On Mon, Dec 18, 2017 at 03:33:34PM +1000, Nicholas Piggin wrote:

On Sun, 17 Dec 2017 20:58:54 -0600
Josh Poimboeuf [off-list ref] wrote:

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On Fri, Dec 15, 2017 at 07:40:09PM +1000, Nicholas Piggin wrote:

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On Tue, 12 Dec 2017 08:05:01 -0600
Josh Poimboeuf [off-list ref] wrote:
  

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On Tue, Dec 12, 2017 at 12:39:12PM +0100, Torsten Duwe wrote:  

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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".

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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("").

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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.

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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