Faddr2line not working

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1

As per this issue in bcachefs Kernel BUG at bch2_bio_compress · Issue #883 · koverstreet/bcachefs · GitHub , the developer wanted the line number for further investigation.

I obtained the faddr2line script from linux/scripts/faddr2line at master · torvalds/linux · GitHub and then ran this command on regular kernel and xanmod kernel.

/tmp/faddr2line /nix/store/hfgrxxhrzax6n0yp33rxsjq7vzs5w86w-linux-6.14.5-dev/vmlinux bch2_bio_compress+0x527/0x530
/tmp/faddr2line /nix/store/anmfvh9s852h7lkmrv919jvf75bsjhw4-linux-xanmod-6.14.5-dev/vmlinux bch2_bio_compress+0x4a6/0x5b0

I did not find any line numbers.
Instead I got these errors

(faddr2line v6.14) sed: Arg is too long
(faddr2line v6.7)  no match for bch2_bio_compress+0x4a6/0x5b0

I followed this reddit post to get the store path of the current kernel.
https://www.reddit.com/r/NixOS/comments/1gm5xg5/comment/lw0ffzm/?utm_source=share&utm_medium=web3x&utm_name=web3xcss&utm_term=1&utm_content=share_button

❯ ll $(nix-store --realise $(nix --experimental-features nix-command derivation show /run/booted-system/kernel | jq -r '.[].outputs.dev.path'))/vmlinux
warning: you did not specify '--add-root'; the result might be removed by the garbage collector
.r-xr-xr-x 376M root  1 Jan  1970  /nix/store/hfgrxxhrzax6n0yp33rxsjq7vzs5w86w-linux-6.14.5-dev/vmlinux

Is this user(me) error or faddr2line does not work on NixOS kernels?

2

It’s been a while since I needed this, but I have a small wrapper for scripts/decode_stacktrace.sh (which i believe calls faddr2line internally)

{ config, pkgs, lib, ... }:

let
  decode_stacktrace =
    let
      kernel = config.boot.kernelPackages.kernel;
    in
    pkgs.writeShellApplication {
      name = "decode_stacktrace.sh";
      runtimeInputs = with pkgs; [
        stdenv
        gcc
        util-linux
      ];
      text = ''
        exec ${kernel.dev}/lib/modules/${kernel.modDirVersion}/source/scripts/decode_stacktrace.sh \
            ${kernel.dev}/vmlinux \
            /build/source/build/.. \
            /run/booted-system/kernel-modules/lib/modules/${kernel.modDirVersion} \
        ;
      '';
    };
in
{
  environment.systemPackages = [
    decode_stacktrace
  ];
}

I’m also setting the below on my kernel build:

  • I had to build bcachefs as a built-in, not as a module (something about not being able to decompress the modules IIRC)
  • I don’t remember why the other options were necessary
  structuredExtraConfig = with lib.kernel; {
    BCACHEFS_FS = yes;

    # for scripts/decode_stacktrace.sh
    IKHEADERS = module;
    DEBUG_KERNEL = yes;
    DEBUG_INFO = yes;
  };

With this setup, I can do:

# echo bch2_rebalance_thread+0xce/0x130 | decode_stacktrace.sh
bch2_rebalance_thread (fs/bcachefs/rebalance.c:626 (discriminator 1))

or

# cat /sys/fs/bcachefs/*/rebalance_status | decode_stacktrace.sh
pending work:                  528 MiB

waiting
io wait duration:            7.28 GiB
io wait remaining:           3.27 GiB
duration waited:             84 m

bch2_rebalance_thread (fs/bcachefs/rebalance.c:626 (discriminator 1))
kthread (kernel/kthread.c:464)
ret_from_fork (arch/x86/kernel/process.c:153)
ret_from_fork_asm (arch/x86/entry/entry_64.S:258)

Hope this helps!
I’m definitely interested if someone has a cleaner/easier method

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