linux-kernel - Re: [PATCH 0/8] unwind, arm64: add sframe unwinder for kernel

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Message-ID: <mb61p7c5sdhv6.fsf@kernel.org>
Date: Fri, 14 Feb 2025 08:56:45 +0000
From: Puranjay Mohan <puranjay@...nel.org>
To: Song Liu <song@...nel.org>
Cc: Indu Bhagat <indu.bhagat@...cle.com>, Weinan Liu <wnliu@...gle.com>,
 Josh Poimboeuf <jpoimboe@...nel.org>, Steven Rostedt
 <rostedt@...dmis.org>, Peter Zijlstra <peterz@...radead.org>, Mark Rutland
 <mark.rutland@....com>, roman.gushchin@...ux.dev, Will Deacon
 <will@...nel.org>, Ian Rogers <irogers@...gle.com>,
 linux-toolchains@...r.kernel.org, linux-kernel@...r.kernel.org,
 live-patching@...r.kernel.org, joe.lawrence@...hat.com,
 linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 0/8] unwind, arm64: add sframe unwinder for kernel

Song Liu <song@...nel.org> writes:

> On Thu, Feb 13, 2025 at 2:22 PM Puranjay Mohan <puranjay@...nel.org> wrote:
>>
>> Song Liu <song@...nel.org> writes:
>>
>> > On Thu, Feb 13, 2025 at 12:38 AM Puranjay Mohan <puranjay@...nel.org> wrote:
>> > [...]
>> >>
>> >> P.S. - The livepatch doesn't have copy_process() but only copy_signal(),
>> >> yours had copy_process() somehow.
>> >
>> > In my build, copy_signal is inlined to copy_process, unless I add noinline.
>> > If I do add noinline, the issue will not reproduce.
>> >
>> > I tried more combinations. The issue doesn't reproduce if I either
>> > 1) add noinline to copy_signal, so we are not patching the whole
>> >    copy_process function;
>> > or
>> > 2) Switch compiler from gcc 14.2.1 to gcc 11.5.0.
>> >
>> > So it appears something in gcc 14.2.1 is causing live patch to fail
>> > for copy_process().
>>
>> So, can you test your RFC set (without SFRAME) with gcc 14.2.1, so we
>> can be sure that it is not a sframe problem?
>>
>> And about having the .sframe section in the livepatch module, I realised
>> that this set doesn't include support for reading/using sframe data from
>> any module(livepatches included), so the patch I added for generating
>> .sframe in kpatch is irrelevant because it is a no-op with the current setup.
>
> Puranjay,
>
> Could you please try the following?
>
> 1. Use gcc 11.4.1;
> 2. Add __always_inline to copy_signal();
> 3. Build kernel, and livepatch with the same test (we need to
>     add __always_inline to the .patch file).
> 4. Run gdb livepatch-xxx.ko
> 5. In gdb do disassemble copy_process.
>
> In my tests, both gcc-14.2.1 and gcc-11.5.0 generated a .ko file
> that looks weird in gdb-disassemble. Specifically, readels shows
> copy_process is about 5.5kB, but gdb-disassemble only shows
> 140 bytes or so for copy_process. clang doesn't seem to have
> this problem.
>
> I am really curious whether you have the same problem in your
> setup.

Hi Song,

I did this test and found the same issue as you (gdb assembly broken),
but I can see this issue even without the inlining. I think GDB tried to
load the debuginfo and that is somehow broken therefore it fails to
disassemblt properly.

But even with inlining, I couldn't see the warning about the refcount
with my setup.

Thanks,
Puranjay

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