linux-kernel - Re: [patch 07/41] x86/fpu: Simplify PTRACE

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Date:   Sat, 12 Jun 2021 11:13:51 +0200
From:   Borislav Petkov <bp@...e.de>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Yu-cheng Yu <yu-cheng.yu@...el.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Kan Liang <kan.liang@...ux.intel.com>
Subject: Re: [patch 07/41] x86/fpu: Simplify PTRACE_GETREGS code

On Fri, Jun 11, 2021 at 06:15:30PM +0200, Thomas Gleixner wrote:
> From: Dave Hansen <dave.hansen@...ux.intel.com>
> 
> ptrace() has interfaces that let a ptracer inspect a ptracee's register state.
> This includes XSAVE state.  The ptrace() ABI includes a hardware-format XSAVE
> buffer for both the SETREGS and GETREGS interfaces.
> 
> In the old days, the kernel buffer and the ptrace() ABI buffer were the
> same boring non-compacted format.  But, since the advent of supervisor
> states and the compacted format, the kernel buffer has diverged from the
> format presented in the ABI.
> 
> This leads to two paths in the kernel:
> 1. Effectively a verbatim copy_to_user() which just copies the kernel buffer
>    out to userspace.  This is used when the kernel buffer is kept in the
>    non-compacted form which means that it shares a format with the ptrace
>    ABI.
> 2. A one-state-at-a-time path: copy_xstate_to_kernel().  This is theoretically
>    slower since it does a bunch of piecemeal copies.
> 
> Remove the verbatim copy case.  Speed probably does not matter in this path,
> and the vast majority of new hardware will use the one-state-at-a-time path
> anyway.  This ensures greater testing for the "slow" path.
> 
> This also makes enabling PKRU in this interface easier since a single path
> can be patched instead of two.
> 
> Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> ---
> V4: Picked up from Dave's PKRU series
> ---
>  arch/x86/kernel/fpu/regset.c |   22 ++--------------------
>  arch/x86/kernel/fpu/xstate.c |    6 +++---
>  2 files changed, 5 insertions(+), 23 deletions(-)

Reviewed-by: Borislav Petkov <bp@...e.de>

-- 
Regards/Gruss,
    Boris.

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