is stdlib/tst-setcontext7 a bad test?

[Szabolcs Nagy]

The 03/27/2020 11:10, Florian Weimer wrote:
> * Szabolcs Nagy:
> > i'm planing to change swapcontext on aarch64 and i see
> >
> > FAIL: stdlib/tst-setcontext7
> >
> > that test case seems wrong to me, a simplified version is
> >
> > #include <ucontext.h>
> > #include <stdio.h>
> >
> > ucontext_t uc[2];
> > volatile int count = 0;
> >
> > int main ()
> > {
> > 	getcontext (uc+0);
> > 	printf ("%d\n", count);
> > 	if (count)
> > 		setcontext (uc+1);
> > 	count++;
> > 	swapcontext (uc+1, uc+0);
> > 	printf ("done\n");
> > 	return 0;
> > }
> >
> > this seems to work today and prints
> >
> > 0
> > 1
> > done
> >
> > but i don't think we can guarantee that after a
> > swapcontext to a getcontext on the same stack frame
> > resuming the context works: if swapcontext uses the
> > stack (which is what i plan to do) then that stack
> > will be corrupted.
> >
> > does glibc plan to support this usage?
> 
> I think it's expected to work.  This seems to be one of the more
> harmless cases.
> 
> > i.e. should i ensure swapcontext does not use the stack? (and should
> > gcc ensure no stack is used in the example between getcontext and
> > swapcontext?)
> 
> Does the test case pass if you add the returns_twice attribute (or
> __INDIRECT_RETURN) to getcontext as well (swapcontext already has it)?

yes, the returns_twice attribute on swapcontext
makes bti work, but that seems ugly (swapcontext
only returns once).

(getcontext has the return_twice attribute because
it's a gcc builtin.)

the way i planed to handle swapcontext is to do

swapcontext:
	bti c
	stp x29, x30, [sp, #-16]! // save return address
	bl internal_swapcontext
	bti j
	ldp x29, x30, [sp], #16 // restore return address
	ret

then bti j (landing pad for indirect jump) is
in libc code, otherwise the compiler has to
special case swapcontext and emit a bti j after
every call site (getcontext is already special),
with my solution swapcontext is not special.

but if glibc plans to support jumping around
within the same stack frame then my solution
does not work so i will have to rely on the
attribute.