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

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

[Marco Elver]

On Thu, Mar 04, 2021 at 04:59PM +0000, Mark Rutland wrote:
> On Thu, Mar 04, 2021 at 04:30:34PM +0100, Marco Elver wrote:
> > On Thu, 4 Mar 2021 at 15:57, Mark Rutland <mark.rutland@arm.com> wrote:
> > > [adding Mark Brown]
> > >
> > > The bigger problem here is that skipping is dodgy to begin with, and
> > > this is still liable to break in some cases. One big concern is that
> > > (especially with LTO) we cannot guarantee the compiler will not inline
> > > or outline functions, causing the skipp value to be too large or too
> > > small. That's liable to happen to callers, and in theory (though
> > > unlikely in practice), portions of arch_stack_walk() or
> > > stack_trace_save() could get outlined too.
> > >
> > > 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.
> > 
> > Will LTO and friends respect 'noinline'?
> 
> I hope so (and suspect we'd have more problems otherwise), but I don't
> know whether they actually so.
> 
> I suspect even with 'noinline' the compiler is permitted to outline
> portions of a function if it wanted to (and IIUC it could still make
> specialized copies in the absence of 'noclone').
> 
> > One thing I also noticed is that tail calls would also cause the stack
> > trace to appear somewhat incomplete (for some of my tests I've
> > disabled tail call optimizations).
> 
> I assume you mean for a chain A->B->C where B tail-calls C, you get a
> trace A->C? ... or is A going missing too?

Correct, it's just the A->C outcome.

> > Is there a way to also mark a function non-tail-callable?
> 
> I think this can be bodged using __attribute__((optimize("$OPTIONS")))
> on a caller to inhibit TCO (though IIRC GCC doesn't reliably support
> function-local optimization options), but I don't expect there's any way
> to mark a callee as not being tail-callable.

I don't think this is reliable. It'd be
__attribute__((optimize("-fno-optimize-sibling-calls"))), but doesn't
work if applied to the function we do not want to tail-call-optimize,
but would have to be applied to the function that does the tail-calling.
So it's a bit backwards, even if it worked.

> Accoding to the GCC documentation, GCC won't TCO noreturn functions, but
> obviously that's not something we can use generally.
> 
> https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#Common-Function-Attributes

Perhaps we can ask the toolchain folks to help add such an attribute. Or
maybe the feature already exists somewhere, but hidden.

+Cc linux-toolchains@vger.kernel.org

> > But I'm also not sure if with all that we'd be guaranteed the code we
> > want, even though in practice it might.
> 
> True! I'd just like to be on the least dodgy ground we can be.

It's been dodgy for a while, and I'd welcome any low-cost fixes to make
it less dodgy in the short-term at least. :-)

Thanks,
-- Marco

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

[Nick Desaulniers]

On Thu, Mar 4, 2021 at 9:42 AM Marco Elver <elver@google.com> wrote:
>
> On Thu, Mar 04, 2021 at 04:59PM +0000, Mark Rutland wrote:
> > On Thu, Mar 04, 2021 at 04:30:34PM +0100, Marco Elver wrote:
> > > On Thu, 4 Mar 2021 at 15:57, Mark Rutland <mark.rutland@arm.com> wrote:
> > > > [adding Mark Brown]
> > > >
> > > > The bigger problem here is that skipping is dodgy to begin with, and
> > > > this is still liable to break in some cases. One big concern is that
> > > > (especially with LTO) we cannot guarantee the compiler will not inline
> > > > or outline functions, causing the skipp value to be too large or too
> > > > small. That's liable to happen to callers, and in theory (though
> > > > unlikely in practice), portions of arch_stack_walk() or
> > > > stack_trace_save() could get outlined too.
> > > >
> > > > 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.
> > >
> > > Will LTO and friends respect 'noinline'?
> >
> > I hope so (and suspect we'd have more problems otherwise), but I don't
> > know whether they actually so.
> >
> > I suspect even with 'noinline' the compiler is permitted to outline
> > portions of a function if it wanted to (and IIUC it could still make
> > specialized copies in the absence of 'noclone').
> >
> > > One thing I also noticed is that tail calls would also cause the stack
> > > trace to appear somewhat incomplete (for some of my tests I've
> > > disabled tail call optimizations).
> >
> > I assume you mean for a chain A->B->C where B tail-calls C, you get a
> > trace A->C? ... or is A going missing too?
>
> Correct, it's just the A->C outcome.
>
> > > Is there a way to also mark a function non-tail-callable?
> >
> > I think this can be bodged using __attribute__((optimize("$OPTIONS")))
> > on a caller to inhibit TCO (though IIRC GCC doesn't reliably support
> > function-local optimization options), but I don't expect there's any way
> > to mark a callee as not being tail-callable.
>
> I don't think this is reliable. It'd be
> __attribute__((optimize("-fno-optimize-sibling-calls"))), but doesn't
> work if applied to the function we do not want to tail-call-optimize,
> but would have to be applied to the function that does the tail-calling.
> So it's a bit backwards, even if it worked.
>
> > Accoding to the GCC documentation, GCC won't TCO noreturn functions, but
> > obviously that's not something we can use generally.
> >
> > https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#Common-Function-Attributes

include/linux/compiler.h:246:
prevent_tail_call_optimization

commit a9a3ed1eff36 ("x86: Fix early boot crash on gcc-10, third try")

>
> Perhaps we can ask the toolchain folks to help add such an attribute. Or
> maybe the feature already exists somewhere, but hidden.
>
> +Cc linux-toolchains@vger.kernel.org
>
> > > But I'm also not sure if with all that we'd be guaranteed the code we
> > > want, even though in practice it might.
> >
> > True! I'd just like to be on the least dodgy ground we can be.
>
> It's been dodgy for a while, and I'd welcome any low-cost fixes to make
> it less dodgy in the short-term at least. :-)
>
> Thanks,
> -- Marco



-- 
Thanks,
~Nick Desaulniers

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

[Segher Boessenkool]

On Thu, Mar 04, 2021 at 09:54:44AM -0800, Nick Desaulniers wrote:
> On Thu, Mar 4, 2021 at 9:42 AM Marco Elver <elver@google.com> wrote:
> include/linux/compiler.h:246:
> prevent_tail_call_optimization
> 
> commit a9a3ed1eff36 ("x86: Fix early boot crash on gcc-10, third try")

That is much heavier than needed (an mb()).  You can just put an empty
inline asm after a call before a return, and that call cannot be
optimised to a sibling call: (the end of a function is an implicit
return:)

Instead of:

void g(void);
void f(int x)
	if (x)
		g();
}

Do:

void g(void);
void f(int x)
	if (x)
		g();
	asm("");
}

This costs no extra instructions, and certainly not something as heavy
as an mb()!  It works without the "if" as well, of course, but with it
it is a more interesting example of a tail call.


Segher

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

[Christophe Leroy]

Le 04/03/2021 à 20:24, Segher Boessenkool a écrit :
> On Thu, Mar 04, 2021 at 09:54:44AM -0800, Nick Desaulniers wrote:
>> On Thu, Mar 4, 2021 at 9:42 AM Marco Elver <elver@google.com> wrote:
>> include/linux/compiler.h:246:
>> prevent_tail_call_optimization
>>
>> commit a9a3ed1eff36 ("x86: Fix early boot crash on gcc-10, third try")

https://github.com/linuxppc/linux/commit/a9a3ed1eff36

> 
> That is much heavier than needed (an mb()).  You can just put an empty
> inline asm after a call before a return, and that call cannot be
> optimised to a sibling call: (the end of a function is an implicit
> return:)
> 
> Instead of:
> 
> void g(void);
> void f(int x)
> 	if (x)
> 		g();
> }
> 
> Do:
> 
> void g(void);
> void f(int x)
> 	if (x)
> 		g();
> 	asm("");
> }
> 
> This costs no extra instructions, and certainly not something as heavy
> as an mb()!  It works without the "if" as well, of course, but with it
> it is a more interesting example of a tail call.

In the commit mentionned at the top, it is said:

The next attempt to prevent compilers from tail-call optimizing
the last function call cpu_startup_entry(), ... , was to add an empty asm("").

This current solution was short and sweet, and reportedly, is supported
by both compilers but we didn't get very far this time: future (LTO?)
optimization passes could potentially eliminate this, which leads us
to the third attempt: having an actual memory barrier there which the
compiler cannot ignore or move around etc.

Christophe

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

[Segher Boessenkool]

On Fri, Mar 05, 2021 at 07:38:25AM +0100, Christophe Leroy wrote:
> Le 04/03/2021 à 20:24, Segher Boessenkool a écrit :
> https://github.com/linuxppc/linux/commit/a9a3ed1eff36
> 
> >
> >That is much heavier than needed (an mb()).  You can just put an empty
> >inline asm after a call before a return, and that call cannot be
> >optimised to a sibling call: (the end of a function is an implicit
> >return:)

> In the commit mentionned at the top, it is said:
> 
> The next attempt to prevent compilers from tail-call optimizing
> the last function call cpu_startup_entry(), ... , was to add an empty 
> asm("").
> 
> This current solution was short and sweet, and reportedly, is supported
> by both compilers but we didn't get very far this time: future (LTO?)
> optimization passes could potentially eliminate this,

This is simply not true.  A volatile inline asm (like this is, all
asm without outputs are) is always run on the reel machine exactly like
on the abstract machine.  LTO can not eliminate it, not more than any
other optimisation can.  The compiler makes no assumption about the
constents of the template of an asm, empty or not.

If you are really scared the compiler violates the rules of GCC inline
asm and thinks it knows what "" means, you can write
  asm(";#");
(that is a comment on all supported archs).

> which leads us
> to the third attempt: having an actual memory barrier there which the
> compiler cannot ignore or move around etc.

Why would it not be allowed to delete this, and delete some other asm?

And the compiler *can* move around asm like this.  But the point is,
it has to stay in order with other side effects, so there cannot be a
sibling call here, the call has to remain: any call contains a sequence
point, so side effects cannot be reordered over it, so the call (being
before the asm) cannot be transformed to a tail call.


Segher

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

[Mark Rutland]

On Thu, Mar 04, 2021 at 06:25:33PM +0100, Marco Elver wrote:
> On Thu, Mar 04, 2021 at 04:59PM +0000, Mark Rutland wrote:
> > On Thu, Mar 04, 2021 at 04:30:34PM +0100, Marco Elver wrote:
> > > On Thu, 4 Mar 2021 at 15:57, Mark Rutland <mark.rutland@arm.com> wrote:
> > > > [adding Mark Brown]
> > > >
> > > > The bigger problem here is that skipping is dodgy to begin with, and
> > > > this is still liable to break in some cases. One big concern is that
> > > > (especially with LTO) we cannot guarantee the compiler will not inline
> > > > or outline functions, causing the skipp value to be too large or too
> > > > small. That's liable to happen to callers, and in theory (though
> > > > unlikely in practice), portions of arch_stack_walk() or
> > > > stack_trace_save() could get outlined too.
> > > >
> > > > 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.
> > > 
> > > Will LTO and friends respect 'noinline'?
> > 
> > I hope so (and suspect we'd have more problems otherwise), but I don't
> > know whether they actually so.
> > 
> > I suspect even with 'noinline' the compiler is permitted to outline
> > portions of a function if it wanted to (and IIUC it could still make
> > specialized copies in the absence of 'noclone').
> > 
> > > One thing I also noticed is that tail calls would also cause the stack
> > > trace to appear somewhat incomplete (for some of my tests I've
> > > disabled tail call optimizations).
> > 
> > I assume you mean for a chain A->B->C where B tail-calls C, you get a
> > trace A->C? ... or is A going missing too?
> 
> Correct, it's just the A->C outcome.

I'd assumed that those cases were benign, e.g. for livepatching what
matters is what can be returned to, so B disappearing from the trace
isn't a problem there.

Is the concern debugability, or is there a functional issue you have in
mind?

> > > Is there a way to also mark a function non-tail-callable?
> > 
> > I think this can be bodged using __attribute__((optimize("$OPTIONS")))
> > on a caller to inhibit TCO (though IIRC GCC doesn't reliably support
> > function-local optimization options), but I don't expect there's any way
> > to mark a callee as not being tail-callable.
> 
> I don't think this is reliable. It'd be
> __attribute__((optimize("-fno-optimize-sibling-calls"))), but doesn't
> work if applied to the function we do not want to tail-call-optimize,
> but would have to be applied to the function that does the tail-calling.

Yup; that's what I meant then I said you could do that on the caller but
not the callee.

I don't follow why you'd want to put this on the callee, though, so I
think I'm missing something. Considering a set of functions in different
compilation units:

  A->B->C->D->E->F->G->H->I->J->K

... if K were marked in this way, and J was compiled with visibility of
this, J would stick around, but J's callers might not, and so the a
trace might see:

  A->J->K

... do you just care about the final caller, i.e. you just need
certainty that J will be in the trace?

If so, we can somewhat bodge that by having K have an __always_inline
wrapper which has a barrier() or similar after the real call to K, so
the call couldn't be TCO'd.

Otherwise I'd expect we'd probably need to disable TCO generally.

> So it's a bit backwards, even if it worked.
> 
> > Accoding to the GCC documentation, GCC won't TCO noreturn functions, but
> > obviously that's not something we can use generally.
> > 
> > https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#Common-Function-Attributes
> 
> Perhaps we can ask the toolchain folks to help add such an attribute. Or
> maybe the feature already exists somewhere, but hidden.
> 
> +Cc linux-toolchains@vger.kernel.org
> 
> > > But I'm also not sure if with all that we'd be guaranteed the code we
> > > want, even though in practice it might.
> > 
> > True! I'd just like to be on the least dodgy ground we can be.
> 
> It's been dodgy for a while, and I'd welcome any low-cost fixes to make
> it less dodgy in the short-term at least. :-)

:)

Thanks,
Mark.

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

[Marco Elver]

On Thu, 4 Mar 2021 at 19:02, Mark Rutland <mark.rutland@arm.com> wrote:
> On Thu, Mar 04, 2021 at 06:25:33PM +0100, Marco Elver wrote:
> > On Thu, Mar 04, 2021 at 04:59PM +0000, Mark Rutland wrote:
> > > On Thu, Mar 04, 2021 at 04:30:34PM +0100, Marco Elver wrote:
> > > > On Thu, 4 Mar 2021 at 15:57, Mark Rutland <mark.rutland@arm.com> wrote:
> > > > > [adding Mark Brown]
> > > > >
> > > > > The bigger problem here is that skipping is dodgy to begin with, and
> > > > > this is still liable to break in some cases. One big concern is that
> > > > > (especially with LTO) we cannot guarantee the compiler will not inline
> > > > > or outline functions, causing the skipp value to be too large or too
> > > > > small. That's liable to happen to callers, and in theory (though
> > > > > unlikely in practice), portions of arch_stack_walk() or
> > > > > stack_trace_save() could get outlined too.
> > > > >
> > > > > 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.
> > > >
> > > > Will LTO and friends respect 'noinline'?
> > >
> > > I hope so (and suspect we'd have more problems otherwise), but I don't
> > > know whether they actually so.
> > >
> > > I suspect even with 'noinline' the compiler is permitted to outline
> > > portions of a function if it wanted to (and IIUC it could still make
> > > specialized copies in the absence of 'noclone').
> > >
> > > > One thing I also noticed is that tail calls would also cause the stack
> > > > trace to appear somewhat incomplete (for some of my tests I've
> > > > disabled tail call optimizations).
> > >
> > > I assume you mean for a chain A->B->C where B tail-calls C, you get a
> > > trace A->C? ... or is A going missing too?
> >
> > Correct, it's just the A->C outcome.
>
> I'd assumed that those cases were benign, e.g. for livepatching what
> matters is what can be returned to, so B disappearing from the trace
> isn't a problem there.
>
> Is the concern debugability, or is there a functional issue you have in
> mind?

For me, it's just been debuggability, and reliable test cases.

> > > > Is there a way to also mark a function non-tail-callable?
> > >
> > > I think this can be bodged using __attribute__((optimize("$OPTIONS")))
> > > on a caller to inhibit TCO (though IIRC GCC doesn't reliably support
> > > function-local optimization options), but I don't expect there's any way
> > > to mark a callee as not being tail-callable.
> >
> > I don't think this is reliable. It'd be
> > __attribute__((optimize("-fno-optimize-sibling-calls"))), but doesn't
> > work if applied to the function we do not want to tail-call-optimize,
> > but would have to be applied to the function that does the tail-calling.
>
> Yup; that's what I meant then I said you could do that on the caller but
> not the callee.
>
> I don't follow why you'd want to put this on the callee, though, so I
> think I'm missing something. Considering a set of functions in different
> compilation units:
>
>   A->B->C->D->E->F->G->H->I->J->K

I was having this problem with KCSAN, where the compiler would
tail-call-optimize __tsan_X instrumentation. This would mean that
KCSAN runtime functions ended up in the trace, but the function where
the access happened would not. However, I don't care about the runtime
functions, and instead want to see the function where the access
happened. In that case, I'd like to just mark __tsan_X and any other
kcsan instrumentation functions as do-not-tail-call-optimize, which
would solve the problem.

The solution today is that when you compile a kernel with KCSAN, every
instrumented TU is compiled with -fno-optimize-sibling-calls. The
better solution would be to just mark KCSAN runtime functions somehow,
but permit tail calling other things. Although, I probably still want
to see the full trace, and would decide that having
-fno-optimize-sibling-calls is a small price to pay in a
debug-only-kernel to get complete traces.

> ... if K were marked in this way, and J was compiled with visibility of
> this, J would stick around, but J's callers might not, and so the a
> trace might see:
>
>   A->J->K
>
> ... do you just care about the final caller, i.e. you just need
> certainty that J will be in the trace?

Yes. But maybe it's a special problem that only sanitizers have.

> If so, we can somewhat bodge that by having K have an __always_inline
> wrapper which has a barrier() or similar after the real call to K, so
> the call couldn't be TCO'd.
>
> Otherwise I'd expect we'd probably need to disable TCO generally.

Thanks,
-- Marco

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

[Mark Rutland]

On Thu, Mar 04, 2021 at 07:22:53PM +0100, Marco Elver wrote:
> On Thu, 4 Mar 2021 at 19:02, Mark Rutland <mark.rutland@arm.com> wrote:
> > On Thu, Mar 04, 2021 at 06:25:33PM +0100, Marco Elver wrote:
> > > On Thu, Mar 04, 2021 at 04:59PM +0000, Mark Rutland wrote:
> > > > On Thu, Mar 04, 2021 at 04:30:34PM +0100, Marco Elver wrote:
> > > > > On Thu, 4 Mar 2021 at 15:57, Mark Rutland <mark.rutland@arm.com> wrote:
> > > > > > [adding Mark Brown]
> > > > > >
> > > > > > The bigger problem here is that skipping is dodgy to begin with, and
> > > > > > this is still liable to break in some cases. One big concern is that
> > > > > > (especially with LTO) we cannot guarantee the compiler will not inline
> > > > > > or outline functions, causing the skipp value to be too large or too
> > > > > > small. That's liable to happen to callers, and in theory (though
> > > > > > unlikely in practice), portions of arch_stack_walk() or
> > > > > > stack_trace_save() could get outlined too.
> > > > > >
> > > > > > 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.
> > > > >
> > > > > Will LTO and friends respect 'noinline'?
> > > >
> > > > I hope so (and suspect we'd have more problems otherwise), but I don't
> > > > know whether they actually so.
> > > >
> > > > I suspect even with 'noinline' the compiler is permitted to outline
> > > > portions of a function if it wanted to (and IIUC it could still make
> > > > specialized copies in the absence of 'noclone').
> > > >
> > > > > One thing I also noticed is that tail calls would also cause the stack
> > > > > trace to appear somewhat incomplete (for some of my tests I've
> > > > > disabled tail call optimizations).
> > > >
> > > > I assume you mean for a chain A->B->C where B tail-calls C, you get a
> > > > trace A->C? ... or is A going missing too?
> > >
> > > Correct, it's just the A->C outcome.
> >
> > I'd assumed that those cases were benign, e.g. for livepatching what
> > matters is what can be returned to, so B disappearing from the trace
> > isn't a problem there.
> >
> > Is the concern debugability, or is there a functional issue you have in
> > mind?
> 
> For me, it's just been debuggability, and reliable test cases.
> 
> > > > > Is there a way to also mark a function non-tail-callable?
> > > >
> > > > I think this can be bodged using __attribute__((optimize("$OPTIONS")))
> > > > on a caller to inhibit TCO (though IIRC GCC doesn't reliably support
> > > > function-local optimization options), but I don't expect there's any way
> > > > to mark a callee as not being tail-callable.
> > >
> > > I don't think this is reliable. It'd be
> > > __attribute__((optimize("-fno-optimize-sibling-calls"))), but doesn't
> > > work if applied to the function we do not want to tail-call-optimize,
> > > but would have to be applied to the function that does the tail-calling.
> >
> > Yup; that's what I meant then I said you could do that on the caller but
> > not the callee.
> >
> > I don't follow why you'd want to put this on the callee, though, so I
> > think I'm missing something. Considering a set of functions in different
> > compilation units:
> >
> >   A->B->C->D->E->F->G->H->I->J->K
> 
> I was having this problem with KCSAN, where the compiler would
> tail-call-optimize __tsan_X instrumentation.

Those are compiler-generated calls, right? When those are generated the
compilation unit (and whatever it has included) might not have provided
a prototype anyway, and the compiler has special knowledge of the
functions, so it feels like the compiler would need to inhibit TCO here
for this to be robust. For their intended usage subjecting them to TCO
doesn't seem to make sense AFAICT.

I suspect that compilers have some way of handling that; otherwise I'd
expect to have heard stories of mcount/fentry calls getting TCO'd and
causing problems. So maybe there's an easy fix there?

> This would mean that KCSAN runtime functions ended up in the trace,
> but the function where the access happened would not. However, I don't
> care about the runtime functions, and instead want to see the function
> where the access happened. In that case, I'd like to just mark
> __tsan_X and any other kcsan instrumentation functions as
> do-not-tail-call-optimize, which would solve the problem.

I understand why we don't want to TCO these calls, but given the calls
are implicitly generated, I strongly suspect it's better to fix the
implicit call generation to not be TCO'd to begin with.

> The solution today is that when you compile a kernel with KCSAN, every
> instrumented TU is compiled with -fno-optimize-sibling-calls. The
> better solution would be to just mark KCSAN runtime functions somehow,
> but permit tail calling other things. Although, I probably still want
> to see the full trace, and would decide that having
> -fno-optimize-sibling-calls is a small price to pay in a
> debug-only-kernel to get complete traces.
> 
> > ... if K were marked in this way, and J was compiled with visibility of
> > this, J would stick around, but J's callers might not, and so the a
> > trace might see:
> >
> >   A->J->K
> >
> > ... do you just care about the final caller, i.e. you just need
> > certainty that J will be in the trace?
> 
> Yes. But maybe it's a special problem that only sanitizers have.

I reckon for basically any instrumentation we don't want calls to be
TCO'd, though I'm not immediately sure of cases beyond sanitizers and
mcount/fentry.

Thanks,
Mark.

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

[Marco Elver]

On Thu, 4 Mar 2021 at 19:51, Mark Rutland <mark.rutland@arm.com> wrote:
> On Thu, Mar 04, 2021 at 07:22:53PM +0100, Marco Elver wrote:
> > On Thu, 4 Mar 2021 at 19:02, Mark Rutland <mark.rutland@arm.com> wrote:
> > > On Thu, Mar 04, 2021 at 06:25:33PM +0100, Marco Elver wrote:
> > > > On Thu, Mar 04, 2021 at 04:59PM +0000, Mark Rutland wrote:
> > > > > On Thu, Mar 04, 2021 at 04:30:34PM +0100, Marco Elver wrote:
> > > > > > On Thu, 4 Mar 2021 at 15:57, Mark Rutland <mark.rutland@arm.com> wrote:
> > > > > > > [adding Mark Brown]
> > > > > > >
> > > > > > > The bigger problem here is that skipping is dodgy to begin with, and
> > > > > > > this is still liable to break in some cases. One big concern is that
> > > > > > > (especially with LTO) we cannot guarantee the compiler will not inline
> > > > > > > or outline functions, causing the skipp value to be too large or too
> > > > > > > small. That's liable to happen to callers, and in theory (though
> > > > > > > unlikely in practice), portions of arch_stack_walk() or
> > > > > > > stack_trace_save() could get outlined too.
> > > > > > >
> > > > > > > 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.
> > > > > >
> > > > > > Will LTO and friends respect 'noinline'?
> > > > >
> > > > > I hope so (and suspect we'd have more problems otherwise), but I don't
> > > > > know whether they actually so.
> > > > >
> > > > > I suspect even with 'noinline' the compiler is permitted to outline
> > > > > portions of a function if it wanted to (and IIUC it could still make
> > > > > specialized copies in the absence of 'noclone').
> > > > >
> > > > > > One thing I also noticed is that tail calls would also cause the stack
> > > > > > trace to appear somewhat incomplete (for some of my tests I've
> > > > > > disabled tail call optimizations).
> > > > >
> > > > > I assume you mean for a chain A->B->C where B tail-calls C, you get a
> > > > > trace A->C? ... or is A going missing too?
> > > >
> > > > Correct, it's just the A->C outcome.
> > >
> > > I'd assumed that those cases were benign, e.g. for livepatching what
> > > matters is what can be returned to, so B disappearing from the trace
> > > isn't a problem there.
> > >
> > > Is the concern debugability, or is there a functional issue you have in
> > > mind?
> >
> > For me, it's just been debuggability, and reliable test cases.
> >
> > > > > > Is there a way to also mark a function non-tail-callable?
> > > > >
> > > > > I think this can be bodged using __attribute__((optimize("$OPTIONS")))
> > > > > on a caller to inhibit TCO (though IIRC GCC doesn't reliably support
> > > > > function-local optimization options), but I don't expect there's any way
> > > > > to mark a callee as not being tail-callable.
> > > >
> > > > I don't think this is reliable. It'd be
> > > > __attribute__((optimize("-fno-optimize-sibling-calls"))), but doesn't
> > > > work if applied to the function we do not want to tail-call-optimize,
> > > > but would have to be applied to the function that does the tail-calling.
> > >
> > > Yup; that's what I meant then I said you could do that on the caller but
> > > not the callee.
> > >
> > > I don't follow why you'd want to put this on the callee, though, so I
> > > think I'm missing something. Considering a set of functions in different
> > > compilation units:
> > >
> > >   A->B->C->D->E->F->G->H->I->J->K
> >
> > I was having this problem with KCSAN, where the compiler would
> > tail-call-optimize __tsan_X instrumentation.
>
> Those are compiler-generated calls, right? When those are generated the
> compilation unit (and whatever it has included) might not have provided
> a prototype anyway, and the compiler has special knowledge of the
> functions, so it feels like the compiler would need to inhibit TCO here
> for this to be robust. For their intended usage subjecting them to TCO
> doesn't seem to make sense AFAICT.
>
> I suspect that compilers have some way of handling that; otherwise I'd
> expect to have heard stories of mcount/fentry calls getting TCO'd and
> causing problems. So maybe there's an easy fix there?

I agree, the compiler builtins should be handled by the compiler
directly, perhaps that was a bad example. But we also have "explicit
instrumentation", e.g. everything that's in <linux/instrumented.h>.

> > This would mean that KCSAN runtime functions ended up in the trace,
> > but the function where the access happened would not. However, I don't
> > care about the runtime functions, and instead want to see the function
> > where the access happened. In that case, I'd like to just mark
> > __tsan_X and any other kcsan instrumentation functions as
> > do-not-tail-call-optimize, which would solve the problem.
>
> I understand why we don't want to TCO these calls, but given the calls
> are implicitly generated, I strongly suspect it's better to fix the
> implicit call generation to not be TCO'd to begin with.
>
> > The solution today is that when you compile a kernel with KCSAN, every
> > instrumented TU is compiled with -fno-optimize-sibling-calls. The
> > better solution would be to just mark KCSAN runtime functions somehow,
> > but permit tail calling other things. Although, I probably still want
> > to see the full trace, and would decide that having
> > -fno-optimize-sibling-calls is a small price to pay in a
> > debug-only-kernel to get complete traces.
> >
> > > ... if K were marked in this way, and J was compiled with visibility of
> > > this, J would stick around, but J's callers might not, and so the a
> > > trace might see:
> > >
> > >   A->J->K
> > >
> > > ... do you just care about the final caller, i.e. you just need
> > > certainty that J will be in the trace?
> >
> > Yes. But maybe it's a special problem that only sanitizers have.
>
> I reckon for basically any instrumentation we don't want calls to be
> TCO'd, though I'm not immediately sure of cases beyond sanitizers and
> mcount/fentry.

Thinking about this more, I think it's all debugging tools. E.g.
lockdep, if you lock/unlock at the end of a function, you might tail
call into lockdep. If the compiler applies TCO, and lockdep determines
there's a bug and then shows a trace, you'll have no idea where the
actual bug is. The kernel has lots of debugging facilities that add
instrumentation in this way. So perhaps it's a general debugging-tool
problem (rather than just sanitizers).

Thanks,
-- Marco

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

[Mark Rutland]

On Thu, Mar 04, 2021 at 08:01:29PM +0100, Marco Elver wrote:
> On Thu, 4 Mar 2021 at 19:51, Mark Rutland <mark.rutland@arm.com> wrote:
> > On Thu, Mar 04, 2021 at 07:22:53PM +0100, Marco Elver wrote:

> > > I was having this problem with KCSAN, where the compiler would
> > > tail-call-optimize __tsan_X instrumentation.
> >
> > Those are compiler-generated calls, right? When those are generated the
> > compilation unit (and whatever it has included) might not have provided
> > a prototype anyway, and the compiler has special knowledge of the
> > functions, so it feels like the compiler would need to inhibit TCO here
> > for this to be robust. For their intended usage subjecting them to TCO
> > doesn't seem to make sense AFAICT.
> >
> > I suspect that compilers have some way of handling that; otherwise I'd
> > expect to have heard stories of mcount/fentry calls getting TCO'd and
> > causing problems. So maybe there's an easy fix there?
> 
> I agree, the compiler builtins should be handled by the compiler
> directly, perhaps that was a bad example. But we also have "explicit
> instrumentation", e.g. everything that's in <linux/instrumented.h>.

True -- I agree for those we want similar, and can see a case for a
no-tco-calls-to-me attribute on functions as with noreturn.

Maybe for now it's worth adding prevent_tail_call_optimization() to the
instrument_*() call wrappers in <linux/instrumented.h>? As those are
__always_inline, that should keep the function they get inlined in
around. Though we probably want to see if we can replace the mb() in
prevent_tail_call_optimization() with something that doesn't require a
real CPU barrier.

[...]

> > I reckon for basically any instrumentation we don't want calls to be
> > TCO'd, though I'm not immediately sure of cases beyond sanitizers and
> > mcount/fentry.
> 
> Thinking about this more, I think it's all debugging tools. E.g.
> lockdep, if you lock/unlock at the end of a function, you might tail
> call into lockdep. If the compiler applies TCO, and lockdep determines
> there's a bug and then shows a trace, you'll have no idea where the
> actual bug is. The kernel has lots of debugging facilities that add
> instrumentation in this way. So perhaps it's a general debugging-tool
> problem (rather than just sanitizers).

This makes sense to me.

Thanks,
Mark.