Re: [PATCH v2] ARM: kprobes: move __kretprobe_trampoline to out of line assembler

[Nick Desaulniers]

On Tue, Sep 27, 2022 at 3:35 PM Russell King (Oracle)
<linux@xxxxxxxxxxxxxxx> 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


>
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--
Thanks,
~Nick Desaulniers