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    SubjectRe: [PATCH 07/12] perf_events: add LBR software filter support for Intel X86
    (2011/10/10 23:45), Andi Kleen wrote:
    >> Ah, nice. Maybe we need another test binary, since current one is
    >> just ensuring the output of objdump and decoder is same.
    >> anyway it's not so difficult if it feeds random binaries to
    >> ensure the decoder doesn't access bad address.
    >
    > Pure /dev/urandom is not good because it cannot be ever reproduced.
    > Better use a PRNG with random seed from urandom, but print the seed.

    Sure,

    > In addition to random I would do fuzzing: take an existing stream
    > and just corrupt some bits and groups of bits. This will exercise different
    > paths. In fact fuzzing is probably better than random for most tests.
    >
    > Not sure it's needed to run on every build though, just checks
    > now and then should be sufficient.

    I made a test for that, and I didn't hit any random
    bytes which can not be decoded after I've tested 100,000,000.
    (with previous hardening patch)

    since the number of the combination is 2^128 (MAX_INSN_SIZE is 16),
    too huge to check all of them. Of course, the real number of
    possible combination should be smaller than that.

    - Because this decoder don't evaluate but just decode,
    we can skip immediates and operands.
    - If we hit the non-oprand opcode, mod/rm, sib, or displacement
    in the middle of byte stream, we can stop trying decode tail bytes.

    So I think there is smarter way to cover all possible combination
    for the decoder than feeding random bytes.

    Anyway, the decoder code can evolve in the future, I think it
    should be put into the linux kernel.

    Thank you,

    --
    Masami HIRAMATSU
    Software Platform Research Dept. Linux Technology Center
    Hitachi, Ltd., Yokohama Research Laboratory
    E-mail: masami.hiramatsu.pt@hitachi.com


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