Re: [PATCH] core: Support heap-based trampolines

[Martin Uecker <ma.uecker@gmail.com>]

> 
> > On 17 Jul 2023, 
> 

> >> You mention setjmp/longjmp - on darwin and other platforms
> requiring
> >> non-stack based trampolines
> >> does the system runtime provide means to deal with this issue like
> an
> >> alternate allocation method
> >> or a way to register cleanup?
> > 
> > There is an alternate mechanism relying on system libraries that is
> possible on darwin specifically (I don’t know for other targets) but
> it will only work for signed binaries, and would require us to
> codesign everything produced by gcc. During development, it was
> deemed too big an ask and the current strategy was chosen (Iain can
> surely add more background on that if needed).
> 
> I do not think that this solves the setjump/longjump issue - since
> there’s still a notional allocation that takes place (it’s just that
> the mechanism for determining permissions is different).
> 
> It is also a big barrier for the general user - and prevents normal
> folks from distributing GCC - since codesigning requires an external
> certificate (i.e. I would really rather avoid it).
> 
> >> Was there ever an attempt to provide a "generic" trampoline driven
> by
> >> a more complex descriptor?
> 
> We did look at the “unused address bits” mechanism that Ada has used
> - but that is not really available to a non-private ABI (unless the
> system vendor agrees to change ABI to leave a bit spare) for the base
> arch either the bits are not there (e.g. X86) or reserved (e.g.
> AArch64).
> 
> Andrew Burgess did the original work he might have comments on
> alternatives we tried
> 

For reference, I proposed a patch for this in 2018. It was not
accepted because minimum alignment for functions would increase
for some archs:

https://gcc.gnu.org/legacy-ml/gcc-patches/2018-12/msg01532.html



> >> (well, it could be a bytecode interpreter and the trampoline being
> >> bytecode on the stack?!)
> > 
> > My own opinion is that executable stack should go away on all
> targets at some point, so a truly generic solution to the problem
> would be great.
> 
> indeed it would.
> 

I think we need a solution rather sooner than later on all archs.

Martin

> > Having something that works reliably across all targets, like you
> suggest, is a much bigger project that this patch, and I am not aware
> of any previous attempt at it.
> 
> The bytecode interpreter idea is neat;  (a) I wonder about
> performance and (b) it is, as FX says, a bigger project - certainly
> bigger than the voluntary Darwin time available :(
> 
> Iain
> 
> > 
> > 
> >> Otherwise I suggest to split the patch into libgcc, generic and
> target parts.
> > 
> > 
>