Re: [PATCH 1/3] powerpc: Split user/kernel definitions of struct pt_regs

On Monday 15 October 2018 04:38 PM, Michael Ellerman wrote:

Madhavan Srinivasan [off-list ref] writes:

quoted

On Saturday 13 October 2018 04:26 PM, Michael Ellerman wrote:

quoted

We use a shared definition for struct pt_regs in uapi/asm/ptrace.h.
That means the layout of the structure is ABI, ie. we can't change it.

That would be fine if it was only used to describe the user-visible
register state of a process, but it's also the struct we use in the
kernel to describe the registers saved in an interrupt frame.

We'd like more flexibility in the content (and possibly layout) of the
kernel version of the struct, but currently that's not possible.

So split the definition into a user-visible definition which remains
unchanged, and a kernel internal one.

At the moment they're still identical, and we check that at build
time. That's because we have code (in ptrace etc.) that assumes that
they are the same. We will fix that code in future patches, and then
we can break the strict symmetry between the two structs.

Nice and awesome. But just trying to understand. What will
*regs will point to in the "struct sigcontext".

Yeah that's a bit fishy.

It should always point to a user_pt_regs.

So in the kernel we want:

   struct sigcontext {
   	...
   	struct user_pt_regs	__user *regs;

And in userspace we want:

   struct sigcontext {
   	...
   	struct pt_regs	__user *regs;


I think it's not actually broken at the moment, because it's just a
pointer, and we don't do anything based on the sizeof() the type.

yes. This clarifies. But still perf/perf_regs.c needs changes.
Because perf support dumping user_space regs and interrupt regs.
Once again, we dont use any sizeof(), but need to handle the
user_pt_regs changes.

I will have a look at that in the morning.

Thanks for clarification.
Maddy