Re: [bpf-next v3 11/12] bpf: do check_nocsr_stack_contract() for ARG_ANYTHING helper params

[Eduard Zingerman <eddyz87@gmail.com>]

On Mon, 2024-07-15 at 19:00 -0700, Alexei Starovoitov wrote:
> On Mon, Jul 15, 2024 at 4:02 PM Eduard Zingerman <eddyz87@gmail.com> wrote:

[...]

> > This might lead to a surprising behavior in combination with nocsr
> > rewrites, e.g. consider the program below:
> > 
> >      1: r1 = 1;
> >         /* nocsr pattern with stack offset -16 */
> >      2: *(u64 *)(r10 - 16) = r1;
> >      3: call %[bpf_get_smp_processor_id];
> >      4: r1 = *(u64 *)(r10 - 16);
> >      5: r1 = r10;
> >      6: r1 += -8;
> >      7: r2 = 1;
> >      8: r3 = r10;
> >      9: r3 += -16;
> >         /* bpf_probe_read_kernel(dst: &fp[-8], size: 1, src: &fp[-16]) */
> >     10: call %[bpf_probe_read_kernel];
> >     11: exit;
> > 
> > Here nocsr rewrite logic would remove instructions (2) and (4).
> > However, (2) writes a value that is later read by a call at (10).
> 
> This makes no sense to me.
> This bpf prog is broken.
> If probe_read is used to read stack it will read garbage.
> JITs and the verifier are allowed to do any transformation
> that keeps the program semantics and safety.

I tried to run the following program
(should have run it earlier):

SEC("raw_tp")
__retval(42)
__success
int bpf_probe_read_kernel_stack_ptr(void *ctx)
{
	unsigned long a = 17;
	unsigned long b = 42;
	int err;

	err = bpf_probe_read_kernel(&a, 8, &b);
	if (err)
		return -1;
	return a;
}

And indeed, it does not produce expected result,
the retval varies around 22079.
However, I don't really understand why this program is broken.
E.g. from C compiler pov pointer &b escapes, and compiler is not
really allowed to replace object at that offset with garbage.
Is it a limitation of the bpf_probe_read_kernel() that it cannot read
BPF program stack?