linux-kernel - Re: [PATCH v3] On ppc64le we HAVE_RELIABLE

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Message-ID: <20180507154208.jo4s43olrf5a3hw4@treble>
Date:   Mon, 7 May 2018 10:42:08 -0500
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 v3] On ppc64le we HAVE_RELIABLE_STACKTRACE

On Fri, May 04, 2018 at 02:38:34PM +0200, 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.
> 
> Feel free to add other ppc variants, but so far only ppc64le got tested.
> 
> This change also implements save_stack_trace_tsk_reliable() for ppc64le
> that checks for the above conditions, where possible.
> 
> Signed-off-by: Torsten Duwe <duwe@...e.de>
> Signed-off-by: Nicolai Stange <nstange@...e.de>

The subject doesn't actively describe what the patch does, maybe change
it to something like:

  powerpc: Add support for HAVE_RELIABLE_STACKTRACE

or maybe

  powerpc: Add support for livepatch consistency model

Otherwise it looks great to me.

Acked-by: Josh Poimboeuf <jpoimboe@...hat.com>

-- 
Josh