linux-kernel - Re: [PATCH 3/5] objtool: Avoid O(bloody terrible) behaviour -- an ode to libelf

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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