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 <20220927222851.37550-1-ndesaulniers@google.com> <YzN6rH6wOiC8a8sN@shell.armlinux.org.uk>
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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Wed, 28 Sep 2022 09:39:46 -0700
Message-ID: <CAKwvOdkg5FccDAKMnBfX9uEw5YoEDpBvSYoBO4Y1dJT+hkGVVA@mail.gmail.com>
Subject: Re: [PATCH v2] ARM: kprobes: move __kretprobe_trampoline to out of
 line assembler
To:     "Russell King (Oracle)" <linux@armlinux.org.uk>,
        Logan Chien <loganchien@google.com>
Cc:     Masami Hiramatsu <mhiramat@kernel.org>,
        "Naveen N . Rao" <naveen.n.rao@linux.ibm.com>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@intel.com>,
        "David S . Miller" <davem@davemloft.net>,
        Arnd Bergmann <arnd@arndb.de>,
        Nathan Chancellor <nathan@kernel.org>,
        Tom Rix <trix@redhat.com>,
        sparkhuang <huangshaobo6@huawei.com>,
        Ard Biesheuvel <ardb@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Linus Walleij <linus.walleij@linaro.org>,
        Chen Zhongjin <chenzhongjin@huawei.com>,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux-arch@vger.kernel.org, llvm@lists.linux.dev,
        Naresh Kamboju <naresh.kamboju@linaro.org>,
        regressions@lists.linux.dev, lkft-triage@lists.linaro.org,
        Linux Kernel Functional Testing <lkft@linaro.org>
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On Tue, Sep 27, 2022 at 3:35 PM Russell King (Oracle)
<linux@armlinux.org.uk> wrote:
>
> On Tue, Sep 27, 2022 at 03:28:51PM -0700, Nick Desaulniers wrote:
> > commit 1069c1dd20a3 ("ARM: 9231/1: Recover kretprobes return address for
> > EABI stack unwinder")
> > tickled a bug in clang's integrated assembler where the .save and .pad
> > directives must have corresponding .fnstart directives. The integrated
> > assembler is unaware that the compiler will be generating the .fnstart
> > directive.
>
> Has it been confirmed that gcc does generate a .fnstart for naked
> functions?

From what I can tell, the presence of __attribute__((naked)) makes no
difference with regards to the emission of the .fnstart directive for
GCC.

One thing I did notice though: https://godbolt.org/z/Mv5GEobc8
GCC will emit .fnstart directives when -fasynchronous-unwind-tables is
specified for C (omitting the directive otherwise), or regardless of
-fasynchronous-unwind-tables/-fno-asynchronous-unwind-tables for C++.
Clang will unconditionally emit .fnstart directives regardless of language mode.

I don't see -fasynchronous-unwind-tables being specified under
arch/arm/. But there are many instances of
UNWIND(.fnstart)
in various .S files under arch/arm/.

https://sourceware.org/binutils/docs/as/ARM-Unwinding-Tutorial.html
https://sourceware.org/binutils/docs/as/ARM-Directives.html#arm_005ffnstart


>
> --
> RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
> FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!


--
Thanks,
~Nick Desaulniers