BTF loading failing on perf

[Arnaldo Carvalho de Melo <acme@kernel.org>]

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

root@number:~# ls -la /sys/kernel/btf/vmlinux 
-r--r--r--. 1 root root 6519699 Jun  6 12:19 /sys/kernel/btf/vmlinux
root@number:~# uname -a
Linux number 6.14.9-300.fc42.x86_64 #1 SMP PREEMPT_DYNAMIC Thu May 29 14:27:53 UTC 2025 x86_64 GNU/Linux
root@number:~# head /etc/os-release 
NAME="Fedora Linux"
VERSION="42 (Workstation Edition)"
RELEASE_TYPE=stable
ID=fedora
VERSION_ID=42
VERSION_CODENAME=""
PLATFORM_ID="platform:f42"
PRETTY_NAME="Fedora Linux 42 (Workstation Edition)"
ANSI_COLOR="0;38;2;60;110;180"
LOGO=fedora-logo-icon
root@number:~# rpm -q glibc-devel
package glibc-devel is not installed
root@number:~# rpm -q glibc
glibc-2.41-5.fc42.x86_64
root@number:~# 

Reverting the patch below "cures" the problem.

⬢ [acme@toolbx perf-tools]$ git log --oneline -10 tools/lib/bpf/
370118ff875244d4 (HEAD -> perf-tools) Revert "libbpf: Use mmap to parse vmlinux BTF from sysfs"
90b83efa6701656e Merge tag 'bpf-next-6.16' of git://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next
3c0421c93ce4ff0f libbpf: Use mmap to parse vmlinux BTF from sysfs
4e29128a9acec2a6 libbpf/btf: Fix string handling to support multi-split BTF
d0445d7dd3fd9b15 libbpf: Check bpf_map_skeleton link for NULL
fd5fd538a1f4b34c libbpf: Use proper errno value in nlattr
62e23f183839c3d7 libbpf: Improve BTF dedup handling of "identical" BTF types
41d4ce6df3f49453 bpf: Fix uninitialized values in BPF_{CORE,PROBE}_READ
358b1c0f56ebb699 libbpf: Use proper errno value in linker
8e64c387c942229c libbpf: Add identical pointer detection to btf_dedup_is_equiv()
⬢ [acme@toolbx perf-tools]$

root@number:~# perf -v
perf version 6.15.g370118ff8752
root@number:~# perf trace -e connect --max-events=5
     0.000 ( 0.021 ms): DNS Res~ver #4/7932 connect(fd: 102, uservaddr: { .family: LOCAL, path: /run/systemd/r }, addrlen: 42) = 0
     0.304 ( 0.013 ms): systemd-resolv/1420 connect(fd: 25, uservaddr: { .family: INET, port: 53, addr: 186.208.78.194 }, addrlen: 16) = 0
     0.371 ( 0.002 ms): systemd-resolv/1420 connect(fd: 26, uservaddr: { .family: INET, port: 53, addr: 186.208.78.194 }, addrlen: 16) = 0
     0.591 ( 0.009 ms): DNS Res~ver #2/7523 connect(fd: 104, uservaddr: { .family: LOCAL, path: /run/systemd/r }, addrlen: 42) = 0
     0.731 ( 0.006 ms): systemd-resolv/1420 connect(fd: 28, uservaddr: { .family: INET, port: 53, addr: 186.208.78.194 }, addrlen: 16) = 0
root@number:~# 

Ideas?

- Arnaldo

⬢ [acme@toolbx perf-tools]$ git bisect good
3c0421c93ce4ff0f5f2612666122c34fc941d569 is the first bad commit
commit 3c0421c93ce4ff0f5f2612666122c34fc941d569
Author: Lorenz Bauer <lmb@isovalent.com>
Date:   Tue May 20 14:01:19 2025 +0100

    libbpf: Use mmap to parse vmlinux BTF from sysfs
    
    Teach libbpf to use mmap when parsing vmlinux BTF from /sys. We don't
    apply this to fall-back paths on the regular file system because there
    is no way to ensure that modifications underlying the MAP_PRIVATE
    mapping are not visible to the process.
    
    Signed-off-by: Lorenz Bauer <lmb@isovalent.com>
    Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
    Tested-by: Alan Maguire <alan.maguire@oracle.com>
    Acked-by: Andrii Nakryiko <andrii@kernel.org>
    Link: https://lore.kernel.org/bpf/20250520-vmlinux-mmap-v5-3-e8c941acc414@isovalent.com

 tools/lib/bpf/btf.c | 89 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++------------------
 1 file changed, 71 insertions(+), 18 deletions(-)
⬢ [acme@toolbx perf-tools]$