linux-kernel - Re: [PATCH v4 2/5] x86/alternative: add indirect call patching

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Date:   Tue, 14 Nov 2023 16:06:27 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Juergen Gross <jgross@...e.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v4 2/5] x86/alternative: add indirect call patching

On Mon, Oct 30, 2023 at 03:25:05PM +0100, Juergen Gross wrote:
> +#ifdef CONFIG_X86_64
> +	/* ff 15 00 00 00 00   call   *0x0(%rip) */
> +	target = *(void **)(instr + a->instrlen + *(s32 *)(instr + 2));
> +#else
> +	/* ff 15 00 00 00 00   call   *0x0 */
> +	target = *(void **)(*(s32 *)(instr + 2));

Yeah, let's document those a bit better. Either with comments above or
as Peter suggests:

	/* Add 2 to skip opcode and ModRM byte: */
	disp32 = *(s32 *)(instr + 2);

	rip_rela_ptr = (void **)(instr + a->instrlen + disp32);
	target = *rip_rela_ptr;

so that it is crystal clear what we're doing here.

Thx.

-- 
Regards/Gruss,
    Boris.

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