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Peter Zijlstra 13f60e80e1 objtool: Avoid O(bloody terrible) behaviour -- an ode to libelf
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>
Tested-by: Yujie Liu <yujie.liu@intel.com>
Link: https://lkml.kernel.org/r/20221028194453.461658986@infradead.org
History 2022-11-01 13:44:08 +01:00
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Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.