| Message ID | 92065ca054beccd6d0f35efe9715ef965e8d379f.1734045451.git.dxu@dxuuu.xyz |
|---|---|
| State | New |
| Headers | show |
| Series | Support eliding map lookup nullness | expand |
On Thu, Dec 12, 2024 at 08:04:45PM GMT, Eduard Zingerman wrote: > On Thu, 2024-12-12 at 16:22 -0700, Daniel Xu wrote: > > I think these changes are fine in general, but see below. > > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > > index 58b36cc96bd5..4947ef884a18 100644 > > --- a/kernel/bpf/verifier.c > > +++ b/kernel/bpf/verifier.c > > @@ -287,6 +287,7 @@ struct bpf_call_arg_meta { > > u32 ret_btf_id; > > u32 subprogno; > > struct btf_field *kptr_field; > > + s64 const_map_key; > > }; > > > > struct bpf_kfunc_call_arg_meta { > > @@ -9163,6 +9164,53 @@ static int check_reg_const_str(struct bpf_verifier_env *env, > > return 0; > > } > > > > +/* Returns constant key value if possible, else -1 */ > > +static s64 get_constant_map_key(struct bpf_verifier_env *env, > > + struct bpf_reg_state *key, > > + u32 key_size) > > I understand that this is not your use case, but maybe generalize this > a bit by checking maximal register value instead of a constant? I'll check on this. If it works I think you're right - it allows more flexibility while retaining safety. User could define max_entries to be a power of two and then mask key with with 0xFFFF.. to guarantee null free codepaths. > > > +{ > > + struct bpf_func_state *state = func(env, key); > > + struct bpf_reg_state *reg; > > + int zero_size = 0; > > + int stack_off; > > + u8 *stype; > > + int slot; > > + int spi; > > + int i; > > + > > + if (!env->bpf_capable) > > + return -1; > > + if (key->type != PTR_TO_STACK) > > + return -1; > > + if (!tnum_is_const(key->var_off)) > > + return -1; > > + > > + stack_off = key->off + key->var_off.value; > > + slot = -stack_off - 1; > > + spi = slot / BPF_REG_SIZE; > > + > > + /* First handle precisely tracked STACK_ZERO, up to BPF_REG_SIZE */ > > + stype = state->stack[spi].slot_type; > > + for (i = 0; i < BPF_REG_SIZE && stype[i] == STACK_ZERO; i++) > > + zero_size++; > > + if (zero_size == key_size) > > + return 0; > > + > > + if (!is_spilled_reg(&state->stack[spi])) > > + /* Not pointer to stack */ > > + return -1; > > Nit: there is a 'is_spilled_scalar_reg' utility function. Ack. > > > + > > + reg = &state->stack[spi].spilled_ptr; > > + if (reg->type != SCALAR_VALUE) > > + /* Only scalars are valid array map keys */ > > + return -1; > > + else if (!tnum_is_const(reg->var_off)) > > + /* Stack value not statically known */ > > + return -1; > > I think you need to check if size of the spill matches the size of the key. > The mismatch would be unsafe when spill size is smaller than key size. > E.g. consider 1-byte spill with mask 'mmmmmmrr' and a 4-byte key, > at runtime the 'mmmmmm' part might be non-zero, rendering key to be > out of range. Ah great catch. I think you're right.
On Thu, Dec 12, 2024 at 3:23 PM Daniel Xu <dxu@dxuuu.xyz> wrote: > > 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. > > Tests also have to be updated in sync with these changes, as the > verifier is more efficient with this change. Notable, iters.c tests had > to be changed to use a map type that still requires null checks, as it's > exercising verifier tracking logic w.r.t iterators. > > Signed-off-by: Daniel Xu <dxu@dxuuu.xyz> > --- > kernel/bpf/verifier.c | 80 ++++++++++++++++++- > tools/testing/selftests/bpf/progs/iters.c | 14 ++-- > .../selftests/bpf/progs/map_kptr_fail.c | 2 +- > .../selftests/bpf/progs/verifier_map_in_map.c | 2 +- > .../testing/selftests/bpf/verifier/map_kptr.c | 2 +- > 5 files changed, 87 insertions(+), 13 deletions(-) > Eduard has great points. I've added a few more comments below. pw-bot: cr > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > index 58b36cc96bd5..4947ef884a18 100644 > --- a/kernel/bpf/verifier.c > +++ b/kernel/bpf/verifier.c > @@ -287,6 +287,7 @@ struct bpf_call_arg_meta { > u32 ret_btf_id; > u32 subprogno; > struct btf_field *kptr_field; > + s64 const_map_key; > }; > > struct bpf_kfunc_call_arg_meta { > @@ -9163,6 +9164,53 @@ static int check_reg_const_str(struct bpf_verifier_env *env, > return 0; > } > > +/* Returns constant key value if possible, else -1 */ > +static s64 get_constant_map_key(struct bpf_verifier_env *env, > + struct bpf_reg_state *key, > + u32 key_size) > +{ > + struct bpf_func_state *state = func(env, key); > + struct bpf_reg_state *reg; > + int zero_size = 0; > + int stack_off; > + u8 *stype; > + int slot; > + int spi; > + int i; > + > + if (!env->bpf_capable) > + return -1; > + if (key->type != PTR_TO_STACK) > + return -1; > + if (!tnum_is_const(key->var_off)) > + return -1; > + > + stack_off = key->off + key->var_off.value; > + slot = -stack_off - 1; > + spi = slot / BPF_REG_SIZE; > + > + /* First handle precisely tracked STACK_ZERO, up to BPF_REG_SIZE */ > + stype = state->stack[spi].slot_type; > + for (i = 0; i < BPF_REG_SIZE && stype[i] == STACK_ZERO; i++) it's Friday and I'm lazy, but please double-check that this works for both big-endian and little-endian :) with Eduard's suggestion this also becomes interesting when you have 000mmm mix (as one example), because that gives you a small range, and all values might be valid keys for arrays > + zero_size++; > + if (zero_size == key_size) > + return 0; > + > + if (!is_spilled_reg(&state->stack[spi])) > + /* Not pointer to stack */ !is_spilled_reg and "Not pointer to stack" seem to be not exactly the same things? btw, we also have is_spilled_scalar_reg() which you can use here instead of two separate checks? > + return -1; > + > + reg = &state->stack[spi].spilled_ptr; > + if (reg->type != SCALAR_VALUE) > + /* Only scalars are valid array map keys */ > + return -1; > + else if (!tnum_is_const(reg->var_off)) > + /* Stack value not statically known */ > + return -1; > + > + return reg->var_off.value; > +} > + [...]
On Fri, 13 Dec 2024 at 00:24, Daniel Xu <dxu@dxuuu.xyz> wrote: > > 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. > > Tests also have to be updated in sync with these changes, as the > verifier is more efficient with this change. Notable, iters.c tests had > to be changed to use a map type that still requires null checks, as it's > exercising verifier tracking logic w.r.t iterators. > > Signed-off-by: Daniel Xu <dxu@dxuuu.xyz> > --- > kernel/bpf/verifier.c | 80 ++++++++++++++++++- > tools/testing/selftests/bpf/progs/iters.c | 14 ++-- > .../selftests/bpf/progs/map_kptr_fail.c | 2 +- > .../selftests/bpf/progs/verifier_map_in_map.c | 2 +- > .../testing/selftests/bpf/verifier/map_kptr.c | 2 +- > 5 files changed, 87 insertions(+), 13 deletions(-) > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > index 58b36cc96bd5..4947ef884a18 100644 > --- a/kernel/bpf/verifier.c > +++ b/kernel/bpf/verifier.c > @@ -287,6 +287,7 @@ struct bpf_call_arg_meta { > u32 ret_btf_id; > u32 subprogno; > struct btf_field *kptr_field; > + s64 const_map_key; > }; > > struct bpf_kfunc_call_arg_meta { > @@ -9163,6 +9164,53 @@ static int check_reg_const_str(struct bpf_verifier_env *env, > return 0; > } > > +/* Returns constant key value if possible, else -1 */ > +static s64 get_constant_map_key(struct bpf_verifier_env *env, > + struct bpf_reg_state *key, > + u32 key_size) > +{ > + struct bpf_func_state *state = func(env, key); > + struct bpf_reg_state *reg; > + int zero_size = 0; > + int stack_off; > + u8 *stype; > + int slot; > + int spi; > + int i; > + > + if (!env->bpf_capable) > + return -1; > + if (key->type != PTR_TO_STACK) > + return -1; > + if (!tnum_is_const(key->var_off)) > + return -1; > + > + stack_off = key->off + key->var_off.value; > + slot = -stack_off - 1; > + spi = slot / BPF_REG_SIZE; > + > + /* First handle precisely tracked STACK_ZERO, up to BPF_REG_SIZE */ > + stype = state->stack[spi].slot_type; > + for (i = 0; i < BPF_REG_SIZE && stype[i] == STACK_ZERO; i++) > + zero_size++; > + if (zero_size == key_size) > + return 0; > + > + if (!is_spilled_reg(&state->stack[spi])) > + /* Not pointer to stack */ > + return -1; > + > + reg = &state->stack[spi].spilled_ptr; > + if (reg->type != SCALAR_VALUE) > + /* Only scalars are valid array map keys */ > + return -1; > + else if (!tnum_is_const(reg->var_off)) > + /* Stack value not statically known */ > + return -1; > + > + return reg->var_off.value; > +} > + > static int check_func_arg(struct bpf_verifier_env *env, u32 arg, > struct bpf_call_arg_meta *meta, > const struct bpf_func_proto *fn, > @@ -9173,6 +9221,7 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg, > enum bpf_arg_type arg_type = fn->arg_type[arg]; > enum bpf_reg_type type = reg->type; > u32 *arg_btf_id = NULL; > + u32 key_size; > int err = 0; > bool mask; > > @@ -9307,8 +9356,11 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg, > verbose(env, "invalid map_ptr to access map->key\n"); > return -EACCES; > } > - err = check_helper_mem_access(env, regno, meta->map_ptr->key_size, > - BPF_READ, false, NULL); > + key_size = meta->map_ptr->key_size; > + err = check_helper_mem_access(env, regno, key_size, BPF_READ, false, NULL); > + if (err) > + return err; > + meta->const_map_key = get_constant_map_key(env, reg, key_size); > break; > case ARG_PTR_TO_MAP_VALUE: > if (type_may_be_null(arg_type) && register_is_null(reg)) > @@ -10833,6 +10885,21 @@ static void update_loop_inline_state(struct bpf_verifier_env *env, u32 subprogno > state->callback_subprogno == subprogno); > } > > +/* 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; > + } > +} > + > static int get_helper_proto(struct bpf_verifier_env *env, int func_id, > const struct bpf_func_proto **ptr) > { > @@ -11199,10 +11266,17 @@ static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn > "kernel subsystem misconfigured verifier\n"); > return -EINVAL; > } > + > + if (func_id == BPF_FUNC_map_lookup_elem && > + can_elide_value_nullness(meta.map_ptr->map_type) && > + meta.const_map_key >= 0 && > + meta.const_map_key < meta.map_ptr->max_entries) > + ret_flag &= ~PTR_MAYBE_NULL; I think we probably need mark_chain_precision applied on the constant key since its concrete value is made use of here to prevent pruning on it. If it's already happening and I missed it, I think we should atleast add a comment. For context of a similar case with tail calls, see commit cc52d9140aa9 ("bpf: Fix record_func_key to perform backtracking on r3") for what happens when it is missed. > + > regs[BPF_REG_0].map_ptr = meta.map_ptr; > regs[BPF_REG_0].map_uid = meta.map_uid; > regs[BPF_REG_0].type = PTR_TO_MAP_VALUE | ret_flag; > - if (!type_may_be_null(ret_type) && > + if (!type_may_be_null(ret_flag) && > btf_record_has_field(meta.map_ptr->record, BPF_SPIN_LOCK)) { > regs[BPF_REG_0].id = ++env->id_gen; > } > diff --git a/tools/testing/selftests/bpf/progs/iters.c b/tools/testing/selftests/bpf/progs/iters.c > index 7c969c127573..190822b2f08b 100644 > --- a/tools/testing/selftests/bpf/progs/iters.c > +++ b/tools/testing/selftests/bpf/progs/iters.c > @@ -524,11 +524,11 @@ int iter_subprog_iters(const void *ctx) > } > > struct { > - __uint(type, BPF_MAP_TYPE_ARRAY); > + __uint(type, BPF_MAP_TYPE_HASH); > __type(key, int); > __type(value, int); > __uint(max_entries, 1000); > -} arr_map SEC(".maps"); > +} hash_map SEC(".maps"); > > SEC("?raw_tp") > __failure __msg("invalid mem access 'scalar'") > @@ -539,7 +539,7 @@ int iter_err_too_permissive1(const void *ctx) > > MY_PID_GUARD(); > > - map_val = bpf_map_lookup_elem(&arr_map, &key); > + map_val = bpf_map_lookup_elem(&hash_map, &key); > if (!map_val) > return 0; > > @@ -561,12 +561,12 @@ int iter_err_too_permissive2(const void *ctx) > > MY_PID_GUARD(); > > - map_val = bpf_map_lookup_elem(&arr_map, &key); > + map_val = bpf_map_lookup_elem(&hash_map, &key); > if (!map_val) > return 0; > > bpf_repeat(1000000) { > - map_val = bpf_map_lookup_elem(&arr_map, &key); > + map_val = bpf_map_lookup_elem(&hash_map, &key); > } > > *map_val = 123; > @@ -585,7 +585,7 @@ int iter_err_too_permissive3(const void *ctx) > MY_PID_GUARD(); > > bpf_repeat(1000000) { > - map_val = bpf_map_lookup_elem(&arr_map, &key); > + map_val = bpf_map_lookup_elem(&hash_map, &key); > found = true; > } > > @@ -606,7 +606,7 @@ int iter_tricky_but_fine(const void *ctx) > MY_PID_GUARD(); > > bpf_repeat(1000000) { > - map_val = bpf_map_lookup_elem(&arr_map, &key); > + map_val = bpf_map_lookup_elem(&hash_map, &key); > if (map_val) { > found = true; > break; > diff --git a/tools/testing/selftests/bpf/progs/map_kptr_fail.c b/tools/testing/selftests/bpf/progs/map_kptr_fail.c > index c2a6bd392e48..4c0ff01f1a96 100644 > --- a/tools/testing/selftests/bpf/progs/map_kptr_fail.c > +++ b/tools/testing/selftests/bpf/progs/map_kptr_fail.c > @@ -345,7 +345,7 @@ int reject_indirect_global_func_access(struct __sk_buff *ctx) > } > > SEC("?tc") > -__failure __msg("Unreleased reference id=5 alloc_insn=") > +__failure __msg("Unreleased reference id=4 alloc_insn=") > int kptr_xchg_ref_state(struct __sk_buff *ctx) > { > struct prog_test_ref_kfunc *p; > diff --git a/tools/testing/selftests/bpf/progs/verifier_map_in_map.c b/tools/testing/selftests/bpf/progs/verifier_map_in_map.c > index 4eaab1468eb7..7d088ba99ea5 100644 > --- a/tools/testing/selftests/bpf/progs/verifier_map_in_map.c > +++ b/tools/testing/selftests/bpf/progs/verifier_map_in_map.c > @@ -47,7 +47,7 @@ l0_%=: r0 = 0; \ > > SEC("xdp") > __description("map in map state pruning") > -__success __msg("processed 26 insns") > +__success __msg("processed 15 insns") > __log_level(2) __retval(0) __flag(BPF_F_TEST_STATE_FREQ) > __naked void map_in_map_state_pruning(void) > { > diff --git a/tools/testing/selftests/bpf/verifier/map_kptr.c b/tools/testing/selftests/bpf/verifier/map_kptr.c > index f420c0312aa0..4b39f8472f9b 100644 > --- a/tools/testing/selftests/bpf/verifier/map_kptr.c > +++ b/tools/testing/selftests/bpf/verifier/map_kptr.c > @@ -373,7 +373,7 @@ > .prog_type = BPF_PROG_TYPE_SCHED_CLS, > .fixup_map_kptr = { 1 }, > .result = REJECT, > - .errstr = "Unreleased reference id=5 alloc_insn=20", > + .errstr = "Unreleased reference id=4 alloc_insn=20", > .fixup_kfunc_btf_id = { > { "bpf_kfunc_call_test_acquire", 15 }, > } > -- > 2.46.0 > >
On Sat, 2024-12-14 at 00:10 +0100, Kumar Kartikeya Dwivedi wrote: [...] > > @@ -11199,10 +11266,17 @@ static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn > > "kernel subsystem misconfigured verifier\n"); > > return -EINVAL; > > } > > + > > + if (func_id == BPF_FUNC_map_lookup_elem && > > + can_elide_value_nullness(meta.map_ptr->map_type) && > > + meta.const_map_key >= 0 && > > + meta.const_map_key < meta.map_ptr->max_entries) > > + ret_flag &= ~PTR_MAYBE_NULL; > > I think we probably need mark_chain_precision applied on the constant > key since its concrete value is made use of here to prevent pruning on > it. If it's already happening and I missed it, I think we should > atleast add a comment. > > For context of a similar case with tail calls, see commit > cc52d9140aa9 ("bpf: Fix record_func_key to perform backtracking on r3") > for what happens when it is missed. Great point, I'm sure this does not happen. [...]
On Fri, 2024-12-13 at 15:14 -0800, Eduard Zingerman wrote:
[...]
> Great point, I'm sure this does not happen.
I mean, mark_chain_precision() does not happen at the moment.
On Fri, Dec 13, 2024 at 03:02:11PM GMT, Andrii Nakryiko wrote: > On Thu, Dec 12, 2024 at 3:23 PM Daniel Xu <dxu@dxuuu.xyz> wrote: > > > > 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. > > > > Tests also have to be updated in sync with these changes, as the > > verifier is more efficient with this change. Notable, iters.c tests had > > to be changed to use a map type that still requires null checks, as it's > > exercising verifier tracking logic w.r.t iterators. > > > > Signed-off-by: Daniel Xu <dxu@dxuuu.xyz> > > --- > > kernel/bpf/verifier.c | 80 ++++++++++++++++++- > > tools/testing/selftests/bpf/progs/iters.c | 14 ++-- > > .../selftests/bpf/progs/map_kptr_fail.c | 2 +- > > .../selftests/bpf/progs/verifier_map_in_map.c | 2 +- > > .../testing/selftests/bpf/verifier/map_kptr.c | 2 +- > > 5 files changed, 87 insertions(+), 13 deletions(-) > > > > Eduard has great points. I've added a few more comments below. > > pw-bot: cr > > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > > index 58b36cc96bd5..4947ef884a18 100644 > > --- a/kernel/bpf/verifier.c > > +++ b/kernel/bpf/verifier.c > > @@ -287,6 +287,7 @@ struct bpf_call_arg_meta { > > u32 ret_btf_id; > > u32 subprogno; > > struct btf_field *kptr_field; > > + s64 const_map_key; > > }; > > > > struct bpf_kfunc_call_arg_meta { > > @@ -9163,6 +9164,53 @@ static int check_reg_const_str(struct bpf_verifier_env *env, > > return 0; > > } > > > > +/* Returns constant key value if possible, else -1 */ > > +static s64 get_constant_map_key(struct bpf_verifier_env *env, > > + struct bpf_reg_state *key, > > + u32 key_size) > > +{ > > + struct bpf_func_state *state = func(env, key); > > + struct bpf_reg_state *reg; > > + int zero_size = 0; > > + int stack_off; > > + u8 *stype; > > + int slot; > > + int spi; > > + int i; > > + > > + if (!env->bpf_capable) > > + return -1; > > + if (key->type != PTR_TO_STACK) > > + return -1; > > + if (!tnum_is_const(key->var_off)) > > + return -1; > > + > > + stack_off = key->off + key->var_off.value; > > + slot = -stack_off - 1; > > + spi = slot / BPF_REG_SIZE; > > + > > + /* First handle precisely tracked STACK_ZERO, up to BPF_REG_SIZE */ > > + stype = state->stack[spi].slot_type; > > + for (i = 0; i < BPF_REG_SIZE && stype[i] == STACK_ZERO; i++) > > it's Friday and I'm lazy, but please double-check that this works for > both big-endian and little-endian :) Any tips? Are the existing tests running thru s390x hosts in CI sufficient or should I add some tests writen in C (and not BPF assembler)? I can never think about endianness correctly... > > with Eduard's suggestion this also becomes interesting when you have > 000mmm mix (as one example), because that gives you a small range, and > all values might be valid keys for arrays Can you define what "small range" means? What range is there with 0's? Any pointers would be helpful. > > > + zero_size++; > > + if (zero_size == key_size) > > + return 0; > > + > > + if (!is_spilled_reg(&state->stack[spi])) > > + /* Not pointer to stack */ > > !is_spilled_reg and "Not pointer to stack" seem to be not exactly the > same things? You're right - comment is not helpful. I'll make the change to use is_spilled_scalar_reg() which is probably as clear as it gets. [..]
On Fri, 2024-12-13 at 19:44 -0700, Daniel Xu wrote: [...] > > > + /* First handle precisely tracked STACK_ZERO, up to BPF_REG_SIZE */ > > > + stype = state->stack[spi].slot_type; > > > + for (i = 0; i < BPF_REG_SIZE && stype[i] == STACK_ZERO; i++) > > > > it's Friday and I'm lazy, but please double-check that this works for > > both big-endian and little-endian :) > > Any tips? Are the existing tests running thru s390x hosts in CI > sufficient or should I add some tests writen in C (and not BPF > assembler)? I can never think about endianness correctly... I think that if test operates on a key like: valid key 15 v 0000000f <-- written to stack as a single u64 value ^^^^^^^ stack zero marks and is executed (e.g. using __retval annotation), then CI passing for s390 should be enough. There is a guide on how to gen a s390 environment locally: https://docs.kernel.org/bpf/s390.html I used it recently to build a vmlinux for s390 with no or minimal issues. Used it to boot long time ago, but don't remember if there were any surprises. > > with Eduard's suggestion this also becomes interesting when you have > > 000mmm mix (as one example), because that gives you a small range, and > > all values might be valid keys for arrays > > Can you define what "small range" means? What range is there with 0's? > Any pointers would be helpful. I think Andrii means that each 'm' adds 8 bits of range. E.g. range for 0000_000m is 0-255, range for 0000_00mm is 0-65535, etc. [...]
On Fri, Dec 13, 2024 at 7:13 PM Eduard Zingerman <eddyz87@gmail.com> wrote: > > On Fri, 2024-12-13 at 19:44 -0700, Daniel Xu wrote: > > [...] > > > > > + /* First handle precisely tracked STACK_ZERO, up to BPF_REG_SIZE */ > > > > + stype = state->stack[spi].slot_type; > > > > + for (i = 0; i < BPF_REG_SIZE && stype[i] == STACK_ZERO; i++) > > > > > > it's Friday and I'm lazy, but please double-check that this works for > > > both big-endian and little-endian :) > > > > Any tips? Are the existing tests running thru s390x hosts in CI > > sufficient or should I add some tests writen in C (and not BPF > > assembler)? I can never think about endianness correctly... > > I think that if test operates on a key like: > > valid key 15 > v > 0000000f <-- written to stack as a single u64 value > ^^^^^^^ > stack zero marks > > and is executed (e.g. using __retval annotation), > then CI passing for s390 should be enough. +1, something like that where for big-endian it will be all zero while for little endian it would be 0xf (and then make sure that the test should *fail* by making sure that 0xf is not a valid index, so NULL check is necessary) > > There is a guide on how to gen a s390 environment locally: > https://docs.kernel.org/bpf/s390.html > I used it recently to build a vmlinux for s390 with no or minimal > issues. Used it to boot long time ago, but don't remember if there > were any surprises. > > > > with Eduard's suggestion this also becomes interesting when you have > > > 000mmm mix (as one example), because that gives you a small range, and > > > all values might be valid keys for arrays > > > > Can you define what "small range" means? What range is there with 0's? > > Any pointers would be helpful. > > I think Andrii means that each 'm' adds 8 bits of range. > E.g. range for 0000_000m is 0-255, range for 0000_00mm is 0-65535, etc. yes, exactly, thank you, Eduard! > > [...] >
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 58b36cc96bd5..4947ef884a18 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -287,6 +287,7 @@ struct bpf_call_arg_meta { u32 ret_btf_id; u32 subprogno; struct btf_field *kptr_field; + s64 const_map_key; }; struct bpf_kfunc_call_arg_meta { @@ -9163,6 +9164,53 @@ static int check_reg_const_str(struct bpf_verifier_env *env, return 0; } +/* Returns constant key value if possible, else -1 */ +static s64 get_constant_map_key(struct bpf_verifier_env *env, + struct bpf_reg_state *key, + u32 key_size) +{ + struct bpf_func_state *state = func(env, key); + struct bpf_reg_state *reg; + int zero_size = 0; + int stack_off; + u8 *stype; + int slot; + int spi; + int i; + + if (!env->bpf_capable) + return -1; + if (key->type != PTR_TO_STACK) + return -1; + if (!tnum_is_const(key->var_off)) + return -1; + + stack_off = key->off + key->var_off.value; + slot = -stack_off - 1; + spi = slot / BPF_REG_SIZE; + + /* First handle precisely tracked STACK_ZERO, up to BPF_REG_SIZE */ + stype = state->stack[spi].slot_type; + for (i = 0; i < BPF_REG_SIZE && stype[i] == STACK_ZERO; i++) + zero_size++; + if (zero_size == key_size) + return 0; + + if (!is_spilled_reg(&state->stack[spi])) + /* Not pointer to stack */ + return -1; + + reg = &state->stack[spi].spilled_ptr; + if (reg->type != SCALAR_VALUE) + /* Only scalars are valid array map keys */ + return -1; + else if (!tnum_is_const(reg->var_off)) + /* Stack value not statically known */ + return -1; + + return reg->var_off.value; +} + static int check_func_arg(struct bpf_verifier_env *env, u32 arg, struct bpf_call_arg_meta *meta, const struct bpf_func_proto *fn, @@ -9173,6 +9221,7 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg, enum bpf_arg_type arg_type = fn->arg_type[arg]; enum bpf_reg_type type = reg->type; u32 *arg_btf_id = NULL; + u32 key_size; int err = 0; bool mask; @@ -9307,8 +9356,11 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg, verbose(env, "invalid map_ptr to access map->key\n"); return -EACCES; } - err = check_helper_mem_access(env, regno, meta->map_ptr->key_size, - BPF_READ, false, NULL); + key_size = meta->map_ptr->key_size; + err = check_helper_mem_access(env, regno, key_size, BPF_READ, false, NULL); + if (err) + return err; + meta->const_map_key = get_constant_map_key(env, reg, key_size); break; case ARG_PTR_TO_MAP_VALUE: if (type_may_be_null(arg_type) && register_is_null(reg)) @@ -10833,6 +10885,21 @@ static void update_loop_inline_state(struct bpf_verifier_env *env, u32 subprogno state->callback_subprogno == subprogno); } +/* 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; + } +} + static int get_helper_proto(struct bpf_verifier_env *env, int func_id, const struct bpf_func_proto **ptr) { @@ -11199,10 +11266,17 @@ static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn "kernel subsystem misconfigured verifier\n"); return -EINVAL; } + + if (func_id == BPF_FUNC_map_lookup_elem && + can_elide_value_nullness(meta.map_ptr->map_type) && + meta.const_map_key >= 0 && + meta.const_map_key < meta.map_ptr->max_entries) + ret_flag &= ~PTR_MAYBE_NULL; + regs[BPF_REG_0].map_ptr = meta.map_ptr; regs[BPF_REG_0].map_uid = meta.map_uid; regs[BPF_REG_0].type = PTR_TO_MAP_VALUE | ret_flag; - if (!type_may_be_null(ret_type) && + if (!type_may_be_null(ret_flag) && btf_record_has_field(meta.map_ptr->record, BPF_SPIN_LOCK)) { regs[BPF_REG_0].id = ++env->id_gen; } diff --git a/tools/testing/selftests/bpf/progs/iters.c b/tools/testing/selftests/bpf/progs/iters.c index 7c969c127573..190822b2f08b 100644 --- a/tools/testing/selftests/bpf/progs/iters.c +++ b/tools/testing/selftests/bpf/progs/iters.c @@ -524,11 +524,11 @@ int iter_subprog_iters(const void *ctx) } struct { - __uint(type, BPF_MAP_TYPE_ARRAY); + __uint(type, BPF_MAP_TYPE_HASH); __type(key, int); __type(value, int); __uint(max_entries, 1000); -} arr_map SEC(".maps"); +} hash_map SEC(".maps"); SEC("?raw_tp") __failure __msg("invalid mem access 'scalar'") @@ -539,7 +539,7 @@ int iter_err_too_permissive1(const void *ctx) MY_PID_GUARD(); - map_val = bpf_map_lookup_elem(&arr_map, &key); + map_val = bpf_map_lookup_elem(&hash_map, &key); if (!map_val) return 0; @@ -561,12 +561,12 @@ int iter_err_too_permissive2(const void *ctx) MY_PID_GUARD(); - map_val = bpf_map_lookup_elem(&arr_map, &key); + map_val = bpf_map_lookup_elem(&hash_map, &key); if (!map_val) return 0; bpf_repeat(1000000) { - map_val = bpf_map_lookup_elem(&arr_map, &key); + map_val = bpf_map_lookup_elem(&hash_map, &key); } *map_val = 123; @@ -585,7 +585,7 @@ int iter_err_too_permissive3(const void *ctx) MY_PID_GUARD(); bpf_repeat(1000000) { - map_val = bpf_map_lookup_elem(&arr_map, &key); + map_val = bpf_map_lookup_elem(&hash_map, &key); found = true; } @@ -606,7 +606,7 @@ int iter_tricky_but_fine(const void *ctx) MY_PID_GUARD(); bpf_repeat(1000000) { - map_val = bpf_map_lookup_elem(&arr_map, &key); + map_val = bpf_map_lookup_elem(&hash_map, &key); if (map_val) { found = true; break; diff --git a/tools/testing/selftests/bpf/progs/map_kptr_fail.c b/tools/testing/selftests/bpf/progs/map_kptr_fail.c index c2a6bd392e48..4c0ff01f1a96 100644 --- a/tools/testing/selftests/bpf/progs/map_kptr_fail.c +++ b/tools/testing/selftests/bpf/progs/map_kptr_fail.c @@ -345,7 +345,7 @@ int reject_indirect_global_func_access(struct __sk_buff *ctx) } SEC("?tc") -__failure __msg("Unreleased reference id=5 alloc_insn=") +__failure __msg("Unreleased reference id=4 alloc_insn=") int kptr_xchg_ref_state(struct __sk_buff *ctx) { struct prog_test_ref_kfunc *p; diff --git a/tools/testing/selftests/bpf/progs/verifier_map_in_map.c b/tools/testing/selftests/bpf/progs/verifier_map_in_map.c index 4eaab1468eb7..7d088ba99ea5 100644 --- a/tools/testing/selftests/bpf/progs/verifier_map_in_map.c +++ b/tools/testing/selftests/bpf/progs/verifier_map_in_map.c @@ -47,7 +47,7 @@ l0_%=: r0 = 0; \ SEC("xdp") __description("map in map state pruning") -__success __msg("processed 26 insns") +__success __msg("processed 15 insns") __log_level(2) __retval(0) __flag(BPF_F_TEST_STATE_FREQ) __naked void map_in_map_state_pruning(void) { diff --git a/tools/testing/selftests/bpf/verifier/map_kptr.c b/tools/testing/selftests/bpf/verifier/map_kptr.c index f420c0312aa0..4b39f8472f9b 100644 --- a/tools/testing/selftests/bpf/verifier/map_kptr.c +++ b/tools/testing/selftests/bpf/verifier/map_kptr.c @@ -373,7 +373,7 @@ .prog_type = BPF_PROG_TYPE_SCHED_CLS, .fixup_map_kptr = { 1 }, .result = REJECT, - .errstr = "Unreleased reference id=5 alloc_insn=20", + .errstr = "Unreleased reference id=4 alloc_insn=20", .fixup_kfunc_btf_id = { { "bpf_kfunc_call_test_acquire", 15 }, }
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. Tests also have to be updated in sync with these changes, as the verifier is more efficient with this change. Notable, iters.c tests had to be changed to use a map type that still requires null checks, as it's exercising verifier tracking logic w.r.t iterators. Signed-off-by: Daniel Xu <dxu@dxuuu.xyz> --- kernel/bpf/verifier.c | 80 ++++++++++++++++++- tools/testing/selftests/bpf/progs/iters.c | 14 ++-- .../selftests/bpf/progs/map_kptr_fail.c | 2 +- .../selftests/bpf/progs/verifier_map_in_map.c | 2 +- .../testing/selftests/bpf/verifier/map_kptr.c | 2 +- 5 files changed, 87 insertions(+), 13 deletions(-)