linux-kernel - Re: [PATCH 26/34] mm: implement new mprotect

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Date:	Wed, 9 Dec 2015 07:48:11 -0800
From:	Dave Hansen <dave@...1.net>
To:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
	linux-kernel@...r.kernel.org
Cc:	linux-mm@...ck.org, x86@...nel.org, dave.hansen@...ux.intel.com,
	linux-api@...r.kernel.org
Subject: Re: [PATCH 26/34] mm: implement new mprotect_key() system call

Hi Michael,

Thanks for all the comments!  I'll fix most of it when I post a new
version of the manpage, but I have a few general questions.

On 12/09/2015 03:08 AM, Michael Kerrisk (man-pages) wrote:
>>
>> +is the protection or storage key to assign to the memory.
> 
> Why "protection or storage key" here? This phrasing seems a
> little ambiguous to me, given that we also have a 'prot'
> argument.  I think it would be clearer just to say 
> "protection key". But maybe I'm missing something.

x86 calls it a "protection key" while powerpc calls it a "storage key".
 They're called "protection keys" consistently inside the kernel.

Should we just stick to one name in the manpages?

> * A general overview of why this functionality is useful.

Any preference on a central spot to do the general overview?  Does it go
in one of the manpages I'm already modifying, or a new one?

> * A note on which architectures support/will support
>   this functionality.

x86 only for now.  We might get powerpc support down the road somewhere.

> * Explanation of what a protection domain is.

A protection domain is a unique view of memory and is represented by the
value in the PKRU register.

> * Explanation of how a process (thread?) changes its
>   protection domain.

Changing protection domains is done by pkey_set() system call, or by
using the WRPKRU instruction.  The system call is preferred and less
error-prone since it enforces that a protection is allocated before its
access protection can be modified.

> * Explanation of the relationship between page permission
>   bits (PROT_READ/PROT_WRITE/PROTE_EXEC) and 
>   PKEY_DISABLE_ACCESS and PKEY_DISABLE_WRITE.
>   It's still not clear to me. Do the PKEY_* bits
>   override the PROT_* bits. Or, something else?

Protection keys add access restrictions in addition to existing page
permissions.  They can only take away access; they never grant
additional access.

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