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SubjectRe: [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@suse.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
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