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From:   Youlin Li <liulin063@gmail.com>
To:     ast@kernel.org
Cc:     daniel@iogearbox.net, john.fastabend@gmail.com, andrii@kernel.org,
        martin.lau@linux.dev, song@kernel.org, yhs@fb.com,
        kpsingh@kernel.org, sdf@google.com, haoluo@google.com,
        jolsa@kernel.org, bpf@vger.kernel.org,
        linux-kernel@vger.kernel.org, Youlin Li <liulin063@gmail.com>
Subject: [PATCH bpf] bpf: Do more tight ALU bounds tracking
Date:   Fri, 29 Jul 2022 12:43:17 +0800
Message-Id: <20220729044317.31975-1-liulin063@gmail.com>
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32bit bounds and 64bit bounds are updated separately in
adjust_scalar_min_max_vals() currently, let them learn from each other to
get more tight bounds tracking. Similar operation can be found in
reg_set_min_max().

Before:

    func#0 @0
    0: R1=ctx(off=0,imm=0) R10=fp0
    0: (b7) r0 = 0                        ; R0_w=0
    1: (b7) r1 = 0                        ; R1_w=0
    2: (87) r1 = -r1                      ; R1_w=scalar()
    3: (87) r1 = -r1                      ; R1_w=scalar()
    4: (c7) r1 s>>= 63                    ; R1_w=scalar(smin=-1,smax=0)
    5: (07) r1 += 2                       ; R1_w=scalar(umin=1,umax=2,var_off=(0x0; 0xffffffff))  <--- [*]
    6: (95) exit

It can be seen that even if the 64bit bounds is clear here, the 32bit
bounds is still in the state of 'UNKNOWN'.

After:

    func#0 @0
    0: R1=ctx(off=0,imm=0) R10=fp0
    0: (b7) r0 = 0                        ; R0_w=0
    1: (b7) r1 = 0                        ; R1_w=0
    2: (87) r1 = -r1                      ; R1_w=scalar()
    3: (87) r1 = -r1                      ; R1_w=scalar()
    4: (c7) r1 s>>= 63                    ; R1_w=scalar(smin=-1,smax=0)
    5: (07) r1 += 2                       ; R1_w=scalar(umin=1,umax=2,var_off=(0x0; 0x3))  <--- [*]
    6: (95) exit

Fixes: 3f50f132d840 ("bpf: Verifier, do explicit ALU32 bounds tracking")
Signed-off-by: Youlin Li <liulin063@gmail.com>
---
 kernel/bpf/verifier.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 0efbac0fd126..888aa50fbdc0 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -8934,10 +8934,13 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env,
 		break;
 	}
 
-	/* ALU32 ops are zero extended into 64bit register */
-	if (alu32)
+	if (alu32) {
+		/* ALU32 ops are zero extended into 64bit register */
 		zext_32_to_64(dst_reg);
-	reg_bounds_sync(dst_reg);
+		__reg_combine_32_into_64(dst_reg);
+	} else {
+		__reg_combine_64_into_32(dst_reg);
+	}
 	return 0;
 }
 
-- 
2.25.1