a question about trampolines and non-exec stacks

a question about trampolines and non-exec stacks

[Marc Espie]

OpenBSD moved to a non-exec stacks on a few arches recently...
as a result, trampolines no longer work.

Looking through gcc's code, I see a few instances of trampoline code
that does more or less the right thing.

- I like m68k's FINALIZE_TRAMPOLINE code. Any issue in adding the
same hook to other arches, e.g., i386 ?

- I'm a bit lost as to TRAMPOLINE_SIZE... Is it supposed to be exact,
or should it be conservative ? At a guess, if I add some emit_library_call
to make the stack executable around the area, I believe the trampoline size
ought to be adjusted upwards, right ?

- where to stick a make_stack_executable function ?

I see that nextstep has some explicit code in libgcc2.c, whereas
solaris2 goes through TRANSFER_FROM_TRAMPOLINE... but it looks to me like
either solaris2 is wrong or the description of TRANSFER_FROM_TRAMPOLINE is.

Looking further, basically, things either go through the nextstep model
(FINALIZE_TRAMPOLINE code) or through some TRANSFER_FROM_TRAMPOLINE code
that calls a make_stack_executable function... I think the
FINALIZE_TRAMPOLINE approach is cleaner.

What should I do ?

Re: a question about trampolines and non-exec stacks

[Marc Espie]

In article < 20020829180142.GA6209@tetto.liafa.jussieu.fr > you write:
>- I'm a bit lost as to TRAMPOLINE_SIZE... Is it supposed to be exact,
>or should it be conservative ? At a guess, if I add some emit_library_call
>to make the stack executable around the area, I believe the trampoline size
>ought to be adjusted upwards, right ?
I'm confused. It's obvious the trampoline generating code is not included
in TRAMPOLINE_SIZE. The other point stands, though.

Re: a question about trampolines and non-exec stacks

[Richard Henderson]

On Thu, Aug 29, 2002 at 08:01:42PM +0200, Marc Espie wrote:
> - I like m68k's FINALIZE_TRAMPOLINE code. Any issue in adding the
> same hook to other arches, e.g., i386 ?

Shouldn't be.

> - I'm a bit lost as to TRAMPOLINE_SIZE... Is it supposed to be exact,
> or should it be conservative?

AFAIK, it can be either.

> - where to stick a make_stack_executable function ?

TRANSFER_FROM_TRAMPOLINE.



r~