Messages in this thread | | | Date | Thu, 3 Nov 2022 09:46:09 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH 3/5] objtool: Avoid O(bloody terrible) behaviour -- an ode to libelf |
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On Wed, Nov 02, 2022 at 03:22:22PM -0700, Josh Poimboeuf wrote: > 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.
Yeah, I've been >< close to doing that at least twice now. The only consideration is that we then also must rewrite all relocs but I think we end up doing that anyway.
I'll go do the patch to see what it looks like.
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