bpf: Introduce bpf_arena.

Introduce bpf_arena, which is a sparse shared memory region between the bpf
program and user space.

Use cases:
1. User space mmap-s bpf_arena and uses it as a traditional mmap-ed anonymous
   region, like memcached or any key/value storage. The bpf program implements an
   in-kernel accelerator. XDP prog can search for a key in bpf_arena and return a
   value without going to user space.
2. The bpf program builds arbitrary data structures in bpf_arena (hash tables,
   rb-trees, sparse arrays), while user space occasionally consumes it.
3. bpf_arena is a "heap" of memory from the bpf program's point of view. It is
   not shared with user space.

Initially, the kernel vm_area and user vma are not populated. User space can
fault in pages within the range. While servicing a page fault, bpf_arena logic
will insert a new page into the kernel and user vmas. The bpf program can
allocate pages from that region via bpf_arena_alloc_pages(). This kernel
function will insert pages into the kernel vm_area. The subsequent fault-in
from user space will populate that page into the user vma. The
BPF_F_SEGV_ON_FAULT flag at arena creation time can be used to prevent fault-in
from user space. In such a case, if a page is not allocated by the bpf program
and not present in the kernel vm_area, the user process will segfault. This is
useful for use cases 2 and 3 above.

bpf_arena_alloc_pages() is similar to user space mmap(). It allocates pages
either at a specific address within the arena or allocates a range with the
maple tree. bpf_arena_free_pages() is analogous to munmap(), which frees pages
and removes the range from the kernel vm_area and from user process vmas.

bpf_arena can be used as a bpf program "heap" of up to 4GB. The memory is not
shared with user space. This is use case 3. In such a case, the
BPF_F_NO_USER_CONV flag is recommended. It will tell the verifier to treat the
rX = bpf_arena_cast_user(rY) instruction as a 32-bit move wX = wY, which will
improve bpf prog performance. Otherwise, bpf_arena_cast_user is translated by
JIT to conditionally add the upper 32 bits of user vm_start (if the pointer is
not NULL) to arena pointers before they are stored into memory. This way, user
space sees them as valid 64-bit pointers.

Diff llvm/llvm-project#79902 taught LLVM BPF backend to
generate the bpf_cast_kern() instruction before dereference of the arena
pointer and the bpf_cast_user() instruction when the arena pointer is formed.
In a typical bpf program there will be very few bpf_cast_user().

From LLVM's point of view, arena pointers are tagged as
__attribute__((address_space(1))). Hence, clang provides helpful diagnostics
when pointers cross address space. Libbpf and the kernel support only
address_space == 1. All other address space identifiers are reserved.

rX = bpf_cast_kern(rY, addr_space) tells the verifier that
rX->type = PTR_TO_ARENA. Any further operations on PTR_TO_ARENA register have
to be in the 32-bit domain. The verifier will mark load/store through
PTR_TO_ARENA with PROBE_MEM32. JIT will generate them as
kern_vm_start + 32bit_addr memory accesses. The behavior is similar to
copy_from_kernel_nofault() except that no address checks are necessary. The
address is guaranteed to be in the 4GB range. If the page is not present, the
destination register is zeroed on read, and the operation is ignored on write.

rX = bpf_cast_user(rY, addr_space) tells the verifier that
rX->type = unknown scalar. If arena->map_flags has BPF_F_NO_USER_CONV set, then
the verifier converts cast_user to mov32. Otherwise, JIT will emit native code
equivalent to:
rX = (u32)rY;
if (rX)
  rX |= arena->user_vm_start & ~(u64)~0U;

After such conversion, the pointer becomes a valid user pointer within
bpf_arena range. The user process can access data structures created in
bpf_arena without any additional computations. For example, a linked list built
by a bpf program can be walked natively by user space.

Signed-off-by: Alexei Starovoitov <[email protected]>

Author: Alexei Starovoitov

include/linux/bpf.h

@@ -37,6 +37,7 @@ struct perf_event;
 struct bpf_prog;
 struct bpf_prog_aux;
 struct bpf_map;
+struct bpf_arena;
 struct sock;
 struct seq_file;
 struct btf;
@@ -531,8 +532,8 @@ void bpf_list_head_free(const struct btf_field *field, void *list_head,
 			struct bpf_spin_lock *spin_lock);
 void bpf_rb_root_free(const struct btf_field *field, void *rb_root,
 		      struct bpf_spin_lock *spin_lock);
-
-
+u64 bpf_arena_get_kern_vm_start(struct bpf_arena *arena);
+u64 bpf_arena_get_user_vm_start(struct bpf_arena *arena);
 int bpf_obj_name_cpy(char *dst, const char *src, unsigned int size);
 
 struct bpf_offload_dev;

include/linux/bpf_types.h

@@ -132,6 +132,7 @@ BPF_MAP_TYPE(BPF_MAP_TYPE_STRUCT_OPS, bpf_struct_ops_map_ops)
 BPF_MAP_TYPE(BPF_MAP_TYPE_RINGBUF, ringbuf_map_ops)
 BPF_MAP_TYPE(BPF_MAP_TYPE_BLOOM_FILTER, bloom_filter_map_ops)
 BPF_MAP_TYPE(BPF_MAP_TYPE_USER_RINGBUF, user_ringbuf_map_ops)
+BPF_MAP_TYPE(BPF_MAP_TYPE_ARENA, arena_map_ops)
 
 BPF_LINK_TYPE(BPF_LINK_TYPE_RAW_TRACEPOINT, raw_tracepoint)
 BPF_LINK_TYPE(BPF_LINK_TYPE_TRACING, tracing)

include/uapi/linux/bpf.h

@@ -983,6 +983,7 @@ enum bpf_map_type {
 	BPF_MAP_TYPE_BLOOM_FILTER,
 	BPF_MAP_TYPE_USER_RINGBUF,
 	BPF_MAP_TYPE_CGRP_STORAGE,
+	BPF_MAP_TYPE_ARENA,
 	__MAX_BPF_MAP_TYPE
 };
 
@@ -1370,6 +1371,12 @@ enum {
 
 /* BPF token FD is passed in a corresponding command's token_fd field */
 	BPF_F_TOKEN_FD          = (1U << 16),
+
+/* When user space page faults in bpf_arena send SIGSEGV instead of inserting new page */
+	BPF_F_SEGV_ON_FAULT	= (1U << 17),
+
+/* Do not translate kernel bpf_arena pointers to user pointers */
+	BPF_F_NO_USER_CONV	= (1U << 18),
 };
 
 /* Flags for BPF_PROG_QUERY. */

kernel/bpf/Makefile

@@ -15,6 +15,9 @@ obj-${CONFIG_BPF_LSM}	  += bpf_inode_storage.o
 obj-$(CONFIG_BPF_SYSCALL) += disasm.o mprog.o
 obj-$(CONFIG_BPF_JIT) += trampoline.o
 obj-$(CONFIG_BPF_SYSCALL) += btf.o memalloc.o
+ifeq ($(CONFIG_MMU)$(CONFIG_64BIT),yy)
+obj-$(CONFIG_BPF_SYSCALL) += arena.o
+endif
 obj-$(CONFIG_BPF_JIT) += dispatcher.o
 ifeq ($(CONFIG_NET),y)
 obj-$(CONFIG_BPF_SYSCALL) += devmap.o