libbpf: Fix unbounded memory access in bpf_usdt_arg()

Loading programs that use bpf_usdt_arg() on s390x fails with:

    ; if (arg_num >= BPF_USDT_MAX_ARG_CNT || arg_num >= spec->arg_cnt)
    128: (79) r1 = *(u64 *)(r10 -24)      ; frame1: R1_w=scalar(umax=4294967295,var_off=(0x0; 0xffffffff)) R10=fp0
    129: (25) if r1 > 0xb goto pc+83      ; frame1: R1_w=scalar(umax=11,var_off=(0x0; 0xf))
    ...
    ; arg_spec = &spec->args[arg_num];
    135: (79) r1 = *(u64 *)(r10 -24)      ; frame1: R1_w=scalar(umax=4294967295,var_off=(0x0; 0xffffffff)) R10=fp0
    ...
    ; switch (arg_spec->arg_type) {
    139: (61) r1 = *(u32 *)(r2 +8)
    R2 unbounded memory access, make sure to bounds check any such access

The reason is that, even though the C code enforces that
arg_num < BPF_USDT_MAX_ARG_CNT, the verifier cannot propagate this
constraint to the arg_spec assignment yet. Help it by forcing r1 back
to stack after comparison.
Signed-off-by: NIlya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20230128000650.1516334-23-iii@linux.ibm.comSigned-off-by: NAlexei Starovoitov <ast@kernel.org>

tools/lib/bpf/usdt.bpf.h

@@ -130,7 +130,10 @@ int bpf_usdt_arg(struct pt_regs *ctx, __u64 arg_num, long *res)
 	if (!spec)
 		return -ESRCH;
 
-	if (arg_num >= BPF_USDT_MAX_ARG_CNT || arg_num >= spec->arg_cnt)
+	if (arg_num >= BPF_USDT_MAX_ARG_CNT)
+		return -ENOENT;
+	barrier_var(arg_num);
+	if (arg_num >= spec->arg_cnt)
 		return -ENOENT;
 
 	arg_spec = &spec->args[arg_num];