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 12:24:22PM -0700, Linus Torvalds wrote:
> On Tue, Aug 23, 2011 at 12:18 PM, H. Peter Anvin <hpa@zytor.com> wrote:
> >
> > 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.
> 
> Well, seriously, UML already looks at the word at "ip-2" for other
> reasons. So it isn't like there is any point in adding more support to
> just give you that information in another form.

That is done only for task singlestepped in the guest:
        /*
         * This closes a way to execute a system call on the host.  If
         * you set a breakpoint on a system call instruction and singlestep
         * from it, the tracing thread used to PTRACE_SINGLESTEP the process
         * rather than PTRACE_SYSCALL it, allowing the system call to execute
         * on the host.  The tracing thread will check this flag and
         * PTRACE_SYSCALL if necessary.
         */
        if (current->ptrace & PT_DTRACE)
                current->thread.singlestep_syscall =
                        is_syscall(PT_REGS_IP(&current->thread.regs));
with PT_DTRACE set by uml user_enable_single_step()

And it's not cheap - doing that on each syscall will be unpleasant...
Frankly, I'd rather stopped telling the uml userland about vdso in such
setups.  And anything that plays with SYSCALL outside of vdso... <shrug>
we already have a "don't run it native on 32bit", adding "don't run
it on 32bit uml on amd64 host" is not too serious.  At least for now...