linux-kernel - Re: [PATCH v4 3/3] selftests: livepatch: test livepatching a kprobed function

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Message-ID: <alpine.LSU.2.21.2410021724040.23724@pobox.suse.cz>
Date: Wed, 2 Oct 2024 17:29:01 +0200 (CEST)
From: Miroslav Benes <mbenes@...e.cz>
To: Michael Vetter <mvetter@...e.com>
cc: linux-kselftest@...r.kernel.org, live-patching@...r.kernel.org, 
    linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 3/3] selftests: livepatch: test livepatching a kprobed
 function

Hi,

On Mon, 30 Sep 2024, Michael Vetter wrote:

> The test proves that a function that is being kprobed and uses a
> post_handler cannot be livepatched.
> 
> Only one ftrace_ops with FTRACE_OPS_FL_IPMODIFY set may be registered
> to any given function at a time.
> 
> Signed-off-by: Michael Vetter <mvetter@...e.com>

since my memory is short, I wondered why you need a separate module to 
register a kprobe for cmdline_proc_show() in vmlinux and why it cannot be 
achieved through tracefs. So... the test ensures that 
FTRACE_OPS_FL_IPMODIFY is exclusive. A kprobe sets FTRACE_OPS_FL_IPMODIFY 
flag if and only if post_handler is not NULL which also indicates a 
situation when regs->ip may be changed in that kprobe. This is something 
which cannot be done in tracefs and hence the module.

It was not clear to me from the changelog.

Miroslav