selftests/bpf: add a test for a bug in liveness-based pruning

Writes in straight-line code should not prevent reads from propagating
 along jumps.  With current verifier code, the jump from 3 to 5 does not
 add a read mark on 3:R0 (because 5:R0 has a write mark), meaning that
 the jump from 1 to 3 gets pruned as safe even though R0 is NOT_INIT.

Verifier output:
0: (61) r2 = *(u32 *)(r1 +0)
1: (35) if r2 >= 0x0 goto pc+1
 R1=ctx(id=0,off=0,imm=0) R2=inv(id=0,umax_value=4294967295,var_off=(0x0; 0xffffffff)) R10=fp0
2: (b7) r0 = 0
3: (35) if r2 >= 0x0 goto pc+1
 R0=inv0 R1=ctx(id=0,off=0,imm=0) R2=inv(id=0,umax_value=4294967295,var_off=(0x0; 0xffffffff)) R10=fp0
4: (b7) r0 = 0
5: (95) exit

from 3 to 5: safe

from 1 to 3: safe
processed 8 insns, stack depth 0

Signed-off-by: Edward Cree <ecree@solarflare.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/tools/testing/selftests/bpf/test_verifier.c b/tools/testing/selftests/bpf/test_verifier.c
index c03542c..c912734 100644 (file)
--- a/tools/testing/selftests/bpf/test_verifier.c
+++ b/tools/testing/selftests/bpf/test_verifier.c
@@ -6487,6 +6487,22 @@ static struct bpf_test tests[] = {
                .result = REJECT,
                .prog_type = BPF_PROG_TYPE_LWT_IN,
        },
+       {
+               "liveness pruning and write screening",
+               .insns = {
+                       /* Get an unknown value */
+                       BPF_LDX_MEM(BPF_W, BPF_REG_2, BPF_REG_1, 0),
+                       /* branch conditions teach us nothing about R2 */
+                       BPF_JMP_IMM(BPF_JGE, BPF_REG_2, 0, 1),
+                       BPF_MOV64_IMM(BPF_REG_0, 0),
+                       BPF_JMP_IMM(BPF_JGE, BPF_REG_2, 0, 1),
+                       BPF_MOV64_IMM(BPF_REG_0, 0),
+                       BPF_EXIT_INSN(),
+               },
+               .errstr = "R0 !read_ok",
+               .result = REJECT,
+               .prog_type = BPF_PROG_TYPE_LWT_IN,
+       },
 };
 
 static int probe_filter_length(const struct bpf_insn *fp)