Messages in this thread | | | Date | Wed, 2 Nov 2022 15:22:22 -0700 | From | Josh Poimboeuf <> | Subject | Re: [PATCH 3/5] objtool: Avoid O(bloody terrible) behaviour -- an ode to libelf |
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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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