selftests/bpf: Fix raw_tp null handling test

Commit b2fc4b1, backport of upstream commit 838a10b ("bpf:
Augment raw_tp arguments with PTR_MAYBE_NULL"), was missing the changes
to tools/testing/selftests/bpf/progs/raw_tp_null.c, and cause the test
to fail with the following error (see link below for the complete log)

  Error: #205 raw_tp_null
  libbpf: prog 'test_raw_tp_null': BPF program load failed: Permission denied
  libbpf: prog 'test_raw_tp_null': -- BEGIN PROG LOAD LOG --
  0: R1=ctx() R10=fp0
  ; int BPF_PROG(test_raw_tp_null, struct sk_buff *skb) @ raw_tp_null.c:13
  0: (79) r6 = *(u64 *)(r1 +0)
  func 'bpf_testmod_test_raw_tp_null' arg0 has btf_id 2081 type STRUCT 'sk_buff'
  1: R1=ctx() R6_w=trusted_ptr_or_null_sk_buff(id=1)
  ; struct task_struct *task = bpf_get_current_task_btf(); @ raw_tp_null.c:15
  1: (85) call bpf_get_current_task_btf#158     ; R0_w=trusted_ptr_task_struct()
  ; if (task->pid != tid) @ raw_tp_null.c:17
  2: (61) r1 = *(u32 *)(r0 +1416)       ; R0_w=trusted_ptr_task_struct() R1_w=scalar(smin=0,smax=umax=0xffffffff,var_off=(0x0; 0xffffffff))
  3: (18) r2 = 0xffffa3bb801c6000       ; R2_w=map_value(map=raw_tp_n.bss,ks=4,vs=8)
  5: (61) r2 = *(u32 *)(r2 +0)          ; R2_w=scalar(smin=0,smax=umax=0xffffffff,var_off=(0x0; 0xffffffff))
  6: (5e) if w1 != w2 goto pc+11        ; R1_w=scalar(smin=0,smax=umax=0xffffffff,var_off=(0x0; 0xffffffff)) R2_w=scalar(smin=0,smax=umax=0xffffffff,var_off=(0x0; 0xffffffff))
  ; i = i + skb->mark + 1; @ raw_tp_null.c:20
  7: (61) r2 = *(u32 *)(r6 +164)
  R6 invalid mem access 'trusted_ptr_or_null_'
  processed 7 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0
  -- END PROG LOAD LOG --
  libbpf: prog 'test_raw_tp_null': failed to load: -13
  libbpf: failed to load object 'raw_tp_null'
  libbpf: failed to load BPF skeleton 'raw_tp_null': -13
  test_raw_tp_null:FAIL:raw_tp_null__open_and_load unexpected error: -13

Bring the missing changes in to fix the test failure.

Link: https://github.com/shunghsiyu/libbpf/actions/runs/14522396622/job/40766998873
Fixes: b2fc4b1 ("bpf: Augment raw_tp arguments with PTR_MAYBE_NULL")
Signed-off-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: shunghsiyu

tools/testing/selftests/bpf/progs/raw_tp_null.c

@@ -3,6 +3,7 @@
 
 #include <vmlinux.h>
 #include <bpf/bpf_tracing.h>
+#include "bpf_misc.h"
 
 char _license[] SEC("license") = "GPL";
 
@@ -17,16 +18,14 @@ int BPF_PROG(test_raw_tp_null, struct sk_buff *skb)
 	if (task->pid != tid)
 		return 0;
 
-	i = i + skb->mark + 1;
-	/* The compiler may move the NULL check before this deref, which causes
-	 * the load to fail as deref of scalar. Prevent that by using a barrier.
+	/* If dead code elimination kicks in, the increment +=2 will be
+	 * removed. For raw_tp programs attaching to tracepoints in kernel
+	 * modules, we mark input arguments as PTR_MAYBE_NULL, so branch
+	 * prediction should never kick in.
 	 */
-	barrier();
-	/* If dead code elimination kicks in, the increment below will
-	 * be removed. For raw_tp programs, we mark input arguments as
-	 * PTR_MAYBE_NULL, so branch prediction should never kick in.
-	 */
-	if (!skb)
-		i += 2;
+	asm volatile ("%[i] += 1; if %[ctx] != 0 goto +1; %[i] += 2;"
+			: [i]"+r"(i)
+			: [ctx]"r"(skb)
+			: "memory");
 	return 0;
 }