Re: [PATCH v2 2/2] x86/cfi,bpf: Fix BPF JIT call

Peter Zijlstra
2023-12-08T20:52:55Z

On Fri, Dec 08, 2023 at 12:41:03PM -0800, Alexei Starovoitov wrote:
> On Fri, Dec 8, 2023 at 12:35 PM Peter Zijlstra <peterz@infradead.org> wrote:

> > -__bpf_kfunc void bpf_task_release(struct task_struct *p)
> > +__bpf_kfunc void bpf_task_release(void *p)
>
> Yeah. That won't work. We need a wrapper.
> Since bpf prog is also calling it directly.
> In progs/task_kfunc_common.h
> void bpf_task_release(struct task_struct *p) __ksym;
>
> than later both libbpf and the verifier check that
> what bpf prog is calling actually matches the proto
> of what is in the kernel.
> Effectively we're doing strong prototype check at load time.

I'm still somewhat confused on how this works, where does BPF get the
address of the function from? and what should I call the wrapper?

> btw instead of EXPORT_SYMBOL_GPL(bpf_task_release)
> can __ADDRESSABLE be used ?
> Since it's not an export symbol.

No __ADDRESSABLE() is expressly ignored, but we have IBT_NOSEAL() that
should do it. I'll rename the thing and lift it out of x86 to avoid
breaking all other arch builds.