Re: Error: bug: failed to retrieve CAP_BPF status: Invalid argument

["Toke Høiland-Jørgensen" <toke@kernel.org>]

>Vincent Li <vincent.mc.li@gmail.com> writes:

> On Mon, Aug 15, 2022 at 5:05 PM Quentin Monnet <quentin@isovalent.com> wrote:
>>
>> On Mon, 15 Aug 2022 at 23:26, Vincent Li <vincent.mc.li@gmail.com> wrote:
>> >
>> > On Mon, Aug 15, 2022 at 3:18 PM Quentin Monnet <quentin@isovalent.com> wrote:
>> > >
>> > > Hi Vincent,
>> > >
>> > > On Mon, 15 Aug 2022 at 18:46, Vincent Li <vincent.mc.li@gmail.com> wrote:
>> > > >
>> > > > Hi,
>> > > >
>> > > > I compile and run kernel 5.18.0 in Centos 8 from bpf-next in my dev
>> > > > machine, I also compiled bpftool from bpf-next on same machine, when
>> > > > run bpftool on same machine, I got :
>> > > >
>> > > > ./bpftool feature probe
>> > > >
>> > > > Error: bug: failed to retrieve CAP_BPF status: Invalid argument
>> > > >
>> > > > where bpftool to retrieve CAP_BPF ? from running kernel or from somewhere else?
>> > >
>> > > Yes, bpftool calls cap_get_proc() to get the capabilities of the
>> > > current process. From what I understand of your output, it looks like
>> > > capget() returns CAP_BPF: I believe the "0x1c0" value at the end is
>> > > (1<<(CAP_CHECKPOINT_RESTORE-32)) + (1<<(CAP_BPF-32)) +
>> > > (1<<(CAP_PERFMON-32)). You could probably check this with a more
>> > > recent version of strace.
>> > >
>> > > Then assuming you do retrieve CAP_BPF from capget(), I don't know why
>> > > cap_get_flag() in bpftool fails to retrieve the capability state. It
>> > > would be worth running bpftool in GDB to check what happens. The check
>> > > in libcap is here [0] but I don't see where we would fail to provide
>> > > valid arguments. Just in case, could you please let me know what
>> > > version of libcap you're using when compiling bpftool?
>> >
>> > I think I installed libcap through centos distro
>> >
>> > [root@centos-dev ~]# rpm -qi libcap.x86_64
>> >
>> > Name        : libcap
>> >
>> > Version     : 2.26
>>
>> So we investigated this on Slack. The issue is related to libcap (and
>> to how libcap is built on CentOS); it is fixed in libcap 2.30 and
>> older.
>>
>> For the record, this is the commit that fixed it:
>> https://git.kernel.org/pub/scm/libs/libcap/libcap.git/commit/?id=f1f62a748d7c67361e91e32d26abafbfb03eeee4
>>
>> Before this, cap_get_flag() would compare its argument "value" (in our
>> case, CAP_BPF == 39) with __CAP_BITS. This __CAP_BITS constant is
>> defined in libcap/cap_names.h, generated by libcap/_makenames.c from
>> the list in libcap/cap_names.list.h. The latter header file is itself
>> generated in libcap/Makefile from the UAPI header at
>> $(KERNEL_HEADERS)/linux/capability.h, which defaults to the local
>> libcap/include/uapi/linux/capability.h.
>>
>> On your CentOS, the libcap version may have been compiled without
>> setting KERNEL_HEADERS to make it point to the correct system UAPI
>> header (or the header could be too old, but looking at it, it seems
>> that it does have CAP_BPF), in which case it defaulted to libcap's
>> version of the header, which in 2.26 stops at CAP_AUDIT (37). In that
>> case, __CAP_BITS is worth 37 and is lower than CAP_BPF, the check in
>> cap_get_flag() fails and we get -EINVAL.
>>
>> The commit referenced above changed the comparison for libcap 2.30+ to
>> compare "value" with __CAP_MAXBITS == 64 instead, which works
>> correctly.
>>
>> Thanks for the report and the shared debug session!
>> Quentin
>
> Thanks Quentin for your quick response and analysis :)

FYI, CAP_BPF should also be fixed in the version of libcap shipped with
RHEL8.5 (version libcap-2.26-5.el8). This should be available in CentOS
Stream as well, so just updating the package should be enough...

-Toke