linux-kernel - Re: [PATCH 15/19] x86, mpx: do 32-bit-only cmpxchg for 32-bit apps

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Date:	Tue, 19 May 2015 10:18:51 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Dave Hansen <dave@...1.net>
cc:	linux-kernel@...r.kernel.org, x86@...nel.org,
	dave.hansen@...ux.intel.com
Subject: Re: [PATCH 15/19] x86, mpx: do 32-bit-only cmpxchg for 32-bit apps

On Mon, 18 May 2015, Dave Hansen wrote:
> From: Dave Hansen <dave.hansen@...ux.intel.com>
> 
> user_atomic_cmpxchg_inatomic() actually looks at sizeof(*ptr) to
> figure out how many bytes to copy.  If we run it on a 64-bit
> kernel with a 64-bit pointer, it will copy a 64-bit bounds
> directory entry.  That's fine, except when we have 32-bit
> programs with 32-bit bounds directory entries and we only *want*
> 32-bits.
> 
> This patch breaks the cmpxchg operation out in to its own
> function and performs the 32-bit type swizzling in there.
> 
> Note, the "64-bit" version of this code _would_ work on a
> 32-bit-only kernel.  The issue this patch addresses is only for
> when the kernel's 'long' is mismatched from the size of the
> bounds directory entry of the process we are working on.
> 
> The new helper modifies 'actual_old_val' or returns an error.
> But gcc doesn't know this, so it warns about 'actual_old_val'
> being unused.  Shut it up with an uninitialized_var().
> 
> Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>

Reviewed-by: Thomas Gleixner <tglx@...utronix.de>
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