linux-kernel - Re: [PATCH 04/24] x86,nospec: Annotate indirect calls/jumps

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Date:   Fri, 26 Jan 2018 10:19:47 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Dave Hansen <dave.hansen@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Jason Baron <jbaron@...mai.com>
Subject: Re: [PATCH 04/24] x86,nospec: Annotate indirect calls/jumps

On Tue, 2018-01-23 at 16:25 +0100, Peter Zijlstra wrote:
> Annotate the indirect calls/jumps in the CALL_NOSPEC/JUMP_NOSPEC
> alternatives.
> 
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>

Reviewed-by: David Woodhouse <dwmw@...zon.co.uk>

However...


>  /*
> + * This should be used immediately before an indirect jump/call. It tells
> + * objtool the subsequent indirect jump/call is vouched safe for retpoline
> + * builds.
> + */
> +.macro ANNOTATE_RETPOLINE_SAFE
> +	.Lannotate_\@:
> +	.pushsection .discard.retpoline_safe
> +	_ASM_PTR .Lannotate_\@
> +	.popsection
> +.endm

Didn't I just see one of those in patch 3? So this makes two...



> @@ -143,6 +155,12 @@
>  	".long 999b - .\n\t"					\
>  	".popsection\n\t"
>  
> +#define ANNOTATE_RETPOLINE_SAFE					\
> +	"999:\n\t"						\
> +	".pushsection .discard.retpoline_safe\n\t"		\
> +	_ASM_PTR " 999b\n\t"					\
> +	".popsection\n\t"
> +
>  #if defined(CONFIG_X86_64) && defined(RETPOLINE)

... three.

Now, I did briefly toy with the idea of using a .macro from both
__ASSEMBLY__ and inline asm, making the latter work by means of 
asm(".include \"asm/nospec-branch.h\");

In the end I just ended up with the __FILL_RETURN_BUFFER CPP macro
which is used from both by other tricks.

Can we look at doing something like that, please?


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