This patch allows progs to elide a null check on statically known map lookup keys. In other words, if the verifier can statically prove that the lookup will be in-bounds, allow the prog to drop the null check.
This is useful for two reasons:
1. Large numbers of nullness checks (especially when they cannot fail) unnecessarily pushes prog towards BPF_COMPLEXITY_LIMIT_JMP_SEQ. 2. It forms a tighter contract between programmer and verifier.
For (1), bpftrace is starting to make heavier use of percpu scratch maps. As a result, for user scripts with large number of unrolled loops, we are starting to hit jump complexity verification errors. These percpu lookups cannot fail anyways, as we only use static key values. Eliding nullness probably results in less work for verifier as well.
For (2), percpu scratch maps are often used as a larger stack, as the currrent stack is limited to 512 bytes. In these situations, it is desirable for the programmer to express: "this lookup should never fail, and if it does, it means I messed up the code". By omitting the null check, the programmer can "ask" the verifier to double check the logic.
Daniel Xu (2): bpf: verifier: Support eliding map lookup nullness bpf: selftests: verifier: Add nullness elision tests
kernel/bpf/verifier.c | 56 +++++++ .../bpf/progs/verifier_array_access.c | 143 ++++++++++++++++++ 2 files changed, 199 insertions(+)
Test that nullness elision works for common use cases. For example, we want to check that both full and subreg stack slots recognized. As well as multiple lookups. And obviously some bound checks.
Signed-off-by: Daniel Xu dxu@dxuuu.xyz --- .../bpf/progs/verifier_array_access.c | 143 ++++++++++++++++++ 1 file changed, 143 insertions(+)
diff --git a/tools/testing/selftests/bpf/progs/verifier_array_access.c b/tools/testing/selftests/bpf/progs/verifier_array_access.c index 95d7ecc12963..85bf50750a8e 100644 --- a/tools/testing/selftests/bpf/progs/verifier_array_access.c +++ b/tools/testing/selftests/bpf/progs/verifier_array_access.c @@ -28,6 +28,20 @@ struct { __uint(map_flags, BPF_F_WRONLY_PROG); } map_array_wo SEC(".maps");
+struct { + __uint(type, BPF_MAP_TYPE_PERCPU_ARRAY); + __uint(max_entries, 2); + __type(key, int); + __type(value, struct test_val); +} map_array_pcpu SEC(".maps"); + +struct { + __uint(type, BPF_MAP_TYPE_ARRAY); + __uint(max_entries, 2); + __type(key, int); + __type(value, struct test_val); +} map_array SEC(".maps"); + struct { __uint(type, BPF_MAP_TYPE_HASH); __uint(max_entries, 1); @@ -526,4 +540,133 @@ l0_%=: exit; \ : __clobber_all); }
+SEC("socket") +__description("valid map access into an array using constant without nullness") +__success +__retval(4) +__naked void an_array_with_a_constant_no_nullness(void) +{ + asm volatile (" \ + r1 = 1; \ + *(u64*)(r10 - 8) = r1; \ + r2 = r10; \ + r2 += -8; \ + r1 = %[map_array] ll; \ + call %[bpf_map_lookup_elem]; \ + r1 = %[test_val_foo]; \ + *(u64*)(r0 + 0) = r1; \ + r0 = *(u64*)(r0 + 0); \ + exit; \ +" : + : __imm(bpf_map_lookup_elem), + __imm_addr(map_array), + __imm_const(test_val_foo, offsetof(struct test_val, foo)) + : __clobber_all); +} + +SEC("socket") +__description("valid multiple map access into an array using constant without nullness") +__success +__retval(8) +__naked void multiple_array_with_a_constant_no_nullness(void) +{ + asm volatile (" \ + r1 = 1; \ + *(u64*)(r10 - 8) = r1; \ + r2 = r10; \ + r2 += -8; \ + r1 = %[map_array] ll; \ + call %[bpf_map_lookup_elem]; \ + r6 = %[test_val_foo]; \ + *(u64*)(r0 + 0) = r6; \ + r7 = *(u64*)(r0 + 0); \ + r1 = 0; \ + *(u64*)(r10 - 16) = r1; \ + r2 = r10; \ + r2 += -16; \ + r1 = %[map_array] ll; \ + call %[bpf_map_lookup_elem]; \ + *(u64*)(r0 + 0) = r6; \ + r1 = *(u64*)(r0 + 0); \ + r7 += r1; \ + r0 = r7; \ + exit; \ +" : + : __imm(bpf_map_lookup_elem), + __imm_addr(map_array), + __imm_const(test_val_foo, offsetof(struct test_val, foo)) + : __clobber_all); +} + +SEC("socket") +__description("valid map access into an array using 32-bit constant without nullness") +__success +__retval(4) +__naked void an_array_with_a_32bit_constant_no_nullness(void) +{ + asm volatile (" \ + r1 = 1; \ + *(u32*)(r10 - 4) = r1; \ + r2 = r10; \ + r2 += -4; \ + r1 = %[map_array] ll; \ + call %[bpf_map_lookup_elem]; \ + r1 = %[test_val_foo]; \ + *(u64*)(r0 + 0) = r1; \ + r0 = *(u64*)(r0 + 0); \ + exit; \ +" : + : __imm(bpf_map_lookup_elem), + __imm_addr(map_array), + __imm_const(test_val_foo, offsetof(struct test_val, foo)) + : __clobber_all); +} + +SEC("socket") +__description("valid map access into a pcpu array using constant without nullness") +__success +__retval(4) +__naked void a_pcpu_array_with_a_constant_no_nullness(void) +{ + asm volatile (" \ + r1 = 1; \ + *(u64*)(r10 - 8) = r1; \ + r2 = r10; \ + r2 += -8; \ + r1 = %[map_array_pcpu] ll; \ + call %[bpf_map_lookup_elem]; \ + r1 = %[test_val_foo]; \ + *(u64*)(r0 + 0) = r1; \ + r0 = *(u64*)(r0 + 0); \ + exit; \ +" : + : __imm(bpf_map_lookup_elem), + __imm_addr(map_array_pcpu), + __imm_const(test_val_foo, offsetof(struct test_val, foo)) + : __clobber_all); +} + +SEC("socket") +__description("invalid map access into an array using constant without nullness") +__failure __msg("R0 invalid mem access 'map_value_or_null'") +__naked void an_array_with_a_constant_no_nullness_out_of_bounds(void) +{ + asm volatile (" \ + r1 = 3; \ + *(u64*)(r10 - 8) = r1; \ + r2 = r10; \ + r2 += -8; \ + r1 = %[map_array] ll; \ + call %[bpf_map_lookup_elem]; \ + r1 = %[test_val_foo]; \ + *(u64*)(r0 + 0) = r1; \ + r0 = *(u64*)(r0 + 0); \ + exit; \ +" : + : __imm(bpf_map_lookup_elem), + __imm_addr(map_array), + __imm_const(test_val_foo, offsetof(struct test_val, foo)) + : __clobber_all); +} + char _license[] SEC("license") = "GPL";
linux-kselftest-mirror@lists.linaro.org
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Daniel Xu