Linker scripts and Section Reordering.

[Sriraman Tallam <tmsriram at google dot com>]

Hi,

  Using linker scripts along with section reordering, either through
--section-ordering-file or the plugin, can cause bugs during segment
address computation.

  Linker scripts fire first, in file layout.cc,  function
"relaxation_loop_body" calls function
"set_section_addresses_from_script" and computes an output section
address for ".text". This can also sort text input sections if
specified via the linker script.

   Section ordering sorting comes later and is called in a chain of
events starting from function "set_segment_offsets" which is called
again from function "relaxation_loop_body" in layout.cc. With section
ordering sorting, the output_section size can change, considering
alignment. If the resulting size happens to be smaller than what was
computed earlier, we get an error  of the form "address of section
<section-name> moves backward from XXX to YYY". This error will
correspond to the section that follows ".text" in the linker script as
its start address begins  where ".text" ends. This error code is
located in file output.cc in function "set_section_list_addresses".

   The other problem is that of handling section sorting when it is
specified via linker scripts and section reordering. We encountered
this problem with the kernel where we cannot do without linker
scripts. We found that plugin based section reordering in the kernel
improves the performance of some applications but we also wanted the
ordering to be compatible with the linker script used.

   To solve these problems, I propose the following simple patch. This
patch will invoke section sorting earlier, before the linker script
can compute the address of the output section. Then, the linker script
can re-sort the sections and the new computed address will be correct
as the order of sections will not change.

  With the patch, the resulting order of input sections is definitely
agreeable with the linker script specification. All other input
sections which the linker script did not care about will be agreeable
with the section ordering spec. If we make the linker script sorting
glob generic, like specify just ".text.*"  within the sections clause,
then we can get the sorting from section ordering alone.


Patch attached.

        * output.h (sort_attached_input_sections): Change to be public.
        * script-sections.cc (set_section_addresses): Call
sort_attached_input_sections
        before linker script assigns section addresses.


Thanks,
-Sri.

Attachment: script_ordering_bug.txt
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