Bug 26391 - Question regarding duplicate symbols
Summary: Question regarding duplicate symbols
Alias: None
Product: binutils
Classification: Unclassified
Component: ld (show other bugs)
Version: 2.36
: P2 normal
Target Milestone: 2.36
Assignee: Not yet assigned to anyone
URL: https://sourceware.org/pipermail/binu...
Depends on:
Reported: 2020-08-14 18:22 UTC by Kristen Carlson Accardi
Modified: 2020-09-13 09:17 UTC (History)
1 user (show)

See Also:
Last reconfirmed: 2020-08-14 00:00:00


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Description Kristen Carlson Accardi 2020-08-14 18:22:27 UTC
I am implementing "function granular kernel address space layout randomization" for Linux, and am trying to solve a problem that I am really hoping there is a compiler or linker option for. My implementation uses gcc's -ffunction-sections to break apart the kernel .text section by function. At kernel boot time, just after decompression, I shuffle all the function sections randomly and re-layout the kernel. Surprisingly, this works pretty well, but I am having a problem with a niche condition that is important to resolve for the live-patching use case. In order to live-patch the kernel, the live-patching code will request the address of symbols by name from the kernel. This works great after randomization because it always receives the current address at runtime rather than relying on any kind of buildtime address. The issue with with the live-patching code's algorithm for resolving duplicate symbol names. If they request a symbol by name from the kernel and there are 3 symbols with the same name, they use the symbol's position in the built binary image to select the correct symbol. This is obviously invalid when the layout has been shuffled randomly. I was hoping there was some kind of compiler or linker option that would add some kind of uniqueness to the symbols to make sure their were never any duplicates, but I am not finding such a thing. Is there something like this already available? Thanks very much!
Comment 1 H.J. Lu 2020-08-14 19:32:20 UTC
Please try


You can pass -z unique to ld to make local symbols unique.
Comment 2 Kristen Carlson Accardi 2020-08-21 22:49:02 UTC
This works for me! Thanks!
Comment 3 H.J. Lu 2020-08-22 15:40:52 UTC
I updated my branch to rename the option name to "-z unique-symbol".
Comment 4 Kristen Carlson Accardi 2020-09-09 15:24:01 UTC
This feature was well received by the livepatch maintainers. It not only solves my individual problem, but also would allow them to more precisely locate duplicate symbols in general for patching. I'd like to request that this feature be implemented in a future release.
Comment 6 H.J. Lu 2020-09-09 16:21:04 UTC
A patch is posted at

Comment 7 cvs-commit@gcc.gnu.org 2020-09-12 12:40:12 UTC
The master branch has been updated by H.J. Lu <hjl@sourceware.org>:


commit 496afd17055aeb7d8f45e01715c475664f2b73bd
Author: H.J. Lu <hjl.tools@gmail.com>
Date:   Sat Sep 12 05:37:30 2020 -0700

    elf: Add -z unique-symbol to avoid duplicated local symbol names
    The symbol string table in the .symtab section is optional and cosmetic.
    The contents of the .symtab section have no impact on run-time execution.
    The symbol names in the symbol string table help distinguish addresses at
    different locations.  Add a linker option, -z unique-symbol, to avoid
    duplicated local symbol names in the symbol string table.
    This feature was well received by the livepatch maintainers.  It not only
    solves the duplicated local symbol name problem, but also would allow
    livepatch to more precisely locate duplicate symbols in general for
            PR ld/26391
            * elflink.c (elf_final_link_info): Add local_hash_table.
            (local_hash_entry): New.
            (local_hash_newfunc): Likewise.
            (elf_link_output_symstrtab): Append ".COUNT" to duplicated local
            (bfd_elf_final_link): Initialize and free local_hash_table for
            "-z unique-symbol".
            PR ld/26391
            * bfdlink.h (bfd_link_info): Add unique_symbol.
            PR ld/26391
            * NEWS: Mention "-z unique-symbol".
            * emultempl/elf.em (gld${EMULATION_NAME}_handle_option): Handle
            "-z unique-symbol" and "-z nounique-symbol".
            * ld.texi: Document "-z unique-symbol" and "-z nounique-symbol".
            * lexsup.c (elf_static_list_options): Add "-z unique-symbol" and
            "-z nounique-symbol".
            * testsuite/ld-elf/elf.exp: Add PR ld/26391 tests.
            * testsuite/ld-elf/pr26391.nd: New file.
            * testsuite/ld-elf/pr26391.out: Likewise.
            * testsuite/ld-elf/pr26391a.c: Likewise.
            * testsuite/ld-elf/pr26391b.c: Likewise.
            * testsuite/ld-elf/pr26391c.c: Likewise.
            * testsuite/ld-elf/pr26391d.c: Likewise.
Comment 8 H.J. Lu 2020-09-12 12:40:54 UTC
Fixed for 2.36.
Comment 9 cvs-commit@gcc.gnu.org 2020-09-13 09:17:14 UTC
The master branch has been updated by Alan Modra <amodra@sourceware.org>:


commit 1f656a652ea8d489b5aa5b7f55c535fc9e6b0b29
Author: Alan Modra <amodra@gmail.com>
Date:   Sun Sep 13 17:56:52 2020 +0930

    Re: elf: Add -z unique-symbol
            PR 26391
            * testsuite/ld-elf/pr26391.nd: Adjust to match powerpc64 function