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From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Sat, 4 Dec 2021 09:45:11 +0100
Message-ID: <CAMj1kXF3ZQ__trDo4PDfBmJsNoiqBkNB2CLK5pTFARTEYzDOzw@mail.gmail.com>
Subject: Re: [PATCH v3 8/9] ARM: kprobes: Make a frame pointer on __kretprobe_trampoline
To:     Arnd Bergmann <arnd@arndb.de>
Cc:     Masami Hiramatsu <mhiramat@kernel.org>,
        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 Fri, 3 Dec 2021 at 21:38, Arnd Bergmann <arnd@arndb.de> wrote:
>
> 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.

Does this help?

diff --git a/arch/arm/probes/kprobes/core.c b/arch/arm/probes/kprobes/core.c
index 9090c3a74dcc..88999ed2cfc4 100644
--- a/arch/arm/probes/kprobes/core.c
+++ b/arch/arm/probes/kprobes/core.c
@@ -408,6 +408,7 @@ void __naked __kprobes __kretprobe_trampoline(void)
 #else
                "mov    pc, lr                  \n\t"
 #endif
+               ".ltorg                         \n\t"
                : : : "memory");
 }