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

Al Viro
2011-08-23T17:08:06Z

On Tue, Aug 23, 2011 at 06:58:18PM +0200, Richard Weinberger wrote:

> What about this hack/solution?
> While booting UML can check whether the host's vDSO contains
> a SYSCALL instruction.
> If so, UML will not make the host's vDSO available to it's
> processes...

Note that this is *only* for 32bit side of things. 64bit one works fine...

I wouldn't search for SYSCALL in vdso, BTW - not when we have a good way
to trigger that crap and recognize it.

At boot time, fork a child. Have it traced with PTRACE_SYSCALL. Let it
put recognizable values in registers and call __kernel_vsyscall(). Then
let the parent do one more PTRACE_SYSCALL, then PTRACE_POKEUSER and set ebp
to 0x69696969. PTRACE_CONT the sucker and let it report what it sees in ecx.
If it's what we'd put there - fine, it looks safe. If it's 0x69696969 -
we have a problem, no vdso for us.