linux-kernel - Re: [PATCH] On ppc64le we HAVE_RELIABLE

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Message-ID: <alpine.LSU.2.21.1712121308440.21907@pobox.suse.cz>
Date:   Tue, 12 Dec 2017 13:12:37 +0100 (CET)
From:   Miroslav Benes <mbenes@...e.cz>
To:     Torsten Duwe <duwe@....de>
cc:     Michael Ellerman <mpe@...erman.id.au>,
        Jiri Kosina <jkosina@...e.cz>,
        Balbir Singh <bsingharora@...il.com>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        live-patching@...r.kernel.org
Subject: Re: [PATCH] On ppc64le we HAVE_RELIABLE_STACKTRACE

On Tue, 12 Dec 2017, Torsten Duwe wrote:

> Hi all,
> 
> The "Power Architecture 64-Bit ELF V2 ABI" says in section 2.3.2.3:
> 
> [...] There are several rules that must be adhered to in order to ensure
> reliable and consistent call chain backtracing:
> 
> * Before a function calls any other function, it shall establish its
>   own stack frame, whose size shall be a multiple of 16 bytes.
> 
>  – In instances where a function’s prologue creates a stack frame, the
>    back-chain word of the stack frame shall be updated atomically with
>    the value of the stack pointer (r1) when a back chain is implemented.
>    (This must be supported as default by all ELF V2 ABI-compliant
>    environments.)
> [...]
>  – The function shall save the link register that contains its return
>    address in the LR save doubleword of its caller’s stack frame before
>    calling another function.
> 
> To me this sounds like the equivalent of HAVE_RELIABLE_STACKTRACE.
> This patch may be unneccessarily limited to ppc64le, but OTOH the only
> user of this flag so far is livepatching, which is only implemented on
> PPCs with 64-LE, a.k.a. ELF ABI v2.
> 
> Signed-off-by: Torsten Duwe <duwe@...e.de>
> 
> ---
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index c51e6ce42e7a..3e3a6ab2e089 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -218,6 +218,7 @@ config PPC
>  	select HAVE_PERF_USER_STACK_DUMP
>  	select HAVE_RCU_TABLE_FREE		if SMP
>  	select HAVE_REGS_AND_STACK_ACCESS_API
> +	select HAVE_RELIABLE_STACKTRACE		if PPC64 && CPU_LITTLE_ENDIAN
>  	select HAVE_SYSCALL_TRACEPOINTS
>  	select HAVE_VIRT_CPU_ACCOUNTING
>  	select HAVE_IRQ_TIME_ACCOUNTING

I think that this is not enough. You need to also implement 
save_stack_trace_tsk_reliable() for powerpc defined as __weak in 
kernel/stacktrace.c. See arch/x86/kernel/stacktrace.c for reference, but I 
think it would be much much simpler here given the changelog description.

Thanks,
Miroslav