linux-kernel - Re: [PATCH] arm64: vdso: Include .eh

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Date:   Fri, 25 Nov 2022 12:27:14 +0000
From:   Szabolcs Nagy <szabolcs.nagy@....com>
To:     Vincent Whitchurch <vincent.whitchurch@...s.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, <kernel@...s.com>,
        Tamas Zsoldos <tamas.zsoldos@....com>,
        Daniel Kiss <daniel.kiss@....com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Ard Biesheuvel <ardb@...nel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] arm64: vdso: Include .eh_frame in debug ELF

The 11/25/2022 11:37, Vincent Whitchurch wrote:
> We currently strip out .eh_frame to work around crashes in libgcc when
> it tries to unwind out of signal handlers, see commit 87676cfca141
> ("arm64: vdso: Disable dwarf unwinding through the sigreturn
> trampoline").
> 
> The .eh_frame does however have correct unwind information for the
> functions implemented in C in vgettimeofday.c, but currently this
> information is not available even for offline unwinding using the
> vdso.so.dbg.  As a result of this, perf built with libdw is unable to
> unwind the stack when the PC is inside one of these functions.
> 
> To fix this, strip the .eh_frame section only from the vdso.so and not
> from the vdso.so.dbg.  This can be used by offline unwinders with access
> to the debug symbols, and will not affect libgcc since the section will
> still not be present in the normal vDSO.

adding eh_frame to vdso.so.dbg makes sense.

but if libdw correctly unwinds across a signal handler
then libgcc should be able to do so too.

so maybe eh_frame should be added back to vdso.so just
without frame info for __kernel_rt_sigreturn+NOP to
ensure unwinders use heuristics for sigreturn.
(i dont know if this was considered back when eh_frame
was dropped from the vdso)

> 
> Cc: Tamas Zsoldos <tamas.zsoldos@....com>
> Cc: Szabolcs Nagy <szabolcs.nagy@....com>
> Cc: Daniel Kiss <daniel.kiss@....com>
> Cc: Vincenzo Frascino <vincenzo.frascino@....com>
> Cc: Ard Biesheuvel <ardb@...nel.org>
> Signed-off-by: Vincent Whitchurch <vincent.whitchurch@...s.com>
> ---
>  arch/arm64/kernel/vdso/Makefile   | 2 +-
>  arch/arm64/kernel/vdso/vdso.lds.S | 3 ++-
>  2 files changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/kernel/vdso/Makefile b/arch/arm64/kernel/vdso/Makefile
> index 619e2dc7ee14..91aac17e11bc 100644
> --- a/arch/arm64/kernel/vdso/Makefile
> +++ b/arch/arm64/kernel/vdso/Makefile
> @@ -65,7 +65,7 @@ $(obj)/vdso.so.dbg: $(obj)/vdso.lds $(obj-vdso) FORCE
>  	$(call if_changed,vdsold_and_vdso_check)
>  
>  # Strip rule for the .so file
> -$(obj)/%.so: OBJCOPYFLAGS := -S
> +$(obj)/%.so: OBJCOPYFLAGS := -S --remove-section=.eh_frame --remove-section=.eh_frame_hdr
>  $(obj)/%.so: $(obj)/%.so.dbg FORCE
>  	$(call if_changed,objcopy)
>  
> diff --git a/arch/arm64/kernel/vdso/vdso.lds.S b/arch/arm64/kernel/vdso/vdso.lds.S
> index 6028f1fe2d1c..66abf70efc58 100644
> --- a/arch/arm64/kernel/vdso/vdso.lds.S
> +++ b/arch/arm64/kernel/vdso/vdso.lds.S
> @@ -31,6 +31,8 @@ SECTIONS
>  	.gnu.version	: { *(.gnu.version) }
>  	.gnu.version_d	: { *(.gnu.version_d) }
>  	.gnu.version_r	: { *(.gnu.version_r) }
> +	.eh_frame	: { *(.eh_frame) }
> +	.eh_frame_hdr	: { *(.eh_frame_hdr) }
>  
>  	/*
>  	 * Discard .note.gnu.property sections which are unused and have
> @@ -78,7 +80,6 @@ SECTIONS
>  	/DISCARD/	: {
>  		*(.data .data.* .gnu.linkonce.d.* .sdata*)
>  		*(.bss .sbss .dynbss .dynsbss)
> -		*(.eh_frame .eh_frame_hdr)
>  	}
>  }
>  
> -- 
> 2.34.1
>