Date:   Tue, 30 Jun 2020 15:04:32 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     davem@...emloft.net, daniel@...earbox.net, netdev@...r.kernel.org,
        bpf@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH v5 bpf-next 2/5] bpf: Introduce sleepable BPF programs

On Mon, Jun 29, 2020 at 09:33:40PM -0700, Alexei Starovoitov wrote:
> From: Alexei Starovoitov <ast@...nel.org>
> 
> 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
> rcu_read_lock_trace 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_rcu_tasks_trace() is used to protect the life time of the program.
> The same trampoline can hold both sleepable and non-sleepable progs.
> 
> When rcu_read_lock_trace 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_rcu_tasks_trace();
> 
> Updates to trampoline now has to do synchronize_rcu_tasks_trace() and
> synchronize_rcu_tasks() to wait for sleepable progs to finish and for
> trampoline assembly to finish.
> 
> This is the first step of introducing sleepable progs.
> 
> After that dynamically allocated hash maps can be allowed. All map elements
> would have to be rcu_trace 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@...nel.org>
> Acked-by: Andrii Nakryiko <andriin@...com>

>From an RCU viewpoint:

Acked-by: Paul E. McKenney <paulmck@...nel.org>

I was going to suggest that BPF maps could just use RCU throughout,
but it appears that this would require an asynchronous wait for both
an RCU and an RCU tasks trace grace period.  Such a thing is possible,
but the price is a reference count, multiple rcu_head structures, and
with each rcu_head structure, a pointer to the reference count.  Which
just might not be worth it.

> ---
>  arch/x86/net/bpf_jit_comp.c    | 32 ++++++++++++------
>  include/linux/bpf.h            |  3 ++
>  include/uapi/linux/bpf.h       |  8 +++++
>  init/Kconfig                   |  1 +
>  kernel/bpf/arraymap.c          |  1 +
>  kernel/bpf/hashtab.c           | 12 +++----
>  kernel/bpf/syscall.c           | 13 +++++--
>  kernel/bpf/trampoline.c        | 28 +++++++++++++--
>  kernel/bpf/verifier.c          | 62 +++++++++++++++++++++++++++++++++-
>  tools/include/uapi/linux/bpf.h |  8 +++++
>  10 files changed, 144 insertions(+), 24 deletions(-)
> 
> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index 42b6709e6dc7..7d9ea7b41c71 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -1379,10 +1379,15 @@ static int invoke_bpf_prog(const struct btf_func_model *m, u8 **pprog,
>  	u8 *prog = *pprog;
>  	int cnt = 0;
>  
> -	if (emit_call(&prog, __bpf_prog_enter, prog))
> -		return -EINVAL;
> -	/* remember prog start time returned by __bpf_prog_enter */
> -	emit_mov_reg(&prog, true, BPF_REG_6, BPF_REG_0);
> +	if (p->aux->sleepable) {
> +		if (emit_call(&prog, __bpf_prog_enter_sleepable, prog))
> +			return -EINVAL;
> +	} else {
> +		if (emit_call(&prog, __bpf_prog_enter, prog))
> +			return -EINVAL;
> +		/* remember prog start time returned by __bpf_prog_enter */
> +		emit_mov_reg(&prog, true, BPF_REG_6, BPF_REG_0);
> +	}
>  
>  	/* arg1: lea rdi, [rbp - stack_size] */
>  	EMIT4(0x48, 0x8D, 0x7D, -stack_size);
> @@ -1402,13 +1407,18 @@ static int invoke_bpf_prog(const struct btf_func_model *m, u8 **pprog,
>  	if (mod_ret)
>  		emit_stx(&prog, BPF_DW, BPF_REG_FP, BPF_REG_0, -8);
>  
> -	/* arg1: mov rdi, progs[i] */
> -	emit_mov_imm64(&prog, BPF_REG_1, (long) p >> 32,
> -		       (u32) (long) p);
> -	/* arg2: mov rsi, rbx <- start time in nsec */
> -	emit_mov_reg(&prog, true, BPF_REG_2, BPF_REG_6);
> -	if (emit_call(&prog, __bpf_prog_exit, prog))
> -		return -EINVAL;
> +	if (p->aux->sleepable) {
> +		if (emit_call(&prog, __bpf_prog_exit_sleepable, prog))
> +			return -EINVAL;
> +	} else {
> +		/* arg1: mov rdi, progs[i] */
> +		emit_mov_imm64(&prog, BPF_REG_1, (long) p >> 32,
> +			       (u32) (long) p);
> +		/* arg2: mov rsi, rbx <- start time in nsec */
> +		emit_mov_reg(&prog, true, BPF_REG_2, BPF_REG_6);
> +		if (emit_call(&prog, __bpf_prog_exit, prog))
> +			return -EINVAL;
> +	}
>  
>  	*pprog = prog;
>  	return 0;
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 3d2ade703a35..e2b1581b2195 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -495,6 +495,8 @@ int arch_prepare_bpf_trampoline(void *image, void *image_end,
>  /* these two functions are called from generated trampoline */
>  u64 notrace __bpf_prog_enter(void);
>  void notrace __bpf_prog_exit(struct bpf_prog *prog, u64 start);
> +void notrace __bpf_prog_enter_sleepable(void);
> +void notrace __bpf_prog_exit_sleepable(void);
>  
>  struct bpf_ksym {
>  	unsigned long		 start;
> @@ -687,6 +689,7 @@ struct bpf_prog_aux {
>  	bool offload_requested;
>  	bool attach_btf_trace; /* true if attaching to BTF-enabled raw tp */
>  	bool func_proto_unreliable;
> +	bool sleepable;
>  	enum bpf_tramp_prog_type trampoline_prog_type;
>  	struct bpf_trampoline *trampoline;
>  	struct hlist_node tramp_hlist;
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 0cb8ec948816..73f9e3f84b77 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -332,6 +332,14 @@ enum bpf_link_type {
>  /* The verifier internal test flag. Behavior is undefined */
>  #define BPF_F_TEST_STATE_FREQ	(1U << 3)
>  
> +/* 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/init/Kconfig b/init/Kconfig
> index a46aa8f3174d..62687583f822 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1663,6 +1663,7 @@ config BPF_SYSCALL
>  	bool "Enable bpf() system call"
>  	select BPF
>  	select IRQ_WORK
> +	select TASKS_TRACE_RCU
>  	default n
>  	help
>  	  Enable the bpf() system call that allows to manipulate eBPF
> diff --git a/kernel/bpf/arraymap.c b/kernel/bpf/arraymap.c
> index c66e8273fccd..b07abcd58785 100644
> --- a/kernel/bpf/arraymap.c
> +++ b/kernel/bpf/arraymap.c
> @@ -10,6 +10,7 @@
>  #include <linux/filter.h>
>  #include <linux/perf_event.h>
>  #include <uapi/linux/btf.h>
> +#include <linux/rcupdate_trace.h>
>  
>  #include "map_in_map.h"
>  
> diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
> index d4378d7d442b..65a7919e189d 100644
> --- a/kernel/bpf/hashtab.c
> +++ b/kernel/bpf/hashtab.c
> @@ -9,6 +9,7 @@
>  #include <linux/rculist_nulls.h>
>  #include <linux/random.h>
>  #include <uapi/linux/btf.h>
> +#include <linux/rcupdate_trace.h>
>  #include "percpu_freelist.h"
>  #include "bpf_lru_list.h"
>  #include "map_in_map.h"
> @@ -577,8 +578,7 @@ 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());
> +	WARN_ON_ONCE(!rcu_read_lock_held() && !rcu_read_lock_trace_held());
>  
>  	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() && !rcu_read_lock_trace_held());
>  
>  	key_size = map->key_size;
>  
> @@ -1026,7 +1026,7 @@ static int htab_lru_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() && !rcu_read_lock_trace_held());
>  
>  	key_size = map->key_size;
>  
> @@ -1214,7 +1214,7 @@ static int htab_map_delete_elem(struct bpf_map *map, void *key)
>  	u32 hash, key_size;
>  	int ret = -ENOENT;
>  
> -	WARN_ON_ONCE(!rcu_read_lock_held());
> +	WARN_ON_ONCE(!rcu_read_lock_held() && !rcu_read_lock_trace_held());
>  
>  	key_size = map->key_size;
>  
> @@ -1246,7 +1246,7 @@ static int htab_lru_map_delete_elem(struct bpf_map *map, void *key)
>  	u32 hash, key_size;
>  	int ret = -ENOENT;
>  
> -	WARN_ON_ONCE(!rcu_read_lock_held());
> +	WARN_ON_ONCE(!rcu_read_lock_held() && !rcu_read_lock_trace_held());
>  
>  	key_size = map->key_size;
>  
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index 8da159936bab..782b2b029539 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -29,6 +29,7 @@
>  #include <linux/bpf_lsm.h>
>  #include <linux/poll.h>
>  #include <linux/bpf-netns.h>
> +#include <linux/rcupdate_trace.h>
>  
>  #define IS_FD_ARRAY(map) ((map)->map_type == BPF_MAP_TYPE_PERF_EVENT_ARRAY || \
>  			  (map)->map_type == BPF_MAP_TYPE_CGROUP_ARRAY || \
> @@ -1728,10 +1729,14 @@ static void __bpf_prog_put_noref(struct bpf_prog *prog, bool deferred)
>  	btf_put(prog->aux->btf);
>  	bpf_prog_free_linfo(prog);
>  
> -	if (deferred)
> -		call_rcu(&prog->aux->rcu, __bpf_prog_put_rcu);
> -	else
> +	if (deferred) {
> +		if (prog->aux->sleepable)
> +			call_rcu_tasks_trace(&prog->aux->rcu, __bpf_prog_put_rcu);
> +		else
> +			call_rcu(&prog->aux->rcu, __bpf_prog_put_rcu);
> +	} else {
>  		__bpf_prog_put_rcu(&prog->aux->rcu);
> +	}
>  }
>  
>  static void __bpf_prog_put(struct bpf_prog *prog, bool do_idr_lock)
> @@ -2096,6 +2101,7 @@ static int bpf_prog_load(union bpf_attr *attr, union bpf_attr __user *uattr)
>  	if (attr->prog_flags & ~(BPF_F_STRICT_ALIGNMENT |
>  				 BPF_F_ANY_ALIGNMENT |
>  				 BPF_F_TEST_STATE_FREQ |
> +				 BPF_F_SLEEPABLE |
>  				 BPF_F_TEST_RND_HI32))
>  		return -EINVAL;
>  
> @@ -2151,6 +2157,7 @@ static int bpf_prog_load(union bpf_attr *attr, union bpf_attr __user *uattr)
>  	}
>  
>  	prog->aux->offload_requested = !!attr->prog_ifindex;
> +	prog->aux->sleepable = attr->prog_flags & BPF_F_SLEEPABLE;
>  
>  	err = security_bpf_prog_alloc(prog->aux);
>  	if (err)
> diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
> index 9be85aa4ec5f..c2b76545153c 100644
> --- a/kernel/bpf/trampoline.c
> +++ b/kernel/bpf/trampoline.c
> @@ -7,6 +7,8 @@
>  #include <linux/rbtree_latch.h>
>  #include <linux/perf_event.h>
>  #include <linux/btf.h>
> +#include <linux/rcupdate_trace.h>
> +#include <linux/rcupdate_wait.h>
>  
>  /* dummy _ops. The verifier will operate on target program's ops. */
>  const struct bpf_verifier_ops bpf_extension_verifier_ops = {
> @@ -210,9 +212,12 @@ static int bpf_trampoline_update(struct bpf_trampoline *tr)
>  	 * updates to trampoline would change the code from underneath the
>  	 * preempted task. Hence wait for tasks to voluntarily schedule or go
>  	 * to userspace.
> +	 * The same trampoline can hold both sleepable and non-sleepable progs.
> +	 * synchronize_rcu_tasks_trace() is needed to make sure all sleepable
> +	 * programs finish executing.
> +	 * Wait for these two grace periods together.
>  	 */
> -
> -	synchronize_rcu_tasks();
> +	synchronize_rcu_mult(call_rcu_tasks, call_rcu_tasks_trace);
>  
>  	err = arch_prepare_bpf_trampoline(new_image, new_image + PAGE_SIZE / 2,
>  					  &tr->func.model, flags, tprogs,
> @@ -344,7 +349,14 @@ void bpf_trampoline_put(struct bpf_trampoline *tr)
>  	if (WARN_ON_ONCE(!hlist_empty(&tr->progs_hlist[BPF_TRAMP_FEXIT])))
>  		goto out;
>  	bpf_image_ksym_del(&tr->ksym);
> -	/* wait for tasks to get out of trampoline before freeing it */
> +	/* This code will be executed when all bpf progs (both sleepable and
> +	 * non-sleepable) went through
> +	 * bpf_prog_put()->call_rcu[_tasks_trace]()->bpf_prog_free_deferred().
> +	 * Hence no need for another synchronize_rcu_tasks_trace() here,
> +	 * but synchronize_rcu_tasks() is still needed, since trampoline
> +	 * may not have had any sleepable programs and we need to wait
> +	 * for tasks to get out of trampoline code before freeing it.
> +	 */
>  	synchronize_rcu_tasks();
>  	bpf_jit_free_exec(tr->image);
>  	hlist_del(&tr->hlist);
> @@ -394,6 +406,16 @@ void notrace __bpf_prog_exit(struct bpf_prog *prog, u64 start)
>  	rcu_read_unlock();
>  }
>  
> +void notrace __bpf_prog_enter_sleepable(void)
> +{
> +	rcu_read_lock_trace();
> +}
> +
> +void notrace __bpf_prog_exit_sleepable(void)
> +{
> +	rcu_read_unlock_trace();
> +}
> +
>  int __weak
>  arch_prepare_bpf_trampoline(void *image, void *image_end,
>  			    const struct btf_func_model *m, u32 flags,
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 7de98906ddf4..05aa990ba9a4 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -9112,6 +9112,23 @@ static int check_map_prog_compatibility(struct bpf_verifier_env *env,
>  		return -EINVAL;
>  	}
>  
> +	if (prog->aux->sleepable)
> +		switch (map->map_type) {
> +		case BPF_MAP_TYPE_HASH:
> +		case BPF_MAP_TYPE_LRU_HASH:
> +		case BPF_MAP_TYPE_ARRAY:
> +			if (!is_preallocated_map(map)) {
> +				verbose(env,
> +					"Sleepable programs can only use preallocated hash maps\n");
> +				return -EINVAL;
> +			}
> +			break;
> +		default:
> +			verbose(env,
> +				"Sleepable programs can only use array and hash maps\n");
> +			return -EINVAL;
> +		}
> +
>  	return 0;
>  }
>  
> @@ -10722,6 +10739,22 @@ static int check_attach_modify_return(struct bpf_prog *prog, unsigned long addr)
>  	return -EINVAL;
>  }
>  
> +/* list of non-sleepable kernel functions that are otherwise
> + * available to attach by bpf_lsm or fmod_ret progs.
> + */
> +static int check_sleepable_blacklist(unsigned long addr)
> +{
> +#ifdef CONFIG_BPF_LSM
> +	if (addr == (long)bpf_lsm_task_free)
> +		return -EINVAL;
> +#endif
> +#ifdef CONFIG_SECURITY
> +	if (addr == (long)security_task_free)
> +		return -EINVAL;
> +#endif
> +	return 0;
> +}
> +
>  static int check_attach_btf_id(struct bpf_verifier_env *env)
>  {
>  	struct bpf_prog *prog = env->prog;
> @@ -10739,6 +10772,12 @@ 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;
> +	}
> +
>  	if (prog->type == BPF_PROG_TYPE_STRUCT_OPS)
>  		return check_struct_ops_btf_id(env);
>  
> @@ -10952,8 +10991,29 @@ 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);
> +				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;
> diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
> index 0cb8ec948816..73f9e3f84b77 100644
> --- a/tools/include/uapi/linux/bpf.h
> +++ b/tools/include/uapi/linux/bpf.h
> @@ -332,6 +332,14 @@ enum bpf_link_type {
>  /* The verifier internal test flag. Behavior is undefined */
>  #define BPF_F_TEST_STATE_FREQ	(1U << 3)
>  
> +/* 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:
>   *
> -- 
> 2.23.0
> 

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