On Fri, Jun 06, 2025 at 09:20:57AM -0700, Andrii Nakryiko wrote:
> On Fri, Jun 6, 2025 at 9:14 AM Leo Yan <leo.yan@xxxxxxx> wrote:
> > 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,
Right, it has to work with older kernels, etc.
> and it clearly doesn't.
> We need something like this:
> +++ 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.
And the only user of the above function is:
btf = btf_parse_raw_mmap(sysfs_btf_path, NULL);
if (IS_ERR(btf))
btf = btf__parse(sysfs_btf_path, NULL);
That expects ERR_PTR() to then use IS_ERR().
I think this could be automated with something like coccinnele(sp)?
Anyway, I tested the patch above and it seems to fix the issue, so:
Reported-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Reviewed-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Tested-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
- Arnaldo
> 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
