is stdlib/tst-setcontext7 a bad test?

[H.J. Lu]

On Fri, Mar 27, 2020 at 4:04 AM Szabolcs Nagy <szabolcs.nagy@arm.com> wrote:
>
> 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.
>

Can't you use indirect_return function attribute like the fix for

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85620

-- 
H.J.