Currently io_uring supports restricting operations on a per-ring basis.
To use those, the ring must be setup in a disabled state by setting
IORING_SETUP_R_DISABLED. Then restrictions can be set for the ring, and
the ring can then be enabled.

This commit adds support for IORING_REGISTER_RESTRICTIONS with ring_fd
== -1, like the other "blind" register opcodes which work on the task
rather than a specific ring. This allows registration of the same kind
of restrictions as can been done on a specific ring, but with the task
itself. Once done, any ring created will inherit these restrictions.

If a restriction filter is registered with a task, then it's inherited
on fork for its children. Children may only further restrict operations,
not extend them.

Inheriting restrictions include both the classic
IORING_REGISTER_RESTRICTIONS based restrictions, as well as the BPF
filters that have been registered with the task via
IORING_REGISTER_BPF_FILTER.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

Currently a few pointer dereferences need to be made to both check if
BPF filters are installed, and then also to retrieve the actual filter
for the opcode. Cache the table in ctx->bpf_filters to avoid that.

Add a bit of debug info on ring exit to show if we ever got this wrong.
Small risk of that given that the table is currently only updated in one
spot, but once task forking is enabled, that will add one more spot.

Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Add support for loading classic BPF programs with io_uring to provide
fine-grained filtering of SQE operations. Unlike
IORING_REGISTER_RESTRICTIONS which only allows bitmap-based allow/deny
of opcodes, BPF filters can inspect request attributes and make dynamic
decisions.

The filter is registered via IORING_REGISTER_BPF_FILTER with a struct
io_uring_bpf:

struct io_uring_bpf_filter {
	__u32	opcode;		/* io_uring opcode to filter */
	__u32	flags;
	__u32	filter_len;	/* number of BPF instructions */
	__u32	resv;
	__u64	filter_ptr;	/* pointer to BPF filter */
	__u64	resv2[5];
};

enum {
	IO_URING_BPF_CMD_FILTER	= 1,
};

struct io_uring_bpf {
	__u16	cmd_type;	/* IO_URING_BPF_* values */
	__u16	cmd_flags;	/* none so far */
	__u32	resv;
	union {
		struct io_uring_bpf_filter	filter;
	};
};

and the filters get supplied a struct io_uring_bpf_ctx:

struct io_uring_bpf_ctx {
	__u64	user_data;
	__u8	opcode;
	__u8	sqe_flags;
	__u8	pdu_size;
	__u8	pad[5];
};

where it's possible to filter on opcode and sqe_flags, with pdu_size
indicating how much extra data is being passed in beyond the pad field.
This will used for specific finer grained filtering inside an opcode.
An example of that for sockets is in one of the following patches.
Anything the opcode supports can end up in this struct, populated by
the opcode itself, and hence can be filtered for.

Filters have the following semantics:
  - Return 1 to allow the request
  - Return 0 to deny the request with -EACCES
  - Multiple filters can be stacked per opcode. All filters must
    return 1 for the opcode to be allowed.
  - Filters are evaluated in registration order (most recent first)

The implementation uses classic BPF (cBPF) rather than eBPF for as
that's required for containers, and since they can be used by any
user in the system.

Signed-off-by: Jens Axboe <axboe@kernel.dk>