linux-kernel - Re: [PATCH 1/3] powerpc/uaccess: Switch __put_user_size_allowed() to __put_user_asm

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Date:   Wed, 04 Nov 2020 00:21:44 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Andreas Schwab <schwab@...ux-m68k.org>
Cc:     Christophe Leroy <christophe.leroy@...roup.eu>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] powerpc/uaccess: Switch __put_user_size_allowed() to __put_user_asm_goto()

Andreas Schwab <schwab@...ux-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