linux-kernel - Re: [PATCH] x86/boot/64: clang: use fixup_pointer() to access next_early

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Message-ID: <20170816221823.fzewgtfijqh4jfdn@node.shutemov.name>
Date:   Thu, 17 Aug 2017 01:18:23 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Alexander Potapenko <glider@...gle.com>
Cc:     tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
        x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/boot/64: clang: use fixup_pointer() to access
 next_early_pgt

On Wed, Aug 16, 2017 at 09:08:08PM +0200, Alexander Potapenko wrote:
> __startup_64() is normally using fixup_pointer() to access globals in a
> position-independent fashion. However |next_early_pgt| was accessed
> directly, which wasn't guaranteed to work.
> 
> Luckily GCC was generating a R_X86_64_PC32 PC-relative relocation for
> |next_early_pgt|, but Clang emitted a R_X86_64_32S, which led to
> accessing invalid memory and rebooting the kernel.

Thanks for tracking this down.

Acked-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>

> Cc: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> Cc: Dmitry Vyukov <dvyukov@...gle.com>
> Cc: Michael Davidson <md@...gle.com>
> Fixes: c88d71508e36 ("x86/boot/64: Rewrite startup_64() in C")

Cc: <stable@...r.kernel.org> # 4.12

-- 
 Kirill A. Shutemov