linux-kernel - Re: [PATCH] vmlinux.lds.h: Force-align ELF notes section to four bytes

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Date:   Thu, 13 Apr 2023 02:21:49 +0000
From:   Joan Bruguera Micó <joanbrugueram@...il.com>
To:     jpoimboe@...nel.org
Cc:     i.pear@...look.com, acme@...nel.org, alan.maguire@...cle.com,
        alexandref75@...il.com, bpf@...r.kernel.org, dxu@...uu.xyz,
        jforbes@...hat.com, linux-kernel@...r.kernel.org,
        olsajiri@...il.com, peterz@...radead.org, ptalbert@...hat.com,
        yhs@...com
Subject: Re: [PATCH] vmlinux.lds.h: Force-align ELF notes section to four bytes

I have done some tests with Arch Linux which is also affected by this issue:

Base scenario: Running Arch Linux updated as of 2023-04-13.
               Building the linux-mainline 6.3rc6-1 from AUR
               (https://aur.archlinux.org/packages/linux-mainline)
               with CONFIG_X86_KERNEL_IBT changed to 'y'
Result:        Build fails with the "btf__dedup failed!" error

Test 1: Update dwarves (=pahole package on Arch Linux) from the current version
        (1:1.24+r29+g02d67c5-1) to the staging version (1:1.25-1).
Result: The build works correctly.

        However, the notes section is still not parsed correctly, as confirmed
        by `readelf -n` or adding a printf near the code on `cus__merging_cu`
        (on `dwarf_loader.c`) pointed to by Tianyi.

        A bisect shows the commit that fixes the build is
        a9498899109d3be14f17abbc322a8f55a1067bee
        "dwarf_loader: Fix for BTF id drift caused by adding unspecified types"
        I don't know why though.

Test 2: Applying Josh's patch to force-align the ELF notes section to 4 bytes
        (clarification: using the base pahole version again here)
Result: The build works correctly.
        I can read the notes correctly using `readelf -n` as well,
        and they are aligned to 4 bytes instead of 8.

Test 3: Similar to Josh's patch, but instead of force-aligning the sections,
        I added `.note.gnu.property` to the `/DISCARD/` list in the line above.
Result: The build works correctly.
        I can read the notes correctly using `readelf -n` as well
        (of course, the GNU notes which made the alignment be 8 bytes are gone;
         I don't know if this has any negative effect).

So, in summary, either upgrading dwarves/pahole from 1.24 to (not yet fully
released?) 1.25, or applying Josh's patch fixes the issue for me.
Despite the new dwarves/pahole version fixing the build, Josh's patch or
something else to fix the notes alignment is needed.