Re: [x86] copy_from{to}_user question

From: Borislav Petkov
Date: Wed Aug 12 2015 - 06:07:41 EST

On Wed, Aug 12, 2015 at 05:01:14PM +0800, yalin wang wrote:
> hi x86 maintainers,
> i have a question about copy_from{to}_user() function,
> i find on other platforms like arm/ arm64 /hexagon,
> all copy_from{to}_user function only check source address for
> copy_from and only check to address for copy_to user function,
> never check both source and dest together,
> but on x86 platform, i see copy_from{to}_user use a generic function
> named copy_user_generic_unrolled() in arch/x86/lib/copy_user_64.S,

That one is the fallback and used only on machines which don't set
X86_FEATURE_REP_GOOD or X86_FEATURE_ERMS. Basically old P4 and K7 and
early K8s.

> it check source and dest address no matter it is copy_from user or
> copy_to_user , is it correct?
> for copy_from_user i think only need check source address is enough,

How else would we be able to use the same function in copy_to and
copy_from variants?


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