Existing verifier does not allow 'ctx + const + const'.  However, due to
compiler optimization, there is a case where BPF compilerit generates
'ctx + const + 0', as shown below:

  599: (1d) if r2 == r4 goto pc+2
   R0=inv(id=0) R1=ctx(id=0,off=40,imm=0)
   R2=inv(id=0,umax_value=4294967295,var_off=(0x0; 0xffffffff))
   R3=inv(id=0,umax_value=65535,var_off=(0x0; 0xffff)) R4=inv0
   R6=ctx(id=0,off=0,imm=0) R7=inv2
  600: (bf) r1 = r6			// r1 is ctx
  601: (07) r1 += 36			// r1 has offset 36
  602: (61) r4 = *(u32 *)(r1 +0)	// r1 + 0
  dereference of modified ctx ptr R1 off=36+0, ctx+const is allowed,
  ctx+const+const is not

The reason for BPF backend generating this code is due optimization
likes this, explained from Yonghong:
    if (...)
        *(ctx + 60)
    else
        *(ctx + 56)

The compiler translates it to
    if (...)
       ptr = ctx + 60
    else
       ptr = ctx + 56
    *(ptr + 0)

So load ptr memory become an example of 'ctx + const + 0'.  This patch
enables support for this case.

Fixes: f8ddadc4db6c7 ("Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net")
Cc: Yonghong Song <redacted>
Signed-off-by: Yifeng Sun <redacted>
Signed-off-by: William Tu <redacted>
---
 kernel/bpf/verifier.c                       |  2 +-
 tools/testing/selftests/bpf/test_verifier.c | 13 +++++++++++++
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 712d8655e916..c9a791b9cf2a 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c

@@ -1638,7 +1638,7 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, u32 regn
 		/* ctx accesses must be at a fixed offset, so that we can
 		 * determine what type of data were returned.
 		 */
-		if (reg->off) {
+		if (reg->off && off != reg->off) {
 			verbose(env,
 				"dereference of modified ctx ptr R%d off=%d+%d, ctx+const is allowed, ctx+const+const is not\n",
 				regno, reg->off, off - reg->off);

diff --git a/tools/testing/selftests/bpf/test_verifier.c b/tools/testing/selftests/bpf/test_verifier.c
index 1acafe26498b..95ad5d5723ae 100644
--- a/tools/testing/selftests/bpf/test_verifier.c
+++ b/tools/testing/selftests/bpf/test_verifier.c

@@ -8452,6 +8452,19 @@ static struct bpf_test tests[] = {
 		.prog_type = BPF_PROG_TYPE_SCHED_CLS,
 	},
 	{
+		"arithmetic ops make PTR_TO_CTX + const + 0 valid",
+		.insns = {
+			BPF_ALU64_IMM(BPF_ADD, BPF_REG_1,
+				      offsetof(struct __sk_buff, data) -
+				      offsetof(struct __sk_buff, mark)),
+			BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_1, 0),