Re: linux-next: Tree for Nov 20 (kcsan + objtool)

[Marco Elver <elver@google.com>]

On Thu, 21 Nov 2019 at 19:37, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
>
> On Wed, Nov 20, 2019 at 08:48:41PM +0100, Marco Elver wrote:
> > On Wed, 20 Nov 2019 at 17:18, Randy Dunlap <rdunlap@infradead.org> wrote:
> > >
> > > On 11/20/19 1:34 AM, Stephen Rothwell wrote:
> > > > Hi all,
> > > >
> > > > Changes since 20191119:
> > > >
> > >
> > > on x86_64:
> > >
> > > kernel/kcsan/core.o: warning: objtool: kcsan_found_watchpoint()+0xa: call to kcsan_is_enabled() with UACCESS enabled
> > > kernel/kcsan/core.o: warning: objtool: __tsan_read1()+0x13: call to find_watchpoint() with UACCESS enabled
> > > kernel/kcsan/core.o: warning: objtool: __tsan_write1()+0x10: call to find_watchpoint() with UACCESS enabled
> > > kernel/kcsan/core.o: warning: objtool: __tsan_read2()+0x13: call to find_watchpoint() with UACCESS enabled
> > > kernel/kcsan/core.o: warning: objtool: __tsan_write2()+0x10: call to find_watchpoint() with UACCESS enabled
> > > kernel/kcsan/core.o: warning: objtool: __tsan_read4()+0x13: call to find_watchpoint() with UACCESS enabled
> > > kernel/kcsan/core.o: warning: objtool: __tsan_write4()+0x10: call to find_watchpoint() with UACCESS enabled
> > > kernel/kcsan/core.o: warning: objtool: __tsan_read8()+0x13: call to find_watchpoint() with UACCESS enabled
> > > kernel/kcsan/core.o: warning: objtool: __tsan_write8()+0x10: call to find_watchpoint() with UACCESS enabled
> > > kernel/kcsan/core.o: warning: objtool: __tsan_read16()+0x13: call to find_watchpoint() with UACCESS enabled
> > > kernel/kcsan/core.o: warning: objtool: __tsan_write16()+0x10: call to find_watchpoint() with UACCESS enabled
> > > kernel/kcsan/core.o: warning: objtool: __tsan_read_range()+0x13: call to find_watchpoint() with UACCESS enabled
> > > kernel/kcsan/core.o: warning: objtool: __tsan_write_range()+0x10: call to find_watchpoint() with UACCESS enabled
> > >
> > > kernel/trace/trace_branch.o: warning: objtool: ftrace_likely_update()+0x361: call to __stack_chk_fail() with UACCESS enabled
> > >
> > >
> > > Full randconfig file is attached.
> >
> > Thanks.
> >
> > This is due to CONFIG_CC_OPTIMIZE_FOR_SIZE=y. It seems the compiler
> > decides to not even inline small static inline functions. I tried to
> > make this go away by adding __always_inline, but then we're also left
> > with atomic64_try_cmpxchg which never gets inlined.
> >
> > The optimized build simply inlines the small static inline functions.
> > We certainly do not want to add more functions to the objtool
> > whitelist, especially those that are private to KCSAN.
> >
> > We could fix it by either:
> >
> > 1. Adding __always_inline to every function used by the KCSAN runtime
> > outside user_access_save + also fix atomic64_try_cmpxchg
> > (atomic-instrumented.h).
> >
> > 2. Just not compile KCSAN with -Os, i.e. have the Makefile strip -Os
> > and replace it with -O2 for kcsan/core.c. #2 is the simpler option,
> > and would probably make KCSAN more effective even with -Os. Although
> > it might violate the assumption of whoever decided they want both
> > CC_OPTIMIZE_FOR_SIZE and KCSAN. It might also mean that future
> > compilers that have a new inlining algorithm will have the same
> > problem.
> >
> > What do people think is better?
>
> I haven't had a chance to look at this yet, and probably won't be able
> to do so until at least Monday...
>
> Adding PeterZ who's the objtool uaccess expert.

Thanks. I have a patch which does #1, since it's probably cleaner. I
will send it out soon.

If there is a better way to resolve this, any suggestions welcome.

Thanks,
-- Marco