Subject: Re: [PATCH v1] powerpc: Include running function as first entry in save_stack_trace() and friends

From: Segher Boessenkool <[email-masked]>
Date: Tue, 9 Mar 2021 16:05:32 -0600

Hi!

On Tue, Mar 09, 2021 at 04:05:23PM +0000, Mark Rutland wrote:
> On Thu, Mar 04, 2021 at 03:54:48PM -0600, Segher Boessenkool wrote:
> > On Thu, Mar 04, 2021 at 02:57:30PM +0000, Mark Rutland wrote:
> > > It looks like GCC is happy to give us the function-entry-time FP if we use
> > > __builtin_frame_address(1),
> > 
> > From the GCC manual:
> >      Calling this function with a nonzero argument can have
> >      unpredictable effects, including crashing the calling program.  As
> >      a result, calls that are considered unsafe are diagnosed when the
> >      '-Wframe-address' option is in effect.  Such calls should only be
> >      made in debugging situations.
> > 
> > It *does* warn (the warning is in -Wall btw), on both powerpc and
> > aarch64.  Furthermore, using this builtin causes lousy code (it forces
> > the use of a frame pointer, which we normally try very hard to optimise
> > away, for good reason).
> > 
> > And, that warning is not an idle warning.  Non-zero arguments to
> > __builtin_frame_address can crash the program.  It won't on simpler
> > functions, but there is no real definition of what a simpler function
> > *is*.  It is meant for debugging, not for production use (this is also
> > why no one has bothered to make it faster).
> >
> > On Power it should work, but on pretty much any other arch it won't.
> 
> I understand this is true generally, and cannot be relied upon in
> portable code. However as you hint here for Power, I believe that on
> arm64 __builtin_frame_address(1) shouldn't crash the program due to the
> way frame records work on arm64, but I'll go check with some local
> compiler folk. I agree that __builtin_frame_address(2) and beyond
> certainly can, e.g.  by NULL dereference and similar.

I still do not know the aarch64 ABI well enough.  If only I had time!

> For context, why do you think this would work on power specifically? I
> wonder if our rationale is similar.

On most 64-bit Power ABIs all stack frames are connected together as a
linked list (which is updated atomically, importantly).  This makes it
possible to always find all previous stack frames.

> Are you aware of anything in particular that breaks using
> __builtin_frame_address(1) in non-portable code, or is this just a
> general sentiment of this not being a supported use-case?

It is not supported, and trying to do it anyway can crash: it can use
random stack contents as pointer!  Not really "random" of course, but
where it thinks to find a pointer into the previous frame, which is not
something it can rely on (unless the ABI guarantees it somehow).

See gcc.gnu.org/PR60109 for example.

> > > Unless we can get some strong guarantees from compiler folk such that we
> > > can guarantee a specific function acts boundary for unwinding (and
> > > doesn't itself get split, etc), the only reliable way I can think to
> > > solve this requires an assembly trampoline. Whatever we do is liable to
> > > need some invasive rework.
> > 
> > You cannot get such a guarantee, other than not letting the compiler
> > see into the routine at all, like with assembler code (not inline asm,
> > real assembler code).
> 
> If we cannot reliably ensure this then I'm happy to go write an assembly
> trampoline to snapshot the state at a function call boundary (where our
> procedure call standard mandates the state of the LR, FP, and frame
> records pointed to by the FP).

Is the frame pointer required?!

> This'll require reworking a reasonable
> amount of code cross-architecture, so I'll need to get some more
> concrete justification (e.g. examples of things that can go wrong in
> practice).

Say you have a function that does dynamic stack allocation, then there
is usually no way to find the previous stack frame (without function-
specific knowledge).  So __builtin_frame_address cannot work (it knows
nothing about frames further up).

Dynamic stack allocation (alloca, or variable length automatic arrays)
is just the most common and most convenient example; it is not the only
case you have problems here.

> > The real way forward is to bite the bullet and to no longer pretend you
> > can do a full backtrace from just the stack contents.  You cannot.
> 
> I think what you mean here is that there's no reliable way to handle the
> current/leaf function, right? If so I do agree.

No, I meant what I said.

There is the separate issue that you do not know where the return
address (etc.) is stored in a function that has not yet done a call
itself, sure.  You cannot assume anything the ABI does not tell you you
can depend on.

> Beyond that I believe that arm64's frame records should be sufficient.

Do you have a simple linked list connecting all frames?  The aarch64 GCC
port does not define anything special here (DYNAMIC_CHAIN_ADDRESS), so
the default will be used: every frame pointer has to point to the
previous one, no exceptions whatsoever.

Segher