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

[Iain Sandoe <iain@sandoe.co.uk>]

> On 17 Jul 2023, at 07:58, Iain Sandoe <iain@sandoe.co.uk> wrote
> 
>> On 17 Jul 2023, at 07:43, FX Coudert <fxcoudert@gmail.com> wrote:
>> 

>> 
>> 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

Although I will comment that the main barrier to data / descriptor based schemes is that we allow recursive use of nested functions and that means that each nest level needs a distinct target address to branch to / call.  [that might also make the bytecode scheme hard(er)]

Iain