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    SubjectRe: [PATCH] Linux Kernel Markers
    * Alan Cox ( wrote:
    > Ar Maw, 2006-09-19 am 13:54 -0400, ysgrifennodd Mathieu Desnoyers:
    > > Very good idea.. However, overwriting the second instruction with a jump could
    > > be dangerous on preemptible and SMP kernels, because we never know if a thread
    > > has an IP in any of its contexts that would return exactly at the middle of the
    > > jump.
    > No: on x86 it is the *same* case for all of these even writing an int3.
    > One byte or a megabyte,
    > You MUST ensure that every CPU executes a serializing instruction before
    > it hits code that was modified by another processor. Otherwise you get
    > CPU errata and the CPU produces results which vendors like to describe
    > as "undefined".
    > Thus you have to serialize, and if you are serializing it really doesn't
    > matter if you write a byte, a paragraph or a page.
    Hi Alan,

    What I am trying to address is not "code patching with INT3", but "code patching
    with a 5 bytes JMP". The errata you point to applies to both and kprobes
    mechanism already takes care of this with the serialization method you describe.

    However, there is a supplemental problem with the fact that a JMP is 5 bytes,
    not 1. You are right about saying that overwriting code with any amount of
    *int3* does not matter, but what happens when you put one or more 5 bytes long
    jumps instead ?

    Think about it : if you are replacing 1-2-3 or 4 bytes long instruction and,
    unluckily, on any stack of any thread preempted from any CPU, you have a
    current instruction pointer pointing at the middle of the region where you want
    to put the 5 bytes JMP, the processor will likely trigger an illegal
    instruction fault when this particular thread is scheduled back.


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