On 8/8/25 5:49 PM, Quentin Monnet wrote:
Please run ./scripts/get_maintainer.pl and Cc all maintainers for your
future submissions, in this case: all BPF maintainers/reviewers.
On 08/08/2025 15:51, Vincent Li wrote:
from bpftool github repo issue [0], when Linux distribution
kernel.kptr_restrict is set to 2, bpftool prog dump jited returns "no
instructions returned", this message can be puzzling to bpftool users
who is not familiar with kernel BPF internal, so add small hint for
bpftool users to check kernel.kptr_restrict setting. Set
kernel.kptr_restrict to expose kernel address to allow bpftool prog
dump jited to dump the jited bpf program instructions.
[0]: https://github.com/libbpf/bpftool/issues/184
Signed-off-by: Vincent Li <vincent.mc.li@xxxxxxxxx.
---
tools/bpf/bpftool/prog.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/tools/bpf/bpftool/prog.c b/tools/bpf/bpftool/prog.c
index 9722d841abc0..7d2337511284 100644
--- a/tools/bpf/bpftool/prog.c
+++ b/tools/bpf/bpftool/prog.c
@@ -714,7 +714,7 @@ prog_dump(struct bpf_prog_info *info, enum dump_mode mode,
if (mode == DUMP_JITED) {
if (info->jited_prog_len == 0 || !info->jited_prog_insns) {
- p_info("no instructions returned");
+ p_info("no instructions returned: set kernel.kptr_restrict to expose kernel addresses");
Can we align this to sth similar as we have further below in that same function?
p_err("error retrieving jit dump: no instructions returned or kernel.kptr_restrict set?");
(I presume the former we'll see for interpreter-only case.)
return -1;
}
buf = u64_to_ptr(info->jited_prog_insns);
Thank you Vincent!
We have the same hint for the xlated dump some 7 lines further in the
file. As we discussed off-list, this hint was initially printed for both
cases, JITed and xlated dump, since commit 7105e828c087 ("bpf: allow for
correlation of maps and helpers in dump") from Daniel, back in 2017. It
was kept for the xlated dump only after commit cae73f233923 ("bpftool:
use bpf_program__get_prog_info_linear() in prog.c:do_dump()"), I believe
by accident.
From what I understand, the kptr restriction should not be relevant in
the case of xlated dump (it does change the information we can print -
it prevents us from retrieving __bpf_call_base from ksyms - but should
not prevent bpftool from retrieving instructions entirely). Daniel, it's
been a while, but do you remember why you printed it for xlated dumps
too? If not, we should probably just keep the hint for the JITed case.
Indeed been a while - my understanding is that we don't return xlated in
case of not having the right capabilities or when the program got constant
blinded so we don't want to expose the latter from the kernel back to user
space when kptr_restrict is set.
