linux-kernel - [PATCH bpf-next v5 4/4] selftests/bpf: add testcase to verifier_bounds.c for BPF

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Date: Tue, 19 Dec 2023 21:48:00 +0800
From: Menglong Dong <menglong8.dong@...il.com>
To: andrii@...nel.org,
	eddyz87@...il.com,
	yonghong.song@...ux.dev,
	alexei.starovoitov@...il.com
Cc: ast@...nel.org,
	daniel@...earbox.net,
	john.fastabend@...il.com,
	martin.lau@...ux.dev,
	song@...nel.org,
	kpsingh@...nel.org,
	sdf@...gle.com,
	haoluo@...gle.com,
	jolsa@...nel.org,
	mykolal@...com,
	shuah@...nel.org,
	menglong8.dong@...il.com,
	bpf@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	linux-kselftest@...r.kernel.org
Subject: [PATCH bpf-next v5 4/4] selftests/bpf: add testcase to verifier_bounds.c for BPF_JNE

Add testcase for the logic that the verifier tracks the BPF_JNE for regs.
The assembly function "reg_not_equal_const()" and "reg_equal_const" that
we add is exactly converted from the following case:

  u32 a = bpf_get_prandom_u32();
  u64 b = 0;

  a %= 8;
  /* the "a > 0" here will be optimized to "a != 0" */
  if (a > 0) {
    /* now the range of a should be [1, 7] */
    bpf_skb_store_bytes(skb, 0, &b, a, 0);
  }

Signed-off-by: Menglong Dong <menglong8.dong@...il.com>
---
v5:
- add some comments to the function that we add
- add reg_not_equal_const()
---
 .../selftests/bpf/progs/verifier_bounds.c     | 62 +++++++++++++++++++
 1 file changed, 62 insertions(+)

diff --git a/tools/testing/selftests/bpf/progs/verifier_bounds.c b/tools/testing/selftests/bpf/progs/verifier_bounds.c
index ec430b71730b..960998f16306 100644
--- a/tools/testing/selftests/bpf/progs/verifier_bounds.c
+++ b/tools/testing/selftests/bpf/progs/verifier_bounds.c
@@ -1075,4 +1075,66 @@ l0_%=:	r0 = 0;						\
 	: __clobber_all);
 }
 
+SEC("tc")
+__description("bounds check with JMP_NE for reg edge")
+__success __retval(0)
+__naked void reg_not_equal_const(void)
+{
+	asm volatile ("					\
+	r6 = r1;					\
+	r1 = 0;						\
+	*(u64*)(r10 - 8) = r1;				\
+	call %[bpf_get_prandom_u32];			\
+	r4 = r0;					\
+	r4 &= 7;					\
+	if r4 != 0 goto l0_%=;				\
+	r0 = 0;						\
+	exit;						\
+l0_%=:	r1 = r6;					\
+	r2 = 0;						\
+	r3 = r10;					\
+	r3 += -8;					\
+	r5 = 0;						\
+	/* The 4th argument of bpf_skb_store_bytes is defined as \
+	 * ARG_CONST_SIZE, so 0 is not allowed. The 'r4 != 0' \
+	 * is providing us this exclusion of zero from initial \
+	 * [0, 7] range.				\
+	 */						\
+	call %[bpf_skb_store_bytes];			\
+	r0 = 0;						\
+	exit;						\
+"	:
+	: __imm(bpf_get_prandom_u32),
+	  __imm(bpf_skb_store_bytes)
+	: __clobber_all);
+}
+
+SEC("tc")
+__description("bounds check with JMP_EQ for reg edge")
+__success __retval(0)
+__naked void reg_equal_const(void)
+{
+	asm volatile ("					\
+	r6 = r1;					\
+	r1 = 0;						\
+	*(u64*)(r10 - 8) = r1;				\
+	call %[bpf_get_prandom_u32];			\
+	r4 = r0;					\
+	r4 &= 7;					\
+	if r4 == 0 goto l0_%=;				\
+	r1 = r6;					\
+	r2 = 0;						\
+	r3 = r10;					\
+	r3 += -8;					\
+	r5 = 0;						\
+	/* Just the same as what we do in reg_not_equal_const() */ \
+	call %[bpf_skb_store_bytes];			\
+l0_%=:	r0 = 0;						\
+	exit;						\
+"	:
+	: __imm(bpf_get_prandom_u32),
+	  __imm(bpf_skb_store_bytes)
+	: __clobber_all);
+}
+
 char _license[] SEC("license") = "GPL";
-- 
2.39.2