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Date:   Wed, 6 Mar 2019 15:40:51 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Julien Thierry <julien.thierry@....com>,
        Will Deacon <will.deacon@....com>,
        Andy Lutomirski <luto@...capital.net>,
        Ingo Molnar <mingo@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        James Morse <james.morse@....com>, valentin.schneider@....com,
        Brian Gerst <brgerst@...il.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>
Subject: Re: [PATCH 1/8] kasan,x86: Frob kasan_report() in an exception

On Wed, Mar 6, 2019 at 3:34 PM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Wed, Mar 06, 2019 at 03:12:37PM +0100, Peter Zijlstra wrote:
> > On Wed, Mar 06, 2019 at 03:01:33PM +0100, Dmitry Vyukov wrote:
> > > On Wed, Mar 6, 2019 at 2:57 PM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > > > I've not found callers of __asan_report_load* with AC=1 in the kernel
> > > > yet. Under what condtions does GCC emit calls to these functions?
> > >
> > > CONFIG_KASAN_INLINE=y
> > > Then compiler inlines fast path into generated code and only calls
> > > into runtime to report errors (also, faster, this should be a default
> > > for anything other than tiny ROM controllers).
> >
> > *sigh*, clearly I've not build enough kernels yet... Lemme go try that.
>
> mm/kasan/generic_report.o: warning: objtool: __asan_report_load1_noabort()+0x0: call to __fentry__() with UACCESS enabled
>
> You want to do:
>
> CFLAGS_REMOVE_f= -pg
>
> like generic.o has?

This should not matter for KASAN itself.
KASAN will call into function tracer, and function tracer will call
into KASAN, but unless function tracer is badly broken and causes a
KASAN report on every invocation, the recursion will end (function
tracer will get to the _report_ function). So we only disabled -pg for
fast paths.
But if it makes things simpler for the objtool, then I think we can
disable function tracer for generic_report.c too.

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