bpf: Add bpf_page_to_pfn helper

In CRIU, we need to be able to determine whether the page pinned by
io_uring is still present in the same range in the process VMA.
/proc/<pid>/pagemap gives us the PFN, hence using this helper we can
establish this mapping easily from the iterator side.

It is a simple wrapper over the in-kernel page_to_pfn helper, and
ensures the passed in pointer is a struct page PTR_TO_BTF_ID. This is
obtained from the bvec of io_uring_ubuf for the CRIU usecase.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

fs/io_uring.c

@@ -11295,6 +11295,23 @@ static struct bpf_iter_reg io_uring_buf_reg_info = {
 	.seq_info	   = &bpf_io_uring_buf_seq_info,
 };
 
+BPF_CALL_1(bpf_page_to_pfn, struct page *, page)
+{
+	/* PTR_TO_BTF_ID can be NULL */
+	if (!page)
+		return U64_MAX;
+	return page_to_pfn(page);
+}
+
+BTF_ID_LIST_SINGLE(btf_page_to_pfn_ids, struct, page)
+
+const struct bpf_func_proto bpf_page_to_pfn_proto = {
+	.func		= bpf_page_to_pfn,
+	.ret_type	= RET_INTEGER,
+	.arg1_type	= ARG_PTR_TO_BTF_ID,
+	.arg1_btf_id	= &btf_page_to_pfn_ids[0],
+};
+
 static int __init io_uring_iter_init(void)
 {
 	io_uring_buf_reg_info.ctx_arg_info[0].btf_id = btf_io_uring_ids[0];

include/linux/bpf.h

@@ -2166,6 +2166,7 @@ extern const struct bpf_func_proto bpf_sk_setsockopt_proto;
 extern const struct bpf_func_proto bpf_sk_getsockopt_proto;
 extern const struct bpf_func_proto bpf_kallsyms_lookup_name_proto;
 extern const struct bpf_func_proto bpf_find_vma_proto;
+extern const struct bpf_func_proto bpf_page_to_pfn_proto;
 
 const struct bpf_func_proto *tracing_prog_func_proto(
   enum bpf_func_id func_id, const struct bpf_prog *prog);

include/uapi/linux/bpf.h

@@ -4960,6 +4960,14 @@ union bpf_attr {
  *		**-ENOENT** if *task->mm* is NULL, or no vma contains *addr*.
  *		**-EBUSY** if failed to try lock mmap_lock.
  *		**-EINVAL** for invalid **flags**.
+ *
+ * long bpf_page_to_pfn(struct page *page)
+ *	Description
+ *		Obtain the page frame number (PFN) for the given *struct page*
+ *		pointer.
+ *	Return
+ *		Page Frame Number corresponding to the page pointed to by the
+ *		*struct page* pointer, or U64_MAX if pointer is NULL.
  */
 #define __BPF_FUNC_MAPPER(FN)		\
 	FN(unspec),			\
@@ -5143,6 +5151,7 @@ union bpf_attr {
 	FN(skc_to_unix_sock),		\
 	FN(kallsyms_lookup_name),	\
 	FN(find_vma),			\
+	FN(page_to_pfn),		\
 	/* */
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper

kernel/trace/bpf_trace.c

@@ -1212,6 +1212,8 @@ bpf_tracing_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 		return &bpf_find_vma_proto;
 	case BPF_FUNC_trace_vprintk:
 		return bpf_get_trace_vprintk_proto();
+	case BPF_FUNC_page_to_pfn:
+		return &bpf_page_to_pfn_proto;
 	default:
 		return bpf_base_func_proto(func_id);
 	}

scripts/bpf_doc.py

@@ -549,6 +549,7 @@ def __init__(self, parser):
             'struct socket',
             'struct file',
             'struct bpf_timer',
+            'struct page',
     ]
     known_types = {
             '...',
@@ -598,6 +599,7 @@ def __init__(self, parser):
             'struct socket',
             'struct file',
             'struct bpf_timer',
+            'struct page',
     }
     mapped_types = {
             'u8': '__u8',

tools/include/uapi/linux/bpf.h

@@ -4960,6 +4960,14 @@ union bpf_attr {
  *		**-ENOENT** if *task->mm* is NULL, or no vma contains *addr*.
  *		**-EBUSY** if failed to try lock mmap_lock.
  *		**-EINVAL** for invalid **flags**.
+ *
+ * long bpf_page_to_pfn(struct page *page)
+ *	Description
+ *		Obtain the page frame number (PFN) for the given *struct page*
+ *		pointer.
+ *	Return
+ *		Page Frame Number corresponding to the page pointed to by the
+ *		*struct page* pointer, or U64_MAX if pointer is NULL.
  */
 #define __BPF_FUNC_MAPPER(FN)		\
 	FN(unspec),			\
@@ -5143,6 +5151,7 @@ union bpf_attr {
 	FN(skc_to_unix_sock),		\
 	FN(kallsyms_lookup_name),	\
 	FN(find_vma),			\
+	FN(page_to_pfn),		\
 	/* */
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper