linux-kernel - Re: [x86, build] 6dafca9780: WARNING:at_arch/x86/kernel/ftrace.c:#ftrace_verify

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Date:   Mon, 1 Mar 2021 14:14:51 -0800
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     kernel test robot <oliver.sang@...el.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Kees Cook <keescook@...omium.org>,
        LKML <linux-kernel@...r.kernel.org>, lkp@...ts.01.org,
        kernel test robot <lkp@...el.com>
Subject: Re: [x86, build] 6dafca9780: WARNING:at_arch/x86/kernel/ftrace.c:#ftrace_verify_code

On Sun, Feb 28, 2021 at 11:25 PM kernel test robot
<oliver.sang@...el.com> wrote:
>
>
> Greeting,
>
> FYI, we noticed the following commit (built with clang-13):
>
> commit: 6dafca97803309c3cb5148d449bfa711e41ddef2 ("x86, build: use objtool mcount")
> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git master

Thanks for the report, I'm able to reproduce the warning.

> [    4.764496] ------------[ ftrace bug ]------------
> [    4.764847] ftrace failed to modify
> [    4.764852] do_sys_open (kbuild/src/consumer/fs/open.c:1186)
> [    4.765483]  actual:   0f:1f:44:00:00
> [    4.765784] Setting ftrace call site to call ftrace function
> [    4.766193] ftrace record flags: 50000001
> [    4.766490]  (1) R
> [    4.766490]  expected tramp: ffffffff81037af0
> [    4.766959] ------------[ cut here ]------------

Basically, the problem is that ftrace_replace_code() expects to find
ideal_nops[NOP_ATOMIC5] here, which in this case is 66:66:66:66:90,
while objtool has replaced the __fentry__ call with 0f:1f:44:00:00.

As ideal_nops changes depending on kernel config and hardware, when
CC_USING_NOP_MCOUNT is defined we could either change
ftrace_nop_replace() to always use P6_NOP5, or skip
ftrace_verify_code() in ftrace_replace_code() for
FTRACE_UPDATE_MAKE_CALL.

Steven, Peter, any thoughts?

Sami