Re: [PATCH bpf-next v3 1/9] bpf: Support __arena and __arena_nullable kfunc argument suffixes
"Kumar Kartikeya Dwivedi" <[email protected]> Wed, 05 Aug 2026 17:53:37 +0200
| Newsgroups | org.kernel.vger.bpf |
|---|---|
| Message-ID | <[email protected]> |
On Tue Aug 4, 2026 at 7:42 PM CEST, Amery Hung wrote: > On Mon, Aug 3, 2026 at 6:12 AM Kumar Kartikeya Dwivedi <[email protected]> wrote: >> >> From: Tejun Heo <[email protected]> >> >> Passing an arena pointer to a kfunc takes two steps today. There is no >> arena pointer argument type, so the pointer crosses the boundary as a >> bare scalar, and the kfunc then offsets it by the arena base and casts >> it before it can touch the memory. Every such kfunc open-codes the same >> translation. >> >> Add the __arena and __arena_nullable argument suffixes to make this more >> convenient. The kfunc declares the parameter by its real pointer type >> and dereferences it directly, with the JIT rebasing the value at the >> call site, rN = kern_vm_start + (u32)rN. No bounds check is needed: the >> u32 offset stays within the guard-padded arena kernel mapping, and a >> fault on an unpopulated page recovers through the per-arena scratch >> page. A suffixed argument accepts a PTR_TO_ARENA or scalar register, >> matching global subprog arena arguments. >> >> __arena rebases unconditionally, so the kfunc never sees NULL and a >> value with zero in the low 32 bits arrives as the arena base. >> __arena_nullable preserves NULL for optional arguments by skipping the >> rebase when the truncated value, arena offset 0, is zero. Keeping the >> plain form NULL-free saves the NULL test on every call. >> >> This patch adds the verifier side: the suffixes are recognized in >> check_kfunc_args() and distilled into argument flags in the function >> model stored in the kfunc descriptor. JITs retrieve the model while >> emitting the call, avoiding per-call state in insn_aux_data. >> >> JITs declare support with bpf_jit_supports_arena_args() and verification >> fails with -ENOTSUPP elsewhere. >> >> Signed-off-by: Tejun Heo <[email protected]> >> Co-developed-by: Kumar Kartikeya Dwivedi <[email protected]> >> Signed-off-by: Kumar Kartikeya Dwivedi <[email protected]> >> --- >> Documentation/bpf/kfuncs.rst | 29 ++++++++++++++++++++++++++ >> include/linux/bpf.h | 6 ++++++ >> include/linux/filter.h | 1 + >> kernel/bpf/btf.c | 18 +++++++++++++++- >> kernel/bpf/core.c | 5 +++++ >> kernel/bpf/verifier.c | 40 +++++++++++++++++++++++++++++++++++- >> 6 files changed, 97 insertions(+), 2 deletions(-) >> >> diff --git a/Documentation/bpf/kfuncs.rst b/Documentation/bpf/kfuncs.rst >> index cbde86d082cc..8e84484636c5 100644 >> --- a/Documentation/bpf/kfuncs.rst >> +++ b/Documentation/bpf/kfuncs.rst >> @@ -278,6 +278,33 @@ An example is given below:: >> ... >> } >> >> +2.3.8 __arena and __arena_nullable Annotations >> +---------------------------------------------- >> + >> +Both annotations indicate that the pointer argument points into the >> +calling program's arena. The JIT rebases the value at the call site so >> +the kfunc receives a directly dereferenceable kernel address, subject to >> +the access rules described in :ref:`BPF_kfunc_arena_access` (at most >> +``GUARD_SZ / 2``, 32 KiB, past the pointer in a single unchecked access). >> + >> +With ``__arena`` the rebase is unconditional and the argument is never >> +NULL: a value whose lower 32 bits are zero arrives as the arena base >> +address (arena offset 0). The kfunc must not check the argument for NULL. >> +With ``__arena_nullable`` such a value arrives as NULL instead and the >> +kfunc must check before dereferencing. >> + >> +An example is given below:: >> + >> + __bpf_kfunc int bpf_process_item(struct item *item__arena) >> + { >> + ... >> + } >> + >> +Calling such a kfunc requires the program to use an arena map and a JIT with >> +arena argument support (currently x86-64); verification fails otherwise. The >> +program can pass any value without compromising the kernel. A value that does >> +not point into the arena is a program bug. >> + >> .. _BPF_kfunc_nodef: >> >> 2.4 Using an existing kernel function >> @@ -515,6 +542,8 @@ In order to accommodate such requirements, the verifier will enforce strict >> PTR_TO_BTF_ID type matching if two types have the exact same name, with one >> being suffixed with ``___init``. >> >> +.. _BPF_kfunc_arena_access: >> + >> 2.8 Accessing arena memory through kfunc arguments >> -------------------------------------------------- >> >> diff --git a/include/linux/bpf.h b/include/linux/bpf.h >> index 356884587ae1..6da8a6be930c 100644 >> --- a/include/linux/bpf.h >> +++ b/include/linux/bpf.h >> @@ -1213,6 +1213,12 @@ struct bpf_prog_offload { >> /* The argument is signed. */ >> #define BTF_FMODEL_SIGNED_ARG BIT(1) >> >> +/* The argument is an arena pointer. */ >> +#define BTF_FMODEL_ARENA_ARG BIT(2) >> + >> +/* The argument is nullable. */ >> +#define BTF_FMODEL_NULLABLE_ARG BIT(3) >> + >> struct btf_func_model { >> u8 ret_size; >> u8 ret_flags; >> diff --git a/include/linux/filter.h b/include/linux/filter.h >> index 32d5297c557e..36ce3403fe59 100644 >> --- a/include/linux/filter.h >> +++ b/include/linux/filter.h >> @@ -1183,6 +1183,7 @@ bool bpf_jit_supports_subprog_tailcalls(void); >> bool bpf_jit_supports_percpu_insn(void); >> bool bpf_jit_supports_kfunc_call(void); >> bool bpf_jit_supports_stack_args(void); >> +bool bpf_jit_supports_arena_args(void); >> bool bpf_jit_supports_far_kfunc_call(void); >> bool bpf_jit_supports_exceptions(void); >> bool bpf_jit_supports_ptr_xchg(void); >> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c >> index 5e8ac45ce56a..bf86df91a70b 100644 >> --- a/kernel/bpf/btf.c >> +++ b/kernel/bpf/btf.c >> @@ -7541,6 +7541,22 @@ static u8 __get_type_fmodel_flags(const struct btf_type *t) >> return flags; >> } >> >> +static u8 __get_arg_fmodel_flags(const struct btf *btf, >> + const struct btf_param *arg, >> + const struct btf_type *t) >> +{ >> + u8 flags = __get_type_fmodel_flags(t); >> + >> + if (btf_param_match_suffix(btf, arg, "__arena") || >> + btf_param_match_suffix(btf, arg, "__arena_nullable")) >> + flags |= BTF_FMODEL_ARENA_ARG; >> + if (btf_param_match_suffix(btf, arg, "__nullable") || >> + btf_param_match_suffix(btf, arg, "__arena_nullable")) >> + flags |= BTF_FMODEL_NULLABLE_ARG; >> + >> + return flags; >> +} >> + >> int btf_distill_func_proto(struct bpf_verifier_log *log, >> struct btf *btf, >> const struct btf_type *func, >> @@ -7606,7 +7622,7 @@ int btf_distill_func_proto(struct bpf_verifier_log *log, >> return -EINVAL; >> } >> m->arg_size[i] = ret; >> - m->arg_flags[i] = __get_type_fmodel_flags(t); >> + m->arg_flags[i] = __get_arg_fmodel_flags(btf, &args[i], t); >> } >> m->nr_args = nargs; >> return 0; >> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c >> index e2076667b245..a3e1fae32eac 100644 >> --- a/kernel/bpf/core.c >> +++ b/kernel/bpf/core.c >> @@ -3308,6 +3308,11 @@ bool __weak bpf_jit_supports_stack_args(void) >> return false; >> } >> >> +bool __weak bpf_jit_supports_arena_args(void) >> +{ >> + return false; >> +} >> + >> bool __weak bpf_jit_supports_far_kfunc_call(void) >> { >> return false; >> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c >> index b274004fccfd..4c50237f49f1 100644 >> --- a/kernel/bpf/verifier.c >> +++ b/kernel/bpf/verifier.c >> @@ -10907,6 +10907,16 @@ static bool is_kfunc_arg_irq_flag(const struct btf *btf, const struct btf_param >> return btf_param_match_suffix(btf, arg, "__irq_flag"); >> } >> >> +static bool is_kfunc_arg_arena(const struct btf *btf, const struct btf_param *arg) >> +{ >> + return btf_param_match_suffix(btf, arg, "__arena"); >> +} >> + >> +static bool is_kfunc_arg_arena_nullable(const struct btf *btf, const struct btf_param *arg) >> +{ >> + return btf_param_match_suffix(btf, arg, "__arena_nullable"); >> +} >> + > > From the verifier's point of view, __arena and __arena_nullable all > maps to KF_ARG_PTR_TO_ARENA and require the same check so maybe > is_kfunc_arg_arena() alone with two suffix matching is good enough, > but it also doesn't hurt to have two separated functions. > >> static bool is_kfunc_arg_scalar_with_name(const struct btf *btf, >> const struct btf_param *arg, >> const char *name) >> @@ -11127,6 +11137,7 @@ enum kfunc_ptr_arg_type { >> KF_ARG_PTR_TO_IRQ_FLAG, >> KF_ARG_PTR_TO_RES_SPIN_LOCK, >> KF_ARG_PTR_TO_TASK_WORK, >> + KF_ARG_PTR_TO_ARENA, >> }; >> >> enum special_kfunc_type { >> @@ -11412,7 +11423,6 @@ get_kfunc_arg_type(struct bpf_verifier_env *env, struct bpf_call_arg_meta *meta, >> reg_arg_name(env, argno), btf_type_str(t)); >> return -EINVAL; >> } >> - >> ref_t = btf_type_skip_modifiers(meta->btf, t->type, NULL); >> ref_tname = btf_name_by_offset(meta->btf, ref_t->name_off); >> >> @@ -11461,6 +11471,9 @@ get_kfunc_arg_type(struct bpf_verifier_env *env, struct bpf_call_arg_meta *meta, >> arg_type = KF_ARG_PTR_TO_RES_SPIN_LOCK; >> else if (is_kfunc_arg_callback(env, meta->btf, &args[arg])) >> arg_type = KF_ARG_PTR_TO_CALLBACK; >> + else if (is_kfunc_arg_arena(meta->btf, &args[arg]) || >> + is_kfunc_arg_arena_nullable(meta->btf, &args[arg])) >> + arg_type = KF_ARG_PTR_TO_ARENA; >> else if (arg + 1 < nargs && >> (is_kfunc_arg_mem_size(meta->btf, &args[arg + 1]) || >> is_kfunc_arg_const_mem_size(meta->btf, &args[arg + 1]))) { >> @@ -12154,6 +12167,31 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_call_arg_me >> >> t = btf_type_skip_modifiers(btf, args[i].type, NULL); >> >> + if (base_type(kf_arg_type) == KF_ARG_PTR_TO_ARENA) { >> + if (!bpf_jit_supports_arena_args()) { >> + verbose(env, "JIT does not support kfunc %s() with arena pointer arguments\n", >> + func_name); >> + return -ENOTSUPP; >> + } > > nit: JIT and register-based arg check can be done earlier in > get_kfunc_arg_type(). We probably will want a consolidated pass > validating bpf_func_proto. > I moved it up there. We still have prog aux argument doing stack argument rejection later, but I will let you figure out the right placement when you rework these bits. Only thing left here now is the register type check. >> + if (!env->prog->aux->arena) { >> + verbose(env, >> + "%s arena pointer requires a program with an associated arena\n", >> + reg_arg_name(env, argno)); >> + return -EINVAL; >> + } >> + if (regno < 0) { >> + verbose(env, "%s arena pointer cannot be a stack argument\n", >> + reg_arg_name(env, argno)); >> + return -EINVAL; >> + } >> + if (reg->type != PTR_TO_ARENA && reg->type != SCALAR_VALUE) { >> + verbose(env, "%s is not a pointer to arena or scalar\n", >> + reg_arg_name(env, argno)); >> + return -EINVAL; >> + } >> + continue; >> + } >> + > > Can we move arena argument checking to the second switch case below? > It only takes adding one condition below. > > This help reduce churn in the future when check_kfunc_args() also > receieve check_reg_type(). > >> if (btf_type_is_ptr(t) && (bpf_register_is_null(reg) || type_may_be_null(reg->type)) && >> !is_kfunc_arg_nullable(meta->btf, &args[i])) { >> verbose(env, "Possibly NULL pointer passed to trusted %s\n", > > if (btf_type_is_ptr(t) && base_type(kf_arg_type) != KF_ARG_PTR_TO_ARENA && > (bpf_register_is_null(reg) || type_may_be_null(reg->type)) && > !type_may_be_null(kf_arg_type)) { > verbose(env, "Possibly NULL pointer passed to trusted %s\n", > > To simplify this and avoid special casing, I changed the kf_arg_type to KF_ARG_PTR_TO_ARENA | PTR_MAYBE_NULL, since known NULL is kosher for both cases and we place no static constraints on the precise value.