linux-kernel - Re: [PATCH v2 12/14] objtool: Add support for more complex UACCESS control

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Date:   Thu, 22 Jun 2023 13:49:24 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Sathvika Vasireddy <sv@...ux.ibm.com>,
        Naveen N Rao <naveen@...nel.org>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2 12/14] objtool: Add support for more complex UACCESS
 control

On Thu, Jun 22, 2023 at 12:54:34PM +0200, Christophe Leroy wrote:
> On x86, UACCESS is controlled by two instructions: STAC and CLAC.
> STAC instruction enables UACCESS while CLAC disables UACCESS.
> This is simple enough for objtool to locate UACCESS enable and
> disable.
> 
> But on powerpc it is a bit more complex, the same instruction is
> used for enabling and disabling UACCESS, and the same instruction
> can be used for many other things. It would be too complex to use
> exclusively instruction decoding.
> 
> To help objtool, mark such instruction into .discard.uaccess_begin
> and .discard.uaccess_end sections, on the same principle as for
> reachable/unreachable instructions. And add ASM_UACCESS_BEGIN
> and ASM_UACCESS_END macros to be used in inline assembly code to
> annotate UACCESS enable and UACCESS disable instructions.
> 
> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
> ---
>  include/linux/objtool.h | 14 +++++++++++++
>  tools/objtool/check.c   | 44 +++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 58 insertions(+)
> 
> diff --git a/include/linux/objtool.h b/include/linux/objtool.h
> index 03f82c2c2ebf..d8fde4158a40 100644
> --- a/include/linux/objtool.h
> +++ b/include/linux/objtool.h
> @@ -57,6 +57,18 @@
>  	".long 998b - .\n\t"						\
>  	".popsection\n\t"
>  
> +#define ASM_UACCESS_BEGIN						\
> +	"998:\n\t"							\
> +	".pushsection .discard.uaccess_begin\n\t"			\
> +	".long 998b - .\n\t"						\
> +	".popsection\n\t"
> +
> +#define ASM_UACCESS_END							\
> +	"998:\n\t"							\
> +	".pushsection .discard.uaccess_end\n\t"				\
> +	".long 998b - .\n\t"						\
> +	".popsection\n\t"
> +
>  #else /* __ASSEMBLY__ */

Yeah, this can work. Josh wanted a more generic hints infra, but I'm not
sure we should make you do that work now.