Re: [PATCH] On ppc64le we HAVE_RELIABLE_STACKTRACE

[Nicholas Piggin]

On Tue, 12 Dec 2017 08:05:01 -0600
Josh Poimboeuf <jpoimboe@xxxxxxxxxx> 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.
> 
> 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.

> 
> Also, what about hand-coded asm?

Should follow the same rules if it uses the stack.

> 
> > 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.

> 
> Furthermore, the "reliable" unwinder needs to have a way to report an
> error if it doesn't reach the end.  This probably just means ensuring
> that it reaches the user mode registers on the stack.
> 
> And as Miroslav mentioned, once that's all done, implement
> save_stack_trace_tsk_reliable().
> 
> I don't think the above is documented anywhere, it would be good to put
> it in the livepatch doc.
> 

Thanks,
Nick