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    SubjectRe: [PATCH 02/02] Elf: Align elf notes properly
    Date
    "Magnus Damm" <magnus.damm@gmail.com> writes:

    > I'm not sure you see all my points. The important parts are the
    > offsets - offset 0 and offset N2 in the description above. The should
    > be aligned somehow. Exactly how to align them depends on if the 64-bit
    > spec is valid or not.
    >
    > My points are:
    >
    > - Some kdump code rounds up the size of "elf note header" today. This
    > is unneccessary for 32 bit alignment and plain wrong for 64 bit
    > alignment. So I think that the code is strange and should be changed
    > regardless if the 64-bit spec is valid or not.

    Sure that is reasonable, if correct.

    > - Many implementations incorrectly calculate N2 as: roundup(sizeof(elf
    > note header)) + roundup(n_namesz).

    I am not certain that is incorrect. roundup(sizeof(elf note header), 4) +
    roundup(n_namesize, 4) will yield something that is properly 4 byte aligned.
    I do agree that implementation is not correct for 8 byte alignment. 8 byte
    alignment does not appear to be in widespread use in the wild.

    > - You say that the size of the notes do not vary and therefore this is
    > a non-issue. I agree that the size does not vary, but I believe that
    > the aligment _is_ an issue. One example is the N2 calculation above,
    > but more importantly the vmcore code that merges the elf note sections
    > into one. You know, if you have more than one cpu you will end up with
    > more than one crash note. And if you run Xen you will have even more
    > crash notes.

    Sure that is clearly an issue.

    > - On top of this I think it would be nice if all this code could be
    > unified to avoid code duplication. But we need to straighten out this
    > and agree on how the aligment should work before the code can be
    > merged into one implementation.

    Sure.

    To verify your claim that 8 byte alignment is correct I checked the
    core dump code in fs/binfmt_elf.c in the linux kernel. That always
    uses 4 byte alignment. Therefore it appears clear that only doing
    4 byte alignment is not a local misreading of the spec, and is used in
    other implementations. If you can find an implementation that uses
    8 byte alignment I am willing to consider it.

    The current situation is that the linux kernel generated application
    core dumps use 4 byte alignment so I expect that is what existing
    applications such as gdb expect.

    Therefore we use 4 byte alignment unless it can be shown that the
    linux core dumps are a fluke and should be fixed.


    Eric
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