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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Fri, 3 Dec 2021 21:37:16 +0100
Message-ID: <CAK8P3a3jCGWjauD0+k5C-VpLDQ8WkcCHs5UpoAb-x_yHU0eEkw@mail.gmail.com>
Subject: Re: [PATCH v3 8/9] ARM: kprobes: Make a frame pointer on __kretprobe_trampoline
To:     Masami Hiramatsu <mhiramat@kernel.org>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        "Naveen N . Rao" <naveen.n.rao@linux.vnet.ibm.com>,
        Ananth N Mavinakayanahalli <ananth@linux.ibm.com>,
        Ingo Molnar <mingo@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Sven Schnelle <svens@linux.ibm.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Russell King <linux@armlinux.org.uk>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>
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On Thu, Oct 21, 2021 at 2:55 AM Masami Hiramatsu <mhiramat@kernel.org> wrote:
>
> Currently kretprobe on ARM just fills r0-r11 of pt_regs, but
> that is not enough for the stacktrace. Moreover, from the user
> kretprobe handler, stacktrace needs a frame pointer on the
> __kretprobe_trampoline.
>
> This adds a frame pointer on __kretprobe_trampoline for both gcc
> and clang case. Those have different frame pointer so we need
> different but similar stack on pt_regs.
>
> Gcc makes the frame pointer (fp) to point the 'pc' address of
> the {fp, ip (=sp), lr, pc}, this means {r11, r13, r14, r15}.
> Thus if we save the r11 (fp) on pt_regs->r12, we can make this
> set on the end of pt_regs.
>
> On the other hand, Clang makes the frame pointer to point the
> 'fp' address of {fp, lr} on stack. Since the next to the
> pt_regs->lr is pt_regs->sp, I reused the pair of pt_regs->fp
> and pt_regs->ip.
> So this stores the 'lr' on pt_regs->ip and make the fp to point
> pt_regs->fp.
>
> For both cases, saves __kretprobe_trampoline address to
> pt_regs->lr, so that the stack tracer can identify this frame
> pointer has been made by the __kretprobe_trampoline.
>
> Note that if the CONFIG_FRAME_POINTER is not set, this keeps
> fp as is.
>
> Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
> Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

This causes a regression that I see in randconfig builds with gcc-11:

/tmp/ccovvQNw.s: Assembler messages:
/tmp/ccovvQNw.s:32: Error: invalid literal constant: pool needs to be closer
make[5]: *** [/git/arm-soc/scripts/Makefile.build:288:
arch/arm/probes/kprobes/core.o] Error 1

I have two randconfigs that reproduce it locally, here is a .config
[1] and assembly
output[2] for reference. I have not done any further analysis, but
maybe you have
an idea.

        Arnd

[1] https://pastebin.com/y4rkH8qX
[2] https://pastebin.com/9mEVU8Rd