Re: [PATCH v2] add powerpc64 port

[Rich Felker <dalias@libc.org>]

On Wed, Apr 27, 2016 at 08:38:19PM -0500, Bobby Bingham wrote:
> > > @@ -141 +141 @@
> > > -mcontext_t: mcontext_t, mcontext_t*, size (*) [1272], align (*) [8]
> > > +mcontext_t: sigcontext, sigcontext*, size (*) [1528], align (*) [8]
> > 
> > IIRC on other archs we made an effort to make the tag here match ABI
> > (duplicating the struct def if needed). Not sure if it matters.
> 
> I can duplicate the structure if you want.  But it looks like glibc used
> to do 'typedef struct sigcontext mcontext_t' as well:
> 
> https://sourceware.org/git/?p=glibc.git;a=blobdiff;f=sysdeps/unix/sysv/linux/powerpc/sys/ucontext.h;h=a499a80ef9994e541b866202ee8440843b004fdd;hp=b75e25a3c84c852b22e1690ff530d3ddb8dff257;hb=5ef6ae4bdb;hpb=39b04aa39823faf1cc414e7f3eca4f43e01426e4

Wow, the glibc stuff is an utter mess and grossly non-conforming. It
doesn't even have a uc_mcontext member of type mcontext_t. At least it
looks like all modern kernels have the mcontext_t at the end where
it's extensible and doesn't break access to uc_sigmask if it changes.
We should probably check and make sure both the current 32-bit powerpc
ucontext_t and your proposed powerpc64 one have the mcontext_t members
(at least the program counter) accessible at the expected locations.
We should probably also let the kernel people know that mcontext_t
(and its location) are ABI whether they like it or not. This is
imposed by POSIX and the glibc hack of making it a pointer is
non-conforming.

> > > @@ -195 +195 @@
> > > -sem_t: sem_t, sem_t*, size (*) [32], align (*) [8]
> > > +sem_t: sem_t, sem_t*, size (*) [32], align (*) [4]
> > 
> > > @@ -229,2 +229,2 @@
> > > -cmsghdr: cmsghdr, cmsghdr*, size (*) [16], align (*) [8]
> > > +cmsghdr: cmsghdr, cmsghdr*, size (*) [16], align (*) [4]
> > 
> > This is likely going to hit the same issue we're trying to debug on
> > mips64.
> 
> The mips64 issue ended up not being alignment related.  Do you still
> want me to do something about this?  And if so, do you have a suggestion?

I'm not entirely sure, but I think you can leave it alone for now. If
it needs fixing here, then it already does on multiple existing archs,
and they should be handled together.

> > > @@ -416 +417 @@
> > > -ucontext_t: ucontext, ucontext*, size (*) [1440], align (*) [8]
> > > +ucontext_t: ucontext, ucontext*, size (*) [1696], align (*) [8]
> > 
> > This may be a real problem. ucontext_t is ABI between kernel and
> > userspace and if it's wrong cancellation won't work right.
> 
> Kernel commit ce48b2100785 expanded the vmx_reserve member of mcontext_t
> by 256 bytes.  The glibc headers haven't been updated for this expansion.

Ah. As noted above, uc_mcontext is at the end so it shouldn't break
anything.

Rich