bpf-restrict-fs fails to load without DYNAMIC_FTRACE_WITH_DIRECT_CALLS on arm64

[Alexei Starovoitov]

On Tue, Jun 10, 2025 at 7:05 PM Nathan Chancellor <nathan at kernel.org> wrote:
>
> On Tue, Jun 10, 2025 at 04:37:24PM -0700, Alexei Starovoitov wrote:
> > On Tue, Jun 10, 2025 at 4:24 PM Nathan Chancellor <nathan at kernel.org> wrote:
> > > I was able to figure out that enabling CONFIG_CFI_CLANG was the culprit
> > > for the change in behavior but it does not appear to be the root cause,
> > > as I can get the same error with GCC and the following diff (which
> > > happens with CFI_CLANG because of the CALL_OPS dependency):
> ...
> > > -       select HAVE_DYNAMIC_FTRACE_WITH_DIRECT_CALLS \
> > > -               if DYNAMIC_FTRACE_WITH_ARGS && DYNAMIC_FTRACE_WITH_CALL_OPS
> > >         select HAVE_DYNAMIC_FTRACE_WITH_CALL_OPS \
> > >                 if (DYNAMIC_FTRACE_WITH_ARGS && !CFI_CLANG && \
> > >                     (CC_IS_CLANG || !CC_OPTIMIZE_FOR_SIZE))
> > >
> ...
> > That's expected.
> > See how kernel/bpf/trampoline.c is using DYNAMIC_FTRACE_WITH_DIRECT_CALLS.
> >
> > Theoretically we can make bpf trampoline work without it,
> > but why bother? Just enable this config.
>
> As I note above, this is incompatible with CONFIG_CFI_CLANG, which is
> more important for my particular area of testing and maintenance. Since
> you note this is expected, I will just go back to ignoring the warning
> in my kernel logs :) thank you for the quick response!

Somebody probably needs to fix CFI_CLANG on arm64 then.
It's not clear to me why dynamic ftrace has to be disabled in such a case.
It's not disabled for CFI_CLANG on x86, right?