linux-kernel - Re: mmotm 2020-05-13-20-30 uploaded (objtool warnings)

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Date:   Fri, 29 May 2020 18:12:53 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Christoph Hellwig <hch@....de>,
        Randy Dunlap <rdunlap@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>, broonie@...nel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-next@...r.kernel.org, mhocko@...e.cz,
        mm-commits@...r.kernel.org, sfr@...b.auug.org.au,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        viro@...iv.linux.org.uk, x86@...nel.org,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: mmotm 2020-05-13-20-30 uploaded (objtool warnings)

On Fri, May 29, 2020 at 11:05:14AM -0500, Josh Poimboeuf wrote:

> It looks to me like GCC is doing the right thing.  That likely()
> translates to:
> 
> #  define likely(x)	(__branch_check__(x, 1, __builtin_constant_p(x)))
> 
> which becomes:
> 
> #define __branch_check__(x, expect, is_constant) ({			\
> 			long ______r;					\
> 			static struct ftrace_likely_data		\
> 				__aligned(4)				\
> 				__section(_ftrace_annotated_branch)	\
> 				______f = {				\
> 				.data.func = __func__,			\
> 				.data.file = __FILE__,			\
> 				.data.line = __LINE__,			\
> 			};						\
> 			______r = __builtin_expect(!!(x), expect);	\
> 			ftrace_likely_update(&______f, ______r,		\
> 					     expect, is_constant);	\
> 			______r;					\
> 		})
> 
> Here 'x' is the call to user_access_begin().  It evaluates 'x' -- and
> thus calls user_access_begin() -- before the call to
> ftrace_likely_update().
> 
> So it's working as designed, right?  The likely() just needs to be
> changed to likely_notrace().

But if !x (ie we fail user_access_begin()), we should not pass STAC() on
the way to out_err. OTOH if x, we should not be jumping to out_err.

I'm most confused... must not stare at asm for a while.