linux-kernel - Re: [PATCH v7 08/14] x86/ftrace: Use text_poke

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Date:   Thu, 6 Dec 2018 00:06:30 +0000
From:   Nadav Amit <namit@...are.com>
To:     Ingo Molnar <mingo@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>
CC:     LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Damian Tometzki <linux_dti@...oud.com>,
        linux-integrity <linux-integrity@...r.kernel.org>,
        LSM List <linux-security-module@...r.kernel.org>
Subject: Re: [PATCH v7 08/14] x86/ftrace: Use text_poke_*() infrastructure

> On Dec 4, 2018, at 5:34 PM, Nadav Amit <namit@...are.com> wrote:
> 
> A following patch is going to make module allocated memory
> non-executable. This requires to modify ftrace and make the memory
> executable again after it is configured.
> 
> In addition, this patch makes ftrace use the general text poking
> infrastructure instead ftrace's homegrown text patching. This provides
> the advantages of having slightly "safer" code patching and avoiding
> races with module removal or other mechanisms that patch the kernel
> code.
> 
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Signed-off-by: Nadav Amit <namit@...are.com>
> ---
> arch/x86/kernel/ftrace.c | 74 +++++++++++++---------------------------
> 1 file changed, 23 insertions(+), 51 deletions(-)

Steven Rostedt pointed that using text_poke() instead of
probe_kernel_write() would introduce considerable overheads. Running:

  # time { echo function > current_tracer; } 

takes 0.24s without this patch and 0.7s with. I don’t know whether to
consider it “so bad”. Obviously we can introduce a batching mechanism and/or
do some micro-optimization (the latter will not buy us much though).

Anyhow, in the meanwhile Steven asked that we’ll leave out the changes in
this patch-set, excluding the set_memory_x() that we need after calling
module_alloc(), and consider them later.