kernel code that one may consider more harmful
> > > > > than this one the easy way. And instead will spent large amounts of
> > > > > time to fix some of then the hard way, and leave the rest as just too
> > > > > hard to debug so let the kernel continue crashing from time to time (I
> > > > > believe a portion of currently open syzbot bugs that developers just
> > > > > left as "I don't see how this can happen" are due to such races).
> > > > >
> > > >
> > > > I still cannot catch. Read/write of sizeof(long) bytes at naturally
> > > > aligned address is atomic, isn't it?
> > >
> > > Nobody guarantees this. According to C non-atomic conflicting
> > > reads/writes of sizeof(long) cause undefined behavior of the whole
> > > program.
> >
> > Yes, but to be fair the kernel has always relied on long accesses to be
> > atomic pretty heavily so that it is now de-facto standard for the kernel
> > AFAICT. I understand this makes life for static checkers hard but such is
> > reality.
> 
> Yes, but nobody never defined what "a long access" means. And if you
> see a function that accepts a long argument and stores it into a long
> field, no, it does not qualify. I bet this will come at surprise to
> lots of developers.

Yes, inlining and other optimizations can screw you.

> Check out this fix and try to extrapolate how this "function stores
> long into a long leads to a serious security bug" can actually be
> applied to whole lot of places after inlining (or when somebody just
> slightly shuffles code in a way that looks totally safe) that also
> kinda look safe and atomic:
> https://lore.kernel.org/patchwork/patch/599779/
> So where is the boundary between "a long access" that is atomic and
> the one that is not necessary atomic?

So I tend to rely on "long access being atomic" for opaque values (no
flags, no counters, ...). Just value that gets fetched from some global
variable / other data structure, stored, read, and possibly compared for
equality. I agree the compiler could still screw you if it could infer how
that value got initially created and try to be clever about it...

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR