linux-kernel - Re: [PATCH v2] On ppc64le we HAVE_RELIABLE

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Date:   Thu, 8 Mar 2018 10:26:16 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Torsten Duwe <duwe@....de>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Jiri Kosina <jkosina@...e.cz>, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, Nicholas Piggin <npiggin@...il.com>,
        live-patching@...r.kernel.org
Subject: Re: [PATCH v2] On ppc64le we HAVE_RELIABLE_STACKTRACE

On Mon, Mar 05, 2018 at 05:49:28PM +0100, Torsten Duwe wrote:
> 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.
> 
> This change also implements save_stack_trace_tsk_reliable() for ppc64
> that checks for the above conditions, where possible.
> 
> Signed-off-by:   Torsten Duwe <duwe@...e.de>

This doesn't seem to address some of my previous concerns:

- Bailing on interrupt/exception frames

- Function graph tracing return address conversion

- kretprobes return address conversion

-- 
Josh