Re: [PATCH v2 bpf-next 05/20] bpf: Introduce bpf_arena.

[David Vernet <void@manifault.com>]

[-- Attachment #1: Type: text/plain, Size: 12246 bytes --]

On Thu, Feb 08, 2024 at 08:05:53PM -0800, Alexei Starovoitov wrote:
> From: Alexei Starovoitov <ast@kernel.org>
> 
> 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 consumes it.
> 3. bpf_arena is a "heap" of memory from the bpf program's point of view.
>    The user space may mmap it, but bpf program will not convert pointers
>    to user base at run-time to improve bpf program speed.
> 
> 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 speed of bpf
> program is more important than ease of sharing 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 https://github.com/llvm/llvm-project/pull/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 (rY)
>   rX |= clear_lo32_bits(arena->user_vm_start); /* replace hi32 bits in rX */
> 
> 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 <ast@kernel.org>
> ---
>  include/linux/bpf.h            |   5 +-
>  include/linux/bpf_types.h      |   1 +
>  include/uapi/linux/bpf.h       |   7 +
>  kernel/bpf/Makefile            |   3 +
>  kernel/bpf/arena.c             | 557 +++++++++++++++++++++++++++++++++
>  kernel/bpf/core.c              |  11 +
>  kernel/bpf/syscall.c           |   3 +
>  kernel/bpf/verifier.c          |   1 +
>  tools/include/uapi/linux/bpf.h |   7 +
>  9 files changed, 593 insertions(+), 2 deletions(-)
>  create mode 100644 kernel/bpf/arena.c
> 
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 8b0dcb66eb33..de557c6c42e0 100644
> --- a/include/linux/bpf.h
> +++ b/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;
> @@ -534,8 +535,8 @@ 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;
> diff --git a/include/linux/bpf_types.h b/include/linux/bpf_types.h
> index 94baced5a1ad..9f2a6b83b49e 100644
> --- a/include/linux/bpf_types.h
> +++ b/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)
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index d96708380e52..f6648851eae6 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/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. */
> diff --git a/kernel/bpf/Makefile b/kernel/bpf/Makefile
> index 4ce95acfcaa7..368c5d86b5b7 100644
> --- a/kernel/bpf/Makefile
> +++ b/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
> diff --git a/kernel/bpf/arena.c b/kernel/bpf/arena.c
> new file mode 100644
> index 000000000000..5c1014471740
> --- /dev/null
> +++ b/kernel/bpf/arena.c
> @@ -0,0 +1,557 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/* Copyright (c) 2024 Meta Platforms, Inc. and affiliates. */
> +#include <linux/bpf.h>
> +#include <linux/btf.h>
> +#include <linux/err.h>
> +#include <linux/btf_ids.h>
> +#include <linux/vmalloc.h>
> +#include <linux/pagemap.h>
> +
> +/*
> + * bpf_arena is a sparsely populated shared memory region between bpf program and
> + * user space process.
> + *
> + * For example on x86-64 the values could be:
> + * user_vm_start 7f7d26200000     // picked by mmap()
> + * kern_vm_start ffffc90001e69000 // picked by get_vm_area()
> + * For user space all pointers within the arena are normal 8-byte addresses.
> + * In this example 7f7d26200000 is the address of the first page (pgoff=0).
> + * The bpf program will access it as: kern_vm_start + lower_32bit_of_user_ptr
> + * (u32)7f7d26200000 -> 26200000
> + * hence
> + * ffffc90001e69000 + 26200000 == ffffc90028069000 is "pgoff=0" within 4Gb
> + * kernel memory region.
> + *
> + * BPF JITs generate the following code to access arena:
> + *   mov eax, eax  // eax has lower 32-bit of user pointer
> + *   mov word ptr [rax + r12 + off], bx
> + * where r12 == kern_vm_start and off is s16.
> + * Hence allocate 4Gb + GUARD_SZ/2 on each side.
> + *
> + * Initially kernel vm_area and user vma are not populated.
> + * User space can fault-in any address which will insert the page
> + * into kernel and user vma.
> + * bpf program can allocate a page via bpf_arena_alloc_pages() kfunc
> + * which will insert it into kernel vm_area.
> + * The later fault-in from user space will populate that page into user vma.
> + */
> +
> +/* number of bytes addressable by LDX/STX insn with 16-bit 'off' field */
> +#define GUARD_SZ (1ull << sizeof(((struct bpf_insn *)0)->off) * 8)
> +#define KERN_VM_SZ ((1ull << 32) + GUARD_SZ)
> +
> +struct bpf_arena {
> +	struct bpf_map map;
> +	u64 user_vm_start;
> +	u64 user_vm_end;
> +	struct vm_struct *kern_vm;
> +	struct maple_tree mt;
> +	struct list_head vma_list;
> +	struct mutex lock;
> +};
> +
> +u64 bpf_arena_get_kern_vm_start(struct bpf_arena *arena)
> +{
> +	return arena ? (u64) (long) arena->kern_vm->addr + GUARD_SZ / 2 : 0;
> +}
> +
> +u64 bpf_arena_get_user_vm_start(struct bpf_arena *arena)
> +{
> +	return arena ? arena->user_vm_start : 0;
> +}
> +
> +static long arena_map_peek_elem(struct bpf_map *map, void *value)
> +{
> +	return -EOPNOTSUPP;
> +}
> +
> +static long arena_map_push_elem(struct bpf_map *map, void *value, u64 flags)
> +{
> +	return -EOPNOTSUPP;
> +}
> +
> +static long arena_map_pop_elem(struct bpf_map *map, void *value)
> +{
> +	return -EOPNOTSUPP;
> +}
> +
> +static long arena_map_delete_elem(struct bpf_map *map, void *value)
> +{
> +	return -EOPNOTSUPP;
> +}
> +
> +static int arena_map_get_next_key(struct bpf_map *map, void *key, void *next_key)
> +{
> +	return -EOPNOTSUPP;
> +}
> +
> +static long compute_pgoff(struct bpf_arena *arena, long uaddr)
> +{
> +	return (u32)(uaddr - (u32)arena->user_vm_start) >> PAGE_SHIFT;
> +}
> +
> +static struct bpf_map *arena_map_alloc(union bpf_attr *attr)
> +{
> +	struct vm_struct *kern_vm;
> +	int numa_node = bpf_map_attr_numa_node(attr);
> +	struct bpf_arena *arena;
> +	u64 vm_range;
> +	int err = -ENOMEM;
> +
> +	if (attr->key_size || attr->value_size || attr->max_entries == 0 ||
> +	    /* BPF_F_MMAPABLE must be set */
> +	    !(attr->map_flags & BPF_F_MMAPABLE) ||
> +	    /* No unsupported flags present */
> +	    (attr->map_flags & ~(BPF_F_SEGV_ON_FAULT | BPF_F_MMAPABLE | BPF_F_NO_USER_CONV)))
> +		return ERR_PTR(-EINVAL);
> +
> +	if (attr->map_extra & ~PAGE_MASK)
> +		/* If non-zero the map_extra is an expected user VMA start address */
> +		return ERR_PTR(-EINVAL);

So I haven't done a thorough review of this patch, beyond trying to
understand the semantics of bpf arenas. On that note, could you please
document the semantics of map_extra with arena maps where map_extra is
defined in [0]?

[0]: https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/tree/include/uapi/linux/bpf.h#n1439

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 228 bytes --]