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Sat, 14 Sep 2024 22:11:25 -0400 (EDT) From: Daniel Xu To: daniel@iogearbox.net, ast@kernel.org, andrii@kernel.org Cc: john.fastabend@gmail.com, martin.lau@linux.dev, eddyz87@gmail.com, song@kernel.org, yonghong.song@linux.dev, kpsingh@kernel.org, sdf@fomichev.me, haoluo@google.com, jolsa@kernel.org, bpf@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@meta.com Subject: [PATCH bpf-next 1/2] bpf: verifier: Support eliding map lookup nullness Date: Sat, 14 Sep 2024 20:11:11 -0600 Message-ID: <25457cde1da9a4d387fdb3ee4628cd5a4ca97590.1726366145.git.dxu@dxuuu.xyz> X-Mailer: git-send-email 2.46.0 In-Reply-To: References: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" This commit 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. Signed-off-by: Daniel Xu --- kernel/bpf/verifier.c | 56 +++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 56 insertions(+) diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 7df5c29293a4..5b5ae3c1a456 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -282,6 +282,7 @@ struct bpf_call_arg_meta { u32 ret_btf_id; u32 subprogno; struct btf_field *kptr_field; + long const_map_key; }; =20 struct bpf_kfunc_call_arg_meta { @@ -10414,6 +10415,45 @@ static void update_loop_inline_state(struct bpf_ve= rifier_env *env, u32 subprogno state->callback_subprogno =3D=3D subprogno); } =20 +/* Returns whether or not the given map type can potentially elide + * lookup return value nullness check. This is possible if the key + * is statically known. + */ +static bool can_elide_value_nullness(enum bpf_map_type type) +{ + switch (type) { + case BPF_MAP_TYPE_ARRAY: + case BPF_MAP_TYPE_PERCPU_ARRAY: + return true; + default: + return false; + } +} + +/* Returns constant key value if possible, else -1 */ +static long get_constant_map_key(struct bpf_verifier_env *env, + struct bpf_reg_state *key) +{ + struct bpf_func_state *state =3D func(env, key); + struct bpf_reg_state *reg; + int stack_off; + int slot; + int spi; + + if (!tnum_is_const(key->var_off)) + return -1; + + stack_off =3D key->off + key->var_off.value; + slot =3D -stack_off - 1; + spi =3D slot / BPF_REG_SIZE; + reg =3D &state->stack[spi].spilled_ptr; + + if (!tnum_is_const(reg->var_off)) + return -1; + + return reg->var_off.value; +} + static int get_helper_proto(struct bpf_verifier_env *env, int func_id, const struct bpf_func_proto **ptr) { @@ -10511,6 +10551,15 @@ static int check_helper_call(struct bpf_verifier_e= nv *env, struct bpf_insn *insn env->insn_aux_data[insn_idx].storage_get_func_atomic =3D true; } =20 + /* Logically we are trying to check on key register state before + * the helper is called, so process here. Otherwise argument processing + * may clobber the spilled key values. + */ + regs =3D cur_regs(env); + if (func_id =3D=3D BPF_FUNC_map_lookup_elem) + meta.const_map_key =3D get_constant_map_key(env, ®s[BPF_REG_2]); + + meta.func_id =3D func_id; /* check args */ for (i =3D 0; i < MAX_BPF_FUNC_REG_ARGS; i++) { @@ -10771,6 +10820,13 @@ static int check_helper_call(struct bpf_verifier_e= nv *env, struct bpf_insn *insn "kernel subsystem misconfigured verifier\n"); return -EINVAL; } + + if (func_id =3D=3D BPF_FUNC_map_lookup_elem && + can_elide_value_nullness(meta.map_ptr->map_type) && + meta.const_map_key >=3D 0 && + meta.const_map_key < meta.map_ptr->max_entries) + ret_flag &=3D ~PTR_MAYBE_NULL; + regs[BPF_REG_0].map_ptr =3D meta.map_ptr; regs[BPF_REG_0].map_uid =3D meta.map_uid; regs[BPF_REG_0].type =3D PTR_TO_MAP_VALUE | ret_flag; --=20 2.46.0 From nobody Fri Nov 29 20:57:20 2024 Received: from fhigh5-smtp.messagingengine.com (fhigh5-smtp.messagingengine.com [103.168.172.156]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 0E3EE10A0C; 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Sat, 14 Sep 2024 22:11:28 -0400 (EDT) From: Daniel Xu To: shuah@kernel.org, andrii@kernel.org, ast@kernel.org, daniel@iogearbox.net, eddyz87@gmail.com Cc: mykolal@fb.com, martin.lau@linux.dev, song@kernel.org, yonghong.song@linux.dev, john.fastabend@gmail.com, kpsingh@kernel.org, sdf@fomichev.me, haoluo@google.com, jolsa@kernel.org, bpf@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@meta.com Subject: [PATCH bpf-next 2/2] bpf: selftests: verifier: Add nullness elision tests Date: Sat, 14 Sep 2024 20:11:12 -0600 Message-ID: <55d878b5b9ee0daef9e19f7c5e13cd78d96a59cd.1726366145.git.dxu@dxuuu.xyz> X-Mailer: git-send-email 2.46.0 In-Reply-To: References: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" 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 --- .../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/to= ols/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"); =20 +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_%=3D: exit; \ : __clobber_all); } =20 +SEC("socket") +__description("valid map access into an array using constant without nulln= ess") +__success +__retval(4) +__naked void an_array_with_a_constant_no_nullness(void) +{ + asm volatile (" \ + r1 =3D 1; \ + *(u64*)(r10 - 8) =3D r1; \ + r2 =3D r10; \ + r2 +=3D -8; \ + r1 =3D %[map_array] ll; \ + call %[bpf_map_lookup_elem]; \ + r1 =3D %[test_val_foo]; \ + *(u64*)(r0 + 0) =3D r1; \ + r0 =3D *(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 with= out nullness") +__success +__retval(8) +__naked void multiple_array_with_a_constant_no_nullness(void) +{ + asm volatile (" \ + r1 =3D 1; \ + *(u64*)(r10 - 8) =3D r1; \ + r2 =3D r10; \ + r2 +=3D -8; \ + r1 =3D %[map_array] ll; \ + call %[bpf_map_lookup_elem]; \ + r6 =3D %[test_val_foo]; \ + *(u64*)(r0 + 0) =3D r6; \ + r7 =3D *(u64*)(r0 + 0); \ + r1 =3D 0; \ + *(u64*)(r10 - 16) =3D r1; \ + r2 =3D r10; \ + r2 +=3D -16; \ + r1 =3D %[map_array] ll; \ + call %[bpf_map_lookup_elem]; \ + *(u64*)(r0 + 0) =3D r6; \ + r1 =3D *(u64*)(r0 + 0); \ + r7 +=3D r1; \ + r0 =3D 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 withou= t nullness") +__success +__retval(4) +__naked void an_array_with_a_32bit_constant_no_nullness(void) +{ + asm volatile (" \ + r1 =3D 1; \ + *(u32*)(r10 - 4) =3D r1; \ + r2 =3D r10; \ + r2 +=3D -4; \ + r1 =3D %[map_array] ll; \ + call %[bpf_map_lookup_elem]; \ + r1 =3D %[test_val_foo]; \ + *(u64*)(r0 + 0) =3D r1; \ + r0 =3D *(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 n= ullness") +__success +__retval(4) +__naked void a_pcpu_array_with_a_constant_no_nullness(void) +{ + asm volatile (" \ + r1 =3D 1; \ + *(u64*)(r10 - 8) =3D r1; \ + r2 =3D r10; \ + r2 +=3D -8; \ + r1 =3D %[map_array_pcpu] ll; \ + call %[bpf_map_lookup_elem]; \ + r1 =3D %[test_val_foo]; \ + *(u64*)(r0 + 0) =3D r1; \ + r0 =3D *(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 nul= lness") +__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 =3D 3; \ + *(u64*)(r10 - 8) =3D r1; \ + r2 =3D r10; \ + r2 +=3D -8; \ + r1 =3D %[map_array] ll; \ + call %[bpf_map_lookup_elem]; \ + r1 =3D %[test_val_foo]; \ + *(u64*)(r0 + 0) =3D r1; \ + r0 =3D *(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") =3D "GPL"; --=20 2.46.0