Re: BTF loading failing on perf

[Leo Yan <leo.yan@arm.com>]

Hi Arnaldo,

On Fri, Jun 06, 2025 at 12:37:55PM -0300, Arnaldo Carvalho de Melo wrote:
> root@number:~# perf trace -e openat --max-events=1
> libbpf: failed to read kernel BTF from '/sys/kernel/btf/vmlinux': -ENODEV
> libbpf: failed to read kernel BTF from '/sys/kernel/btf/vmlinux': -ENODEV
>      0.000 ( 0.016 ms): ptyxis-agent/4375 openat(dfd: CWD, filename: "/proc/6593/cmdline", flags: RDONLY|CLOEXEC) = 13
> root@number:~#
> 
> openat(AT_FDCWD, "/sys/kernel/btf/vmlinux", O_RDONLY) = 258
> mmap(NULL, 6519699, PROT_READ, MAP_PRIVATE, 258, 0) = -1 ENODEV (No such device)
> libbpf: failed to read kernel BTF from '/sys/kernel/btf/vmlinux': -ENODEV

Have you included the commit below in the kernel side?

commit a539e2a6d51d1c12d89eec149ccc72ec561639bc
Author: Lorenz Bauer <lmb@isovalent.com>
Date:   Tue May 20 14:01:17 2025 +0100

    btf: Allow mmap of vmlinux btf
    
    User space needs access to kernel BTF for many modern features of BPF.
    Right now each process needs to read the BTF blob either in pieces or
    as a whole. Allow mmaping the sysfs file so that processes can directly
    access the memory allocated for it in the kernel.
    
    remap_pfn_range is used instead of vm_insert_page due to aarch64
    compatibility issues.
    
    Signed-off-by: Lorenz Bauer <lmb@isovalent.com>
    Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
    Tested-by: Alan Maguire <alan.maguire@oracle.com>
    Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev>
    Link: https://lore.kernel.org/bpf/20250520-vmlinux-mmap-v5-1-e8c941acc414@isovalent.com

Thanks,
Leo