On Thu, May 28, 2020 at 9:39 PM Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> wrote:
>
> From: Alexei Starovoitov <ast@xxxxxxxxxx>
>
> Introduce sleepable BPF programs that can request such property for themselves
> via BPF_F_SLEEPABLE flag at program load time. In such case they will be able
> to use helpers like bpf_copy_from_user() that might sleep. At present only
> fentry/fexit/fmod_ret and lsm programs can request to be sleepable and only
> when they are attached to kernel functions that are known to allow sleeping.
>
> The non-sleepable programs are relying on implicit rcu_read_lock() and
> migrate_disable() to protect life time of programs, maps that they use and
> per-cpu kernel structures used to pass info between bpf programs and the
> kernel. The sleepable programs cannot be enclosed into rcu_read_lock().
> migrate_disable() maps to preempt_disable() in non-RT kernels, so the progs
> should not be enclosed in migrate_disable() as well. Therefore bpf_srcu is used
> to protect the life time of sleepable progs.
>
> There are many networking and tracing program types. In many cases the
> 'struct bpf_prog *' pointer itself is rcu protected within some other kernel
> data structure and the kernel code is using rcu_dereference() to load that
> program pointer and call BPF_PROG_RUN() on it. All these cases are not touched.
> Instead sleepable bpf programs are allowed with bpf trampoline only. The
> program pointers are hard-coded into generated assembly of bpf trampoline and
> synchronize_srcu(&bpf_srcu) is used to protect the life time of the program.
> The same trampoline can hold both sleepable and non-sleepable progs.
>
> When bpf_srcu lock is held it means that some sleepable bpf program is running
> from bpf trampoline. Those programs can use bpf arrays and preallocated hash/lru
> maps. These map types are waiting on programs to complete via
> synchronize_srcu(&bpf_srcu);
>
> Updates to trampoline now has to do synchronize_srcu + synchronize_rcu_tasks
> to wait for sleepable progs to finish and for trampoline assembly to finish.
>
> In the future srcu will be replaced with upcoming rcu_trace.
> That will complete the first step of introducing sleepable progs.
>
> After that dynamically allocated hash maps can be allowed. All map elements
> would have to be srcu protected instead of normal rcu.
> per-cpu maps will be allowed. Either via the following pattern:
> void *elem = bpf_map_lookup_elem(map, key);
> if (elem) {
> // access elem
> bpf_map_release_elem(map, elem);
> }
> where modified lookup() helper will do migrate_disable() and
> new bpf_map_release_elem() will do corresponding migrate_enable().
> Or explicit bpf_migrate_disable/enable() helpers will be introduced.
>
> Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx>
> Acked-by: KP Singh <kpsingh@xxxxxxxxxx>
> ---
> arch/x86/net/bpf_jit_comp.c | 36 ++++++++++++++------
> include/linux/bpf.h | 4 +++
> include/uapi/linux/bpf.h | 8 +++++
> kernel/bpf/arraymap.c | 5 +++
> kernel/bpf/hashtab.c | 19 +++++++----
> kernel/bpf/syscall.c | 12 +++++--
> kernel/bpf/trampoline.c | 33 +++++++++++++++++-
> kernel/bpf/verifier.c | 62 +++++++++++++++++++++++++++++++++-
> tools/include/uapi/linux/bpf.h | 8 +++++
> 9 files changed, 165 insertions(+), 22 deletions(-)
>
[...]
> +/* If BPF_F_SLEEPABLE is used in BPF_PROG_LOAD command, the verifier will
> + * restrict map and helper usage for such programs. Sleepable BPF programs can
> + * only be attached to hooks where kernel execution context allows sleeping.
> + * Such programs are allowed to use helpers that may sleep like
> + * bpf_copy_from_user().
> + */
> +#define BPF_F_SLEEPABLE (1U << 4)
> +
> /* When BPF ldimm64's insn[0].src_reg != 0 then this can have
> * two extensions:
> *
> diff --git a/kernel/bpf/arraymap.c b/kernel/bpf/arraymap.c
> index 11584618e861..26b18b6a3dbc 100644
> --- a/kernel/bpf/arraymap.c
> +++ b/kernel/bpf/arraymap.c
> @@ -393,6 +393,11 @@ static void array_map_free(struct bpf_map *map)
> */
> synchronize_rcu();
>
> + /* arrays could have been used by both sleepable and non-sleepable bpf
> + * progs. Make sure to wait for both prog types to finish executing.
> + */
> + synchronize_srcu(&bpf_srcu);
> +
to minimize churn later on when you switch to rcu_trace, maybe extract
synchronize_rcu() + synchronize_srcu(&bpf_srcu) into a function (e.g.,
something like synchronize_sleepable_bpf?), exposed as an internal
API? That way you also wouldn't need to add bpf_srcu to linux/bpf.h?
> if (array->map.map_type == BPF_MAP_TYPE_PERCPU_ARRAY)
> bpf_array_free_percpu(array);
>
> diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
> index b4b288a3c3c9..b001957fdcbf 100644
> --- a/kernel/bpf/hashtab.c
> +++ b/kernel/bpf/hashtab.c
> @@ -577,8 +577,8 @@ static void *__htab_map_lookup_elem(struct bpf_map *map, void *key)
> struct htab_elem *l;
> u32 hash, key_size;
>
> - /* Must be called with rcu_read_lock. */
> - WARN_ON_ONCE(!rcu_read_lock_held());
> + /* Must be called with s?rcu_read_lock. */
> + WARN_ON_ONCE(!rcu_read_lock_held() && !srcu_read_lock_held(&bpf_srcu));
>
Similar to above, might be worthwhile extracting into a function?
> key_size = map->key_size;
>
> @@ -935,7 +935,7 @@ static int htab_map_update_elem(struct bpf_map *map, void *key, void *value,
> /* unknown flags */
> return -EINVAL;
>
> - WARN_ON_ONCE(!rcu_read_lock_held());
> + WARN_ON_ONCE(!rcu_read_lock_held() && !srcu_read_lock_held(&bpf_srcu));
>
> key_size = map->key_size;
>
[...]
> static int check_attach_btf_id(struct bpf_verifier_env *env)
> {
> struct bpf_prog *prog = env->prog;
> @@ -10549,6 +10582,12 @@ static int check_attach_btf_id(struct bpf_verifier_env *env)
> long addr;
> u64 key;
>
> + if (prog->aux->sleepable && prog->type != BPF_PROG_TYPE_TRACING &&
> + prog->type != BPF_PROG_TYPE_LSM) {
> + verbose(env, "Only fentry/fexit/fmod_ret and lsm programs can be sleepable\n");
> + return -EINVAL;
> + }
BPF_PROG_TYPE_TRACING also includes iterator and raw tracepoint
programs. You mention only fentry/fexit/fmod_ret are allowed. What
about those two? I don't see any explicit checks for iterator and
raw_tracepoint attach types in a switch below, so just checking if
they should be allowed to be sleepable?
Also seems like freplace ones are also sleeepable, if they replace
sleepable programs, right?
> +
> if (prog->type == BPF_PROG_TYPE_STRUCT_OPS)
> return check_struct_ops_btf_id(env);
>
> @@ -10762,8 +10801,29 @@ static int check_attach_btf_id(struct bpf_verifier_env *env)
> if (ret)
> verbose(env, "%s() is not modifiable\n",
> prog->aux->attach_func_name);
> + } else if (prog->aux->sleepable) {
> + switch (prog->type) {
> + case BPF_PROG_TYPE_TRACING:
> + /* fentry/fexit progs can be sleepable only if they are
> + * attached to ALLOW_ERROR_INJECTION or security_*() funcs.
> + */
> + ret = check_attach_modify_return(prog, addr);
I was so confused about this piece... check_attach_modify_return()
should probably be renamed to something else, it's not for fmod_ret
only anymore.
> + if (!ret)
> + ret = check_sleepable_blacklist(addr);
> + break;
> + case BPF_PROG_TYPE_LSM:
> + /* LSM progs check that they are attached to bpf_lsm_*() funcs
> + * which are sleepable too.
> + */
> + ret = check_sleepable_blacklist(addr);
> + break;
> + default:
> + break;
> + }
> + if (ret)
> + verbose(env, "%s is not sleepable\n",
> + prog->aux->attach_func_name);
> }
> -
> if (ret)
> goto out;
> tr->func.addr = (void *)addr;
[...]
