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Date:   Mon, 18 Dec 2017 12:56:22 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     Torsten Duwe <duwe@....de>, linux-kernel@...r.kernel.org,
        Jiri Kosina <jkosina@...e.cz>, live-patching@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] On ppc64le we HAVE_RELIABLE_STACKTRACE

On Mon, Dec 18, 2017 at 03:33:34PM +1000, Nicholas Piggin wrote:
> On Sun, 17 Dec 2017 20:58:54 -0600
> 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".
> > 
> > > > 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?
> 
> I don't believe so.

It's allowed on x86, I don't see why it wouldn't be allowed on powerpc.
As you mentioned, GCC doesn't pay attention to what's inside asm("").

> > > > Also, what about hand-coded asm?  
> > > 
> > > Should follow the same rules if it uses the stack.  
> > 
> > How is that enforced?
> 
> It's not, AFAIK. Gcc doesn't understand what's inside asm("").

Here I was talking about .S files.

> > > > > 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?
> > 
> 
> Then there will be no stack frame, but you still get the caller address
> because it's saved in LR register as part of the function call. Then
> you get the caller's caller in its stack frame.

Ok.  So what about the interrupted function itself?  Looking at the
powerpc version of save_context_stack(), it doesn't do anything special
for exception frames like checking regs->nip.

Though it looks like that should be possible since show_stack() has a
way to identify exception frames.

-- 
Josh

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