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

From: Josh Poimboeuf <[email-masked]>
Date: Thu, 21 Nov 2019 12:37:11 -0600

On Wed, Nov 20, 2019 at 08:48:41PM +0100, Marco Elver wrote:
> On Wed, 20 Nov 2019 at 17:18, Randy Dunlap <[email-masked]> 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.

-- 
Josh