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From:   Michael Ellerman <mpe@ellerman.id.au>
To:     Andreas Schwab <schwab@linux-m68k.org>
Cc:     Christophe Leroy <christophe.leroy@csgroup.eu>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/3] powerpc/uaccess: Switch __put_user_size_allowed() to __put_user_asm_goto()
In-Reply-To: <871rhgwzcg.fsf@igel.home>
References: <94ba5a5138f99522e1562dbcdb38d31aa790dc89.1599216721.git.christophe.leroy__44535.5968013004$1599217383$gmane$org@csgroup.eu> <87mu079ron.fsf@igel.home> <87imav9r64.fsf@igel.home> <87pn53vsep.fsf@mpe.ellerman.id.au> <871rhgwzcg.fsf@igel.home>
Date:   Wed, 04 Nov 2020 00:21:44 +1100
Message-ID: <875z6mvamv.fsf@mpe.ellerman.id.au>
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Andreas Schwab <schwab@linux-m68k.org> writes:
> #
> # Automatically generated file; DO NOT EDIT.
> # Linux/powerpc 5.10.0-rc1 Kernel Configuration
> #
> CONFIG_CC_VERSION_TEXT="gcc-4.9 (SUSE Linux) 4.9.3"

So it seems to be a combination of GCC 4.9 and ...

> # CONFIG_PPC_RADIX_MMU is not set

That ^, which specifically causes PPC_KUAP=n.

When PPC_KUAP=y allow_user_access() inlines an isync and mtspr, both of
which contain a memory clobber, and that seems to hide the bug.

I think for now we just have to stop using asm goto for put_user() on
GCC 4.9.

I'll send a patch for that.

cheers