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Message-ID: <20221102222222.r4xkj6uynlj6n2t6@treble>
Date: Wed, 2 Nov 2022 15:22:22 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, djwong@...nel.org,
yujie.liu@...el.com, tglx@...utronix.de, joao.moreira@...el.com,
samitolvanen@...gle.com
Subject: Re: [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@...radead.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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