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782347b6bc
XDP_REDIRECT works by a three-step process: the bpf_redirect() and bpf_redirect_map() helpers will lookup the target of the redirect and store it (along with some other metadata) in a per-CPU struct bpf_redirect_info. Next, when the program returns the XDP_REDIRECT return code, the driver will call xdp_do_redirect() which will use the information thus stored to actually enqueue the frame into a bulk queue structure (that differs slightly by map type, but shares the same principle). Finally, before exiting its NAPI poll loop, the driver will call xdp_do_flush(), which will flush all the different bulk queues, thus completing the redirect. Pointers to the map entries will be kept around for this whole sequence of steps, protected by RCU. However, there is no top-level rcu_read_lock() in the core code; instead drivers add their own rcu_read_lock() around the XDP portions of the code, but somewhat inconsistently as Martin discovered[0]. However, things still work because everything happens inside a single NAPI poll sequence, which means it's between a pair of calls to local_bh_disable()/local_bh_enable(). So Paul suggested[1] that we could document this intention by using rcu_dereference_check() with rcu_read_lock_bh_held() as a second parameter, thus allowing sparse and lockdep to verify that everything is done correctly. This patch does just that: we add an __rcu annotation to the map entry pointers and remove the various comments explaining the NAPI poll assurance strewn through devmap.c in favour of a longer explanation in filter.c. The goal is to have one coherent documentation of the entire flow, and rely on the RCU annotations as a "standard" way of communicating the flow in the map code (which can additionally be understood by sparse and lockdep). The RCU annotation replacements result in a fairly straight-forward replacement where READ_ONCE() becomes rcu_dereference_check(), WRITE_ONCE() becomes rcu_assign_pointer() and xchg() and cmpxchg() gets wrapped in the proper constructs to cast the pointer back and forth between __rcu and __kernel address space (for the benefit of sparse). The one complication is that xskmap has a few constructions where double-pointers are passed back and forth; these simply all gain __rcu annotations, and only the final reference/dereference to the inner-most pointer gets changed. With this, everything can be run through sparse without eliciting complaints, and lockdep can verify correctness even without the use of rcu_read_lock() in the drivers. Subsequent patches will clean these up from the drivers. [0] https://lore.kernel.org/bpf/20210415173551.7ma4slcbqeyiba2r@kafai-mbp.dhcp.thefacebook.com/ [1] https://lore.kernel.org/bpf/20210419165837.GA975577@paulmck-ThinkPad-P17-Gen-1/ Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net> Link: https://lore.kernel.org/bpf/20210624160609.292325-6-toke@redhat.com
101 lines
2 KiB
C
101 lines
2 KiB
C
/* SPDX-License-Identifier: GPL-2.0 */
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/* AF_XDP internal functions
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* Copyright(c) 2018 Intel Corporation.
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*/
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#ifndef _LINUX_XDP_SOCK_H
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#define _LINUX_XDP_SOCK_H
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#include <linux/workqueue.h>
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#include <linux/if_xdp.h>
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#include <linux/mutex.h>
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#include <linux/spinlock.h>
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#include <linux/mm.h>
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#include <net/sock.h>
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struct net_device;
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struct xsk_queue;
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struct xdp_buff;
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struct xdp_umem {
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void *addrs;
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u64 size;
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u32 headroom;
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u32 chunk_size;
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u32 chunks;
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u32 npgs;
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struct user_struct *user;
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refcount_t users;
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u8 flags;
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bool zc;
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struct page **pgs;
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int id;
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struct list_head xsk_dma_list;
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struct work_struct work;
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};
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struct xsk_map {
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struct bpf_map map;
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spinlock_t lock; /* Synchronize map updates */
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struct xdp_sock __rcu *xsk_map[];
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};
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struct xdp_sock {
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/* struct sock must be the first member of struct xdp_sock */
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struct sock sk;
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struct xsk_queue *rx ____cacheline_aligned_in_smp;
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struct net_device *dev;
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struct xdp_umem *umem;
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struct list_head flush_node;
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struct xsk_buff_pool *pool;
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u16 queue_id;
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bool zc;
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enum {
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XSK_READY = 0,
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XSK_BOUND,
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XSK_UNBOUND,
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} state;
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struct xsk_queue *tx ____cacheline_aligned_in_smp;
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struct list_head tx_list;
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/* Protects generic receive. */
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spinlock_t rx_lock;
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/* Statistics */
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u64 rx_dropped;
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u64 rx_queue_full;
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struct list_head map_list;
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/* Protects map_list */
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spinlock_t map_list_lock;
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/* Protects multiple processes in the control path */
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struct mutex mutex;
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struct xsk_queue *fq_tmp; /* Only as tmp storage before bind */
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struct xsk_queue *cq_tmp; /* Only as tmp storage before bind */
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};
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#ifdef CONFIG_XDP_SOCKETS
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int xsk_generic_rcv(struct xdp_sock *xs, struct xdp_buff *xdp);
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int __xsk_map_redirect(struct xdp_sock *xs, struct xdp_buff *xdp);
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void __xsk_map_flush(void);
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#else
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static inline int xsk_generic_rcv(struct xdp_sock *xs, struct xdp_buff *xdp)
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{
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return -ENOTSUPP;
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}
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static inline int __xsk_map_redirect(struct xdp_sock *xs, struct xdp_buff *xdp)
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{
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return -EOPNOTSUPP;
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}
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static inline void __xsk_map_flush(void)
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{
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}
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#endif /* CONFIG_XDP_SOCKETS */
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#endif /* _LINUX_XDP_SOCK_H */
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