On 1/12/21 7:41 AM, Andrii Nakryiko wrote:
On Mon, Jan 11, 2021 at 5:16 PM Yonghong Song <yhs@xxxxxx> wrote:
On 1/11/21 12:51 PM, Andrii Nakryiko wrote:
On Mon, Jan 11, 2021 at 10:13 AM Yonghong Song <yhs@xxxxxx> wrote:
On 1/9/21 11:03 PM, Andrii Nakryiko wrote:
Empty BTFs do come up (e.g., simple kernel modules with no new types and
strings, compared to the vmlinux BTF) and there is nothing technically wrong
with them. So remove unnecessary check preventing loading empty BTFs.
Reported-by: Christopher William Snowhill <chris@xxxxxxxxxx>
Fixes: ("d8123624506c libbpf: Fix BTF data layout checks and allow empty BTF")
Fixed up Fixes tag ^^^^^ while applying. ;-)
Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
---
tools/lib/bpf/btf.c | 5 -----
1 file changed, 5 deletions(-)
diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
index 3c3f2bc6c652..9970a288dda5 100644
--- a/tools/lib/bpf/btf.c
+++ b/tools/lib/bpf/btf.c
@@ -240,11 +240,6 @@ static int btf_parse_hdr(struct btf *btf)
}
meta_left = btf->raw_size - sizeof(*hdr);
- if (!meta_left) {
- pr_debug("BTF has no data\n");
- return -EINVAL;
- }
Previous kernel patch allows empty btf only if that btf is module (not
base/vmlinux) btf. Here it seems we allow any empty non-module btf to be
loaded into the kernel. In such cases, loading may fail? Maybe we should
detect such cases in libbpf and error out instead of going to kernel and
get error back?
I did this consciously. Kernel is more strict, because there is no
reasonable case when vmlinux BTF or BPF program's BTF can be empty (at
least not that now we have FUNCs in BTF). But allowing libbpf to load
empty BTF generically is helpful for bpftool, as one example, for
inspection. If you do `bpftool btf dump` on empty BTF, it will just
print nothing and you'll know that it's a valid (from BTF header
perspective) BTF, just doesn't have any types (besides VOID). If we
don't allow it, then we'll just get an error and then you'll have to
do painful hex dumping and decoding to see what's wrong.
It is totally okay to allow empty btf in libbpf. I just want to check
if this btf is going to be loaded into the kernel, right before it is
loading whether libbpf could check whether it is a non-module empty btf
or not, if it is, do not go to kernel.
Ok, I see what you are proposing. We can do that, but it's definitely
separate from these bug fixes. But, to be honest, I wouldn't bother
because libbpf will return BTF verification log with a very readable
"No data" message in it.
Right, seems okay to me for this particular case given the user will be
able to make some sense of it from the log.
Thanks,
Daniel
