[PATCH disco 3/3] bpf: improve verifier branch analysis

[Thadeu Lima de Souza Cascardo]

From: Alexei Starovoitov <ast at kernel.org>

pathological bpf programs may try to force verifier to explode in
the number of branch states:
  20: (d5) if r1 s<= 0x24000028 goto pc+0
  21: (b5) if r0 <= 0xe1fa20 goto pc+2
  22: (d5) if r1 s<= 0x7e goto pc+0
  23: (b5) if r0 <= 0xe880e000 goto pc+0
  24: (c5) if r0 s< 0x2100ecf4 goto pc+0
  25: (d5) if r1 s<= 0xe880e000 goto pc+1
  26: (c5) if r0 s< 0xf4041810 goto pc+0
  27: (d5) if r1 s<= 0x1e007e goto pc+0
  28: (b5) if r0 <= 0xe86be000 goto pc+0
  29: (07) r0 += 16614
  30: (c5) if r0 s< 0x6d0020da goto pc+0
  31: (35) if r0 >= 0x2100ecf4 goto pc+0

Teach verifier to recognize always taken and always not taken branches.
This analysis is already done for == and != comparison.
Expand it to all other branches.

It also helps real bpf programs to be verified faster:
                       before  after
bpf_lb-DLB_L3.o         2003    1940
bpf_lb-DLB_L4.o         3173    3089
bpf_lb-DUNKNOWN.o       1080    1065
bpf_lxc-DDROP_ALL.o     29584   28052
bpf_lxc-DUNKNOWN.o      36916   35487
bpf_netdev.o            11188   10864
bpf_overlay.o           6679    6643
bpf_lcx_jit.o           39555   38437

Reported-by: Anatoly Trosinenko <anatoly.trosinenko at gmail.com>
Signed-off-by: Alexei Starovoitov <ast at kernel.org>
Acked-by: Daniel Borkmann <daniel at iogearbox.net>
Acked-by: Edward Cree <ecree at solarflare.com>
Signed-off-by: Daniel Borkmann <daniel at iogearbox.net>
(cherry picked from commit 4f7b3e82589e0de723780198ec7983e427144c0a)
[cascardo: really just pick the test parts, as this was already applied,
but without the test parts]
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo at canonical.com>
---
 tools/testing/selftests/bpf/test_verifier.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/bpf/test_verifier.c b/tools/testing/selftests/bpf/test_verifier.c
index e6c41217c7fa..65d3bcb95835 100644
--- a/tools/testing/selftests/bpf/test_verifier.c
+++ b/tools/testing/selftests/bpf/test_verifier.c
@@ -7854,7 +7854,7 @@ static struct bpf_test tests[] = {
 			BPF_JMP_IMM(BPF_JA, 0, 0, -7),
 		},
 		.fixup_map1 = { 4 },
-		.errstr = "R0 invalid mem access 'inv'",
+		.errstr = "unbounded min value",
 		.errstr_unpriv = "R1 has unknown scalar with mixed signed bounds",
 		.result = REJECT,
 	},
@@ -9831,7 +9831,7 @@ static struct bpf_test tests[] = {
 		"check deducing bounds from const, 5",
 		.insns = {
 			BPF_MOV64_IMM(BPF_REG_0, 0),
-			BPF_JMP_IMM(BPF_JSGE, BPF_REG_0, 0, 1),
+			BPF_JMP_IMM(BPF_JSGE, BPF_REG_0, 1, 1),
 			BPF_ALU64_REG(BPF_SUB, BPF_REG_0, BPF_REG_1),
 			BPF_EXIT_INSN(),
 		},
-- 
2.20.1