On Fri, Jun 6, 2025 at 9:14 AM Leo Yan <leo.yan@xxxxxxx> wrote:
>
> 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?
It doesn't matter, libbpf should silently fallback to non-mmap() way,
and it clearly doesn't.
We need something like this:
diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
index f1d495dc66bb..37682908cb0f 100644
--- a/tools/lib/bpf/btf.c
+++ b/tools/lib/bpf/btf.c
@@ -1384,12 +1384,12 @@ static struct btf *btf_parse_raw_mmap(const
char *path, struct btf *base_btf)
fd = open(path, O_RDONLY);
if (fd < 0)
- return libbpf_err_ptr(-errno);
+ return ERR_PTR(-errno);
if (fstat(fd, &st) < 0) {
err = -errno;
close(fd);
- return libbpf_err_ptr(err);
+ return ERR_PTR(err);
}
data = mmap(NULL, st.st_size, PROT_READ, MAP_PRIVATE, fd, 0);
@@ -1397,7 +1397,7 @@ static struct btf *btf_parse_raw_mmap(const char
*path, struct btf *base_btf)
close(fd);
if (data == MAP_FAILED)
- return libbpf_err_ptr(err);
+ return ERR_PTR(err);
btf = btf_new(data, st.st_size, base_btf, true);
if (IS_ERR(btf))
libbpf_err_ptr() should be used for user-facing API functions, they
return NULL on error and set errno, so checking for IS_ERR() is wrong
here.
Lorenz, can you please test and send a proper fix ASAP?
>
> commit a539e2a6d51d1c12d89eec149ccc72ec561639bc
> Author: Lorenz Bauer <lmb@xxxxxxxxxxxxx>
> 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@xxxxxxxxxxxxx>
> Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
> Tested-by: Alan Maguire <alan.maguire@xxxxxxxxxx>
> Reviewed-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>
> Link: https://lore.kernel.org/bpf/20250520-vmlinux-mmap-v5-1-e8c941acc414@xxxxxxxxxxxxx
>
> Thanks,
> Leo
