NACK: [SRU][R][PATCH 0/1] Enable BTF generation for out-of-tree module builds when kernel dbgsyms are installed

[Jacob Martin]

On 3/17/26 4:18 PM, Jacob Martin wrote:
> BugLink: https://bugs.launchpad.net/bugs/2133794
> 
> [Impact]
> 
> When building out-of-tree kernel modules (via DKMS or manual module
> build) on systems with the debuginfo package (`linux-image-*-dbgsym`)
> installed, BTF (BPF Type Format) generation is skipped due to vmlinux
> not being found in the expected location. This prevents BPF-based
> tracing tools (bpftrace, BCC) from attaching kprobes/kfuncs to
> out-of-tree module functions.
> 
> [Fix]
> 
> The patch "UBUNTU: [Packaging] dbgsym: add vmlinux symlink to headers
> directory" resolves this issue by including a symlink to the vmlinux at
> /usr/src/linux-headers-7.0.0-xx-generic/vmlinux in the
> linux-image-7.0.0-xx-generic-dbgsym package.
> 
> [Test Plan]
> 
> Verify that BTF generation succeeds:
> 
> 1. Install dbgsym and headers for the target kernel.
> 
> $ sudo apt install linux-image-7.0.0-xx-generic-dbgsym linux-headers-7.0.0-xx-generic-dbgsym
> 
> 2. Obtain sources for a test kernel module.
> 
> $ git clone https://github.com/ivbakula/hello-world-kernel-module
> 
> 3. Build this kernel module.
> 
> make -C /lib/modules/7.0.0-xx-generic/build M=hello-world-kernel-module modules
> 
> 4. Verify BTF generation is successful.
> 
> $ objdump --section=.BTF -s ~/hello-world-kernel-module/hello.ko
> 
> /root/hello-world-kernel-module/hello.ko: file format elf64-x86-64
> 
> Contents of section .BTF:
>   0000 9feb0100 18000000 00000000 60000000 ............`...
>   0010 60000000 16000000 00000000 0000000a `...............
>   0020 02000000 00000000 00000003 00000000 ................
>   0030 03000000 01000000 05000000 00000000 ................
>   0040 0000000a 04000000 15000000 0000000c ................
>   0050 09000000 20000000 0000000c 08000000 .... ...........
>   0060 00000000 0000000d 01000000 00000000 ................
>   0070 0000000d 00000000 68656c6c 6f5f6578 ........hello_ex
>   0080 69740068 656c6c6f 5f696e69 7400 it.hello_init.
> 
> Note: if BTF generation is unsuccessful, the output will appear as:
> 
> $ objdump --section=.BTF -s hello.nobtf.ko
> 
> hello.nobtf.ko: file format elf64-x86-64
> 
> objdump: section '.BTF' mentioned in a -j option, but not found in any input file
> 
> [Where issues could occur]
> 
> This modifies the Ubuntu dbgsym packaging. An issue with this fix could
> result in a malformed dbgsym package. On systems with the dbgsym package
> installed, built out-of-tree kernel modules will now include an
> additional .BTF section, which is already true for in-tree kernel
> modules.
> 
> Jacob Martin (1):
>    UBUNTU: [Packaging] dbgsym: add vmlinux symlink to headers directory
> 
>   debian/rules.d/2-binary-arch.mk | 5 ++++-
>   1 file changed, 4 insertions(+), 1 deletion(-)
> 

I'm working on a v2 that uses objcopy to generate a reduced-size (~10MB) 
vmlinux in the kernel headers with the necessary BTF information, rather 
than requiring dbgsyms to be installed, as noted here: 
https://lists.ubuntu.com/archives/kernel-team/2026-March/166404.html.