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

[Al Viro <viro@ZenIV.linux.org.uk>]

On Tue, Aug 23, 2011 at 09:03:04AM -0700, Linus Torvalds wrote:

> Suggested fixes:
> 
>  - instead of blindly doing SETREGS, just write the result registers
> individually like you suggested

Not enough.  There is also a PITA with signal handlers.  There we can't
avoid modifying ebp on the way out of handler (i.e. by emulated sigreturn).
And it lands us straight after syscall insn, with ebp "restored" to the
wrong value.

> OR (and perhaps preferably):
> 
>  - teach UML that when you do 'GETREGS' after a system call trapped,
> we have switched things around to match the "official system call
> order", and UML should just undo our swizzling, and do a "regs.ebp =
> regs.ecx" to make it be what the actual original registers were (or
> whatever the actual correct swizzle is - I didn't think that through
> very much).

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...