diff options
| author | Brendan Jackman <jackmanb@google.com> | 2021-02-02 13:50:02 +0000 |
|---|---|---|
| committer | Alexei Starovoitov <ast@kernel.org> | 2021-02-02 18:23:29 -0800 |
| commit | 37086bfdc737ea6f66bf68dcf16757004d68e1e1 (patch) | |
| tree | 39949b12b17a29dfcfe61ee2c560c3bc137030c6 /tools/testing/selftests/bpf/verifier | |
| parent | 058107abafc75028e3ac95a8d19dfa17c50c676b (diff) | |
bpf: Propagate stack bounds to registers in atomics w/ BPF_FETCH
When BPF_FETCH is set, atomic instructions load a value from memory
into a register. The current verifier code first checks via
check_mem_access whether we can access the memory, and then checks
via check_reg_arg whether we can write into the register.
For loads, check_reg_arg has the side-effect of marking the
register's value as unkonwn, and check_mem_access has the side effect
of propagating bounds from memory to the register. This currently only
takes effect for stack memory.
Therefore with the current order, bounds information is thrown away,
but by simply reversing the order of check_reg_arg
vs. check_mem_access, we can instead propagate bounds smartly.
A simple test is added with an infinite loop that can only be proved
unreachable if this propagation is present. This is implemented both
with C and directly in test_verifier using assembly.
Suggested-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Brendan Jackman <jackmanb@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210202135002.4024825-1-jackmanb@google.com
Diffstat (limited to 'tools/testing/selftests/bpf/verifier')
| -rw-r--r-- | tools/testing/selftests/bpf/verifier/atomic_bounds.c | 27 |
1 files changed, 27 insertions, 0 deletions
diff --git a/tools/testing/selftests/bpf/verifier/atomic_bounds.c b/tools/testing/selftests/bpf/verifier/atomic_bounds.c new file mode 100644 index 000000000000..e82183e4914f --- /dev/null +++ b/tools/testing/selftests/bpf/verifier/atomic_bounds.c @@ -0,0 +1,27 @@ +{ + "BPF_ATOMIC bounds propagation, mem->reg", + .insns = { + /* a = 0; */ + /* + * Note this is implemented with two separate instructions, + * where you might think one would suffice: + * + * BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0), + * + * This is because BPF_ST_MEM doesn't seem to set the stack slot + * type to 0 when storing an immediate. + */ + BPF_MOV64_IMM(BPF_REG_0, 0), + BPF_STX_MEM(BPF_DW, BPF_REG_10, BPF_REG_0, -8), + /* b = atomic_fetch_add(&a, 1); */ + BPF_MOV64_IMM(BPF_REG_1, 1), + BPF_ATOMIC_OP(BPF_DW, BPF_ADD | BPF_FETCH, BPF_REG_10, BPF_REG_1, -8), + /* Verifier should be able to tell that this infinite loop isn't reachable. */ + /* if (b) while (true) continue; */ + BPF_JMP_IMM(BPF_JNE, BPF_REG_1, 0, -1), + BPF_EXIT_INSN(), + }, + .result = ACCEPT, + .result_unpriv = REJECT, + .errstr_unpriv = "back-edge", +}, |