Re: SYSCALL, ptrace and syscall restart breakages (Re: [RFC] weird crap with vdso on uml/i386)

Al Viro
2011-08-21T14:44:13Z

On Sun, Aug 21, 2011 at 07:24:35AM -0400, Andrew Lutomirski wrote:

> I don't see the point of all this hackery at all. sysenter/sysexit
> indeed screws up some registers, but we can return on the iret path in
> the case of restart.

We *do* return on iret path in case of restart, TYVM.

> So why do we lie to ptrace (and iret!) at all? Why not just fill in
> pt_regs with the registers as they were (at least the
> non-clobbered-by-sysenter ones), set the actual C parameters correctly
> to contain the six arguments (in rdi, rsi, etc.), do the syscall, and
> return back to userspace without any funny business? Is there some
> ABI reason that, once we've started lying to tracers, we have to keep
> doing so?

We do not lie to ptrace and iret. At all. We do just what you have
described. And fuck up when restart returns us to the SYSCALL / SYSENTER
instruction again, which expects the different calling conventions,
so the values arranged in registers in the way int 0x80 would expect
do us no good.