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    SubjectRe: arch/x86/kernel/vsyscall_64.c: overeager NOP of syscalls
    On Wed, Feb 20, 2008 at 02:57:34PM +0100, Arne Georg Gleditsch wrote:
    > Hi,
    > I'm looking at 2.6.25-rc2. vsyscall_sysctl_change contains code to NOP
    > out the actual system call instructions of the vsyscall page when
    > vsyscall64 is enabled. This seems to interact badly with the fallback
    > code in do_vgettimeofday which tries to call gettimeofday if the
    > configured clock source does not support vread. (In effect,
    > gettimeofday() becomes a nop and time() always returns 0. Not very
    > useful.)
    > Is there a good reason to keep this? Aren't the instructions in
    > question avoided (or invoked) according to the vsyscall64 flag by the
    > surrounding logic anyway?

    Yes they are. But a system call sequence at a known fixed address
    is potentially useful to exploits. That is why it is nop'ed out when
    it is not needed.


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