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    SubjectRe: [PATCH 02/02] Elf: Align elf notes properly
    On 11/10/06, Eric W. Biederman <ebiederm@xmission.com> wrote:
    > "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 are correct that it only matters if we are interested in 8 byte
    alignment. So it should be a non-issue for the 4-byte aligment case.

    > > - 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.

    Yes, fs/binfmt_elf.c is one of the files that my patch modifies. There
    are several elf note implementations in the kernel, all seem to use
    4-byte aligment.

    Implementations that use 8-byte alignment:

    binutils-2.16.1/bfd/elf.c: elf_core_write_note() is using
    log_file_align which is set to 3 on some 64-bit platforms. 8-byte
    alignment in some cases.

    binutils-2.16.1/binutils/readelf.c: process_corefile_note_segment() is
    always using 4-byte alignment though.

    > 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.

    Most applications probably expect 4-byte aligned data. OTOH, I just
    came across HP's ELF-64 Object File Format document. It says that
    8-byte alignment should be used:

    http://devresource.hp.com/drc/STK/docs/refs/elf-64-hp.pdf

    So now we have two documents that say 8-byte alignment should be used.

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

    Ok. Vivek, Dave, anyone? Comments?

    / magnus
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