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    SubjectRe: [PATCH 3/5] objtool: Avoid O(bloody terrible) behaviour -- an ode to libelf
    On Fri, Oct 28, 2022 at 09:40:25PM +0200, Peter Zijlstra wrote:
    > Due to how gelf_update_sym*() requires an Elf_Data pointer, and how
    > libelf keeps Elf_Data in a linked list per section,
    > elf_update_symbol() ends up having to iterate this list on each
    > update to find the correct Elf_Data for the index'ed symbol.
    >
    > By allocating one Elf_Data per new symbol, the list grows per new
    > symbol, giving an effective O(n^2) insertion time. This is obviously
    > bloody terrible.
    >
    > Therefore over-allocate the Elf_Data when an extention is needed.
    > Except it turns out libelf disregards Elf_Scn::sh_size in favour of
    > the sum of Elf_Data::d_size. IOW it will happily write out all the
    > unused space and fill it with:
    >
    > 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
    >
    > entries (aka zeros). Which obviously violates the STB_LOCAL placement
    > rule, and is a general pain in the backside for not being the desired
    > behaviour.
    >
    > Manually fix-up the Elf_Data size to avoid this problem before calling
    > elf_update().
    >
    > This significantly improves performance when adding a significant
    > number of symbols.
    >
    > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>

    Instead of going through libelf to add each symbol, and
    adjusting/shifting/reallocating the d_buf one symbol at a time, it would
    probably be a lot easier (and faster) to just manually do all that at
    the end, just before writing the file.

    See for example what kpatch does:

    https://github.com/dynup/kpatch/blob/master/kpatch-build/kpatch-elf.c#L725

    --
    Josh

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