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Date:   Mon, 18 Dec 2017 14:39:06 +1100
From:   Balbir Singh <bsingharora@...il.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Nicholas Piggin <npiggin@...il.com>, Torsten Duwe <duwe@....de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Jiri Kosina <jkosina@...e.cz>, live-patching@...r.kernel.org,
        "open list:LINUX FOR POWERPC (32-BIT AND 64-BIT)" 
        <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH] On ppc64le we HAVE_RELIABLE_STACKTRACE

On Mon, Dec 18, 2017 at 1:58 PM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> On Fri, Dec 15, 2017 at 07:40:09PM +1000, Nicholas Piggin wrote:
>> On Tue, 12 Dec 2017 08:05:01 -0600
>> Josh Poimboeuf <jpoimboe@...hat.com> wrote:
>>
>> > On Tue, Dec 12, 2017 at 12:39:12PM +0100, 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.
>> >
>> > What about leaf functions?  If a leaf function doesn't establish a stack
>> > frame, and it has inline asm which contains a blr to another function,
>> > this ABI is broken.
>
> Oops, I meant to say "bl" instead of "blr".

I was wondering why "blr" mattered, but I guess we should speak of the
consistency
model. By walking a stack trace we expect to find whether a function is in use
or not and can/cannot be live-patched at this point in time. Right?

>
>> > Also, even for non-leaf functions, is it possible for GCC to insert the
>> > inline asm before it sets up the stack frame?  (This is an occasional
>> > problem on x86.)
>>
>> Inline asm must not have control transfer out of the statement unless
>> it is asm goto.
>
> Can inline asm have calls to other functions?
>
>> > Also, what about hand-coded asm?
>>
>> Should follow the same rules if it uses the stack.
>
> How is that enforced?
>
>> > > 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.
>> >
>> > In addition to fixing the above issues, the unwinder also needs to
>> > detect interrupts (i.e., preemption) and page faults on the stack of a
>> > blocked task.  If a function were preempted before it created a stack
>> > frame, or if a leaf function blocked on a page fault, the stack trace
>> > will skip the function's caller, so such a trace will need to be
>> > reported to livepatch as unreliable.
>>
>> I don't think there is much problem there for powerpc. Stack frame
>> creation and function call with return pointer are each atomic.
>
> What if the function is interrupted before it creates the stack frame?
>

If it is interrupted, the exception handler will establish a new stack frame.
>From a consistency viewpoint, I guess the question is -- has the function
been entered or considered to be entered when a stack frame has not
yet been established

Balbir Singh.

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