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    SubjectRe: [uml-devel] SYSCALL, ptrace and syscall restart breakages (Re: [RFC] weird crap with vdso on uml/i386)
    Am 23.08.2011 19:07, schrieb Al Viro:
    > 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 know. :)

    > 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.

    Okay, this is a much cleaner approach.
    But first I've to find a machine where I can test the issue.
    At home none on of my x86_64 machines is SYSCALL-based.
    Tomorrow I'll search at the university for one...

    Thanks,
    //richard


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