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

Linus Torvalds
2011-08-23T02:00:48Z

On Mon, Aug 22, 2011 at 6:13 PM, Al Viro <viro@zeniv.linux.org.uk> wrote:
>
> *UGH*.  OK,
>        1) I'm an idiot; int_ret_from_sys_call does *not* usually step on
> rbp (it's callee-saved).  So normally ebp is left as is on the way out,
> which is why we don't see stuff getting buggered left, right and center.

Check.

And the system call restart should actually work fine too, because at
syscall entry we save %ebp *both* in the slot for ebp and ecx when we
enter the first time. So the second time, we'll re-load the third
argument from ebp again, but that's fine - it's still going to be the
right value. Yes? No?

However, I note that the cstar entrypont has a comment about not saving ebp:

* %ebp Arg2 [note: not saved in the stack frame, should not be touched]

which sounds odd. Why don't we save it? If we take a signal handler
there, don't we want %ebp on the kernel stack in pt_regs, in order to
do everything right?

Now I'm *really* confused.

Linus