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

H. Peter Anvin
2011-08-23T19:18:52Z

On 08/23/2011 09:48 AM, Al Viro wrote:
>
> Um... How would it know which syscall variant had that been, to start
> with? For int 0x80 it would need to use registers as-is. For SYSENTER
> it also could use them as-is - ebp will differ from what we put there
> when entering the sucker, but not critically so; on the way out of
> syscall we'll overwrite it anyway immediately (either by pop or mov).
> For SYSCALL... we don't really care about ecx contents prior to entering
> the kernel (and it'll be blown out anyway), and ebp one could be found in
> regs.ecx. So yes, we can do it that way, but... how to tell what variant
> had been triggered? Examining two bytes prior to user eip? Sounds bloody
> brittle...

We could drop that information in a metaregister. It's not backward
compatible, but at least it will be obvious when that information is
available and not.

-hpa