Re: [PATCH 06/14] Fix CFLAGS for UBSAN_BOUNDS on Clang

Marco Elver
2020-09-16T18:53:39Z

On Wed, Sep 16, 2020 at 12:14PM +0000, George Popescu wrote:
> On Wed, Sep 16, 2020 at 10:32:40AM +0200, Marco Elver wrote:
> > On Wed, 16 Sep 2020 at 09:40, George Popescu <georgepope@google.com> wrote:
> > > On Tue, Sep 15, 2020 at 07:32:28PM +0200, Marco Elver wrote:
> > > > On Tue, 15 Sep 2020 at 14:01, George Popescu <georgepope@google.com> wrote:
> > > > > On Tue, Sep 15, 2020 at 01:18:11PM +0200, Marco Elver wrote:
> > > > > > On Tue, 15 Sep 2020 at 12:25, George Popescu <georgepope@google.com> wrote:
> > > > > > > On Mon, Sep 14, 2020 at 03:13:14PM -0700, Kees Cook wrote:
> > > > > > > > On Mon, Sep 14, 2020 at 05:27:42PM +0000, George-Aurelian Popescu wrote:
> > > > > > > > > From: George Popescu <georgepope@google.com>
> > > > > > > > >
> > > > > > > > > When the kernel is compiled with Clang, UBSAN_BOUNDS inserts a brk after
> > > > > > > > > the handler call, preventing it from printing any information processed
> > > > > > > > > inside the buffer.
> > > > > > > > > For Clang -fsanitize=bounds expands to -fsanitize=array-bounds and
> > > > > > > > > -fsanitize=local-bounds, and the latter adds a brk after the handler
> > > > > > > > > call
> > > > > > > >
> > > > > > > This would mean losing the local-bounds coverage. I tried to test it without
> > > > > > > local-bounds and with a locally defined array on the stack and it works fine
> > > > > > > (the handler is called and the error reported). For me it feels like
> > > > > > > --array-bounds and --local-bounds are triggered for the same type of
> > > > > > > undefined_behaviours but they are handling them different.
> > > > > >
> > > > > > Does -fno-sanitize-trap=bounds help?
[...]
> > Your full config would be good, because it includes compiler version etc.
> My full config is:

Thanks. Yes, I can reproduce, and the longer I keep digging I start
wondering why we have local-bounds at all.

It appears that local-bounds finds a tiny subset of the issues that
KASAN finds:

http://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20131021/091536.html
http://llvm.org/viewvc/llvm-project?view=revision&revision=193205

fsanitize=undefined also does not include local-bounds:

https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html#available-checks

And the reason is that we do want to enable KASAN and UBSAN together;
but local-bounds is useless overhead if we already have KASAN.

I'm inclined to say that what you propose is reasonable (but the commit
message needs to be more detailed explaining the relationship with
KASAN) -- but I have no idea if this is going to break somebody's
usecase (e.g. find some OOB bugs, but without KASAN -- but then why not
use KASAN?!)

I'll ask some more people on LLVM side.

Thanks,
-- Marco