6.12-stable review patch. If anyone has any objections, please let me know.
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From: Paul Chaignon paul.chaignon@gmail.com
[ Upstream commit f41345f47fb267a9c95ca710c33448f8d0d81d83 ]
In the following toy program (reg states minimized for readability), R0 and R1 always have different values at instruction 6. This is obvious when reading the program but cannot be guessed from ranges alone as they overlap (R0 in [0; 0xc0000000], R1 in [1024; 0xc0000400]).
0: call bpf_get_prandom_u32#7 ; R0_w=scalar() 1: w0 = w0 ; R0_w=scalar(var_off=(0x0; 0xffffffff)) 2: r0 >>= 30 ; R0_w=scalar(var_off=(0x0; 0x3)) 3: r0 <<= 30 ; R0_w=scalar(var_off=(0x0; 0xc0000000)) 4: r1 = r0 ; R1_w=scalar(var_off=(0x0; 0xc0000000)) 5: r1 += 1024 ; R1_w=scalar(var_off=(0x400; 0xc0000000)) 6: if r1 != r0 goto pc+1
Looking at tnums however, we can deduce that R1 is always different from R0 because their tnums don't agree on known bits. This patch uses this logic to improve is_scalar_branch_taken in case of BPF_JEQ and BPF_JNE.
This change has a tiny impact on complexity, which was measured with the Cilium complexity CI test. That test covers 72 programs with various build and load time configurations for a total of 970 test cases. For 80% of test cases, the patch has no impact. On the other test cases, the patch decreases complexity by only 0.08% on average. In the best case, the verifier needs to walk 3% less instructions and, in the worst case, 1.5% more. Overall, the patch has a small positive impact, especially for our largest programs.
Signed-off-by: Paul Chaignon paul.chaignon@gmail.com Signed-off-by: Daniel Borkmann daniel@iogearbox.net Acked-by: Eduard Zingerman eddyz87@gmail.com Acked-by: Shung-Hsi Yu shung-hsi.yu@suse.com Acked-by: Daniel Borkmann daniel@iogearbox.net Link: https://lore.kernel.org/bpf/be3ee70b6e489c49881cb1646114b1d861b5c334.1755694... Signed-off-by: Sasha Levin sashal@kernel.org --- include/linux/tnum.h | 3 +++ kernel/bpf/tnum.c | 8 ++++++++ kernel/bpf/verifier.c | 4 ++++ 3 files changed, 15 insertions(+)
diff --git a/include/linux/tnum.h b/include/linux/tnum.h index 3c13240077b87..191baf3a8d832 100644 --- a/include/linux/tnum.h +++ b/include/linux/tnum.h @@ -49,6 +49,9 @@ struct tnum tnum_xor(struct tnum a, struct tnum b); /* Multiply two tnums, return @a * @b */ struct tnum tnum_mul(struct tnum a, struct tnum b);
+/* Return true if the known bits of both tnums have the same value */ +bool tnum_overlap(struct tnum a, struct tnum b); + /* Return a tnum representing numbers satisfying both @a and @b */ struct tnum tnum_intersect(struct tnum a, struct tnum b);
diff --git a/kernel/bpf/tnum.c b/kernel/bpf/tnum.c index 9dbc31b25e3d0..8e6d0ac713731 100644 --- a/kernel/bpf/tnum.c +++ b/kernel/bpf/tnum.c @@ -138,6 +138,14 @@ struct tnum tnum_mul(struct tnum a, struct tnum b) return tnum_add(TNUM(acc_v, 0), acc_m); }
+bool tnum_overlap(struct tnum a, struct tnum b) +{ + u64 mu; + + mu = ~a.mask & ~b.mask; + return (a.value & mu) == (b.value & mu); +} + /* Note that if a and b disagree - i.e. one has a 'known 1' where the other has * a 'known 0' - this will return a 'known 1' for that bit. */ diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 709151d33e5e4..218c238d61398 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -14772,6 +14772,8 @@ static int is_scalar_branch_taken(struct bpf_reg_state *reg1, struct bpf_reg_sta */ if (tnum_is_const(t1) && tnum_is_const(t2)) return t1.value == t2.value; + if (!tnum_overlap(t1, t2)) + return 0; /* non-overlapping ranges */ if (umin1 > umax2 || umax1 < umin2) return 0; @@ -14796,6 +14798,8 @@ static int is_scalar_branch_taken(struct bpf_reg_state *reg1, struct bpf_reg_sta */ if (tnum_is_const(t1) && tnum_is_const(t2)) return t1.value != t2.value; + if (!tnum_overlap(t1, t2)) + return 1; /* non-overlapping ranges */ if (umin1 > umax2 || umax1 < umin2) return 1;