linux-kernel - Re: [PATCH v1 06/19] perf/x86/intel/ds: Check insn_get

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Message-ID: <20210104131919.GN3021@hirez.programming.kicks-ass.net>
Date:   Mon, 4 Jan 2021 14:19:19 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 06/19] perf/x86/intel/ds: Check insn_get_length()
 retval

On Wed, Dec 23, 2020 at 06:42:20PM +0100, Borislav Petkov wrote:
> From: Borislav Petkov <bp@...e.de>
> 
> intel_pmu_pebs_fixup_ip() needs only the insn length so use the
> appropriate helper instead of a full decode. A full decode differs only
> in running insn_complete() on the decoded insn but that is not needed
> here.
> 
> Signed-off-by: Borislav Petkov <bp@...e.de>
> ---
>  arch/x86/events/intel/ds.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/events/intel/ds.c b/arch/x86/events/intel/ds.c
> index 67dbc91bccfe..3786b4e07078 100644
> --- a/arch/x86/events/intel/ds.c
> +++ b/arch/x86/events/intel/ds.c
> @@ -1265,14 +1265,14 @@ static int intel_pmu_pebs_fixup_ip(struct pt_regs *regs)
>  		is_64bit = kernel_ip(to) || any_64bit_mode(regs);
>  #endif
>  		insn_init(&insn, kaddr, size, is_64bit);
> -		insn_get_length(&insn);
> +
>  		/*
>  		 * Make sure there was not a problem decoding the
>  		 * instruction and getting the length.  This is
>  		 * doubly important because we have an infinite
>  		 * loop if insn.length=0.
>  		 */
> -		if (!insn.length)
> +		if (insn_get_length(&insn) || !insn.length)

Do we really still need the !insn.length? That is, it *should* be
impossible to not fail insn_get_length() and still have a 0 length,
seeing how x86 doesn't have 0 length instructions.