[PATCH bpf] selftests: bpf: check map in map pruning

[Lorenz Bauer]

Ensure that two registers with a map_value loaded from a nested
map are considered equivalent for the purpose of state pruning
and don't cause the verifier to revisit a pruning point.

This uses a rather crude match on the number of insns visited by
the verifier, which might change in the future. I've therefore
tried to keep the code as "unpruneable" as possible by having
the code paths only converge on the second to last instruction.

Should you require to adjust the test in the future, reducing the
number of processed instructions should always be safe. Increasing
them could cause another regression, so proceed with caution.

Suggested-by: Alexei Starovoitov <ast@xxxxxxxxxx>
Signed-off-by: Lorenz Bauer <lmb@xxxxxxxxxxxxxx>
Link: https://lore.kernel.org/bpf/CACAyw99hVEJFoiBH_ZGyy=+oO-jyydoz6v1DeKPKs2HVsUH28w@xxxxxxxxxxxxxx/
---
 .../selftests/bpf/verifier/map_in_map.c       | 33 +++++++++++++++++++
 1 file changed, 33 insertions(+)

diff --git a/tools/testing/selftests/bpf/verifier/map_in_map.c b/tools/testing/selftests/bpf/verifier/map_in_map.c
index 2798927ee9ff..f46c7121e216 100644
--- a/tools/testing/selftests/bpf/verifier/map_in_map.c
+++ b/tools/testing/selftests/bpf/verifier/map_in_map.c
@@ -18,6 +18,39 @@
 	.fixup_map_in_map = { 3 },
 	.result = ACCEPT,
 },
+{
+	"map in map state pruning",
+	.insns = {
+	BPF_ST_MEM(0, BPF_REG_10, -4, 0),
+	BPF_MOV64_REG(BPF_REG_6, BPF_REG_10),
+	BPF_ALU64_IMM(BPF_ADD, BPF_REG_6, -4),
+	BPF_MOV64_REG(BPF_REG_2, BPF_REG_6),
+	BPF_LD_MAP_FD(BPF_REG_1, 0),
+	BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem),
+	BPF_JMP_IMM(BPF_JNE, BPF_REG_0, 0, 1),
+	BPF_EXIT_INSN(),
+	BPF_MOV64_REG(BPF_REG_2, BPF_REG_6),
+	BPF_MOV64_REG(BPF_REG_1, BPF_REG_0),
+	BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem),
+	BPF_JMP_IMM(BPF_JNE, BPF_REG_0, 0, 11),
+	BPF_MOV64_REG(BPF_REG_2, BPF_REG_6),
+	BPF_LD_MAP_FD(BPF_REG_1, 0),
+	BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem),
+	BPF_JMP_IMM(BPF_JNE, BPF_REG_0, 0, 1),
+	BPF_EXIT_INSN(),
+	BPF_MOV64_REG(BPF_REG_2, BPF_REG_6),
+	BPF_MOV64_REG(BPF_REG_1, BPF_REG_0),
+	BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem),
+	BPF_JMP_IMM(BPF_JNE, BPF_REG_0, 0, 1),
+	BPF_EXIT_INSN(),
+	BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_0, 0),
+	BPF_EXIT_INSN(),
+	},
+	.fixup_map_in_map = { 4, 14 },
+	.flags = BPF_F_TEST_STATE_FREQ,
+	.result = VERBOSE_ACCEPT,
+	.errstr = "processed 25 insns",
+},
 {
 	"invalid inner map pointer",
 	.insns = {
-- 
2.32.0