[PATCH bpf-next v2 0/2] Update a struct_ops link through a pinned path

[PATCH bpf-next v2 0/2] Update a struct_ops link through a pinned path

[Kui-Feng Lee]

Applications already have the ability to update a struct_ops link with
another struct_ops map. However, they were unable to open pinned paths
of the links. This implies that updating a link through its pinned
paths was not feasible. By allowing the "open" operator on pinned
paths, applications can pin a struct_ops link and update the link
through the pinned path later.

---
Changes from v1:

 - Fix a link time error for the case that CONFIG_BPF_JIT is not
   enabled. (Reported by kernel test robot)

v1: https://lore.kernel.org/all/20240417002513.1534535-1-thinker.li@gmail.com/

Kui-Feng Lee (2):
  bpf: enable the "open" operator on a pinned path of a struct_osp link.
  selftests/bpf: open a pinned path of a struct_ops link.

 include/linux/bpf.h                           |  6 ++
 kernel/bpf/bpf_struct_ops.c                   | 10 ++++
 kernel/bpf/inode.c                            | 11 +++-
 kernel/bpf/syscall.c                          | 16 +++++-
 .../selftests/bpf/bpf_testmod/bpf_testmod.c   |  6 ++
 .../bpf/prog_tests/test_struct_ops_module.c   | 56 +++++++++++++++++++
 6 files changed, 101 insertions(+), 4 deletions(-)

-- 
2.34.1

[PATCH bpf-next v2 1/2] bpf: enable the "open" operator on a pinned path of a struct_osp link.

[Kui-Feng Lee]

Add the "open" operator for the inodes of BPF links to allow applications
to obtain a file descriptor of a struct_ops link from a pinned path.

Applications have the ability to update a struct_ops link with another
struct_ops map. However, they were unable to open pinned paths of the links
with this patch. This implies that updating a link through its pinned paths
was not feasible.

This patch adds the "open" operator to bpf_link_ops and uses bpf_link_ops
as the i_fop for inodes of struct_ops links. "open" will be called to open
the pinned path represented by an inode. Additionally, bpf_link_ops will be
used as the f->f_ops of the opened "file" to provide operators for the
"file".

Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
---
 include/linux/bpf.h         |  6 ++++++
 kernel/bpf/bpf_struct_ops.c | 10 ++++++++++
 kernel/bpf/inode.c          | 11 ++++++++---
 kernel/bpf/syscall.c        | 16 +++++++++++++++-
 4 files changed, 39 insertions(+), 4 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 5034c1b4ded7..a0c0234d754b 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -2160,6 +2160,12 @@ extern const struct super_operations bpf_super_ops;
 extern const struct file_operations bpf_map_fops;
 extern const struct file_operations bpf_prog_fops;
 extern const struct file_operations bpf_iter_fops;
+extern const struct file_operations bpf_link_fops;
+
+#ifdef CONFIG_BPF_JIT
+/* Required by bpf_link_open() */
+int bpffs_struct_ops_link_open(struct inode *inode, struct file *filp);
+#endif
 
 #define BPF_PROG_TYPE(_id, _name, prog_ctx_type, kern_ctx_type) \
 	extern const struct bpf_prog_ops _name ## _prog_ops; \
diff --git a/kernel/bpf/bpf_struct_ops.c b/kernel/bpf/bpf_struct_ops.c
index 86c7884abaf8..8be4f755a182 100644
--- a/kernel/bpf/bpf_struct_ops.c
+++ b/kernel/bpf/bpf_struct_ops.c
@@ -1198,3 +1198,13 @@ void bpf_map_struct_ops_info_fill(struct bpf_map_info *info, struct bpf_map *map
 
 	info->btf_vmlinux_id = btf_obj_id(st_map->btf);
 }
+
+int bpffs_struct_ops_link_open(struct inode *inode, struct file *filp)
+{
+	struct bpf_struct_ops_link *link = inode->i_private;
+
+	/* Paired with bpf_link_put_direct() in bpf_link_release(). */
+	bpf_link_inc(&link->link);
+	filp->private_data = link;
+	return 0;
+}
diff --git a/kernel/bpf/inode.c b/kernel/bpf/inode.c
index af5d2ffadd70..b020d761ab0a 100644
--- a/kernel/bpf/inode.c
+++ b/kernel/bpf/inode.c
@@ -360,11 +360,16 @@ static int bpf_mkmap(struct dentry *dentry, umode_t mode, void *arg)
 
 static int bpf_mklink(struct dentry *dentry, umode_t mode, void *arg)
 {
+	const struct file_operations *fops;
 	struct bpf_link *link = arg;
 
-	return bpf_mkobj_ops(dentry, mode, arg, &bpf_link_iops,
-			     bpf_link_is_iter(link) ?
-			     &bpf_iter_fops : &bpffs_obj_fops);
+	if (bpf_link_is_iter(link))
+		fops = &bpf_iter_fops;
+	else if (link->type == BPF_LINK_TYPE_STRUCT_OPS)
+		fops = &bpf_link_fops;
+	else
+		fops = &bpffs_obj_fops;
+	return bpf_mkobj_ops(dentry, mode, arg, &bpf_link_iops, fops);
 }
 
 static struct dentry *
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 7d392ec83655..265e2faf317d 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -3108,7 +3108,21 @@ static void bpf_link_show_fdinfo(struct seq_file *m, struct file *filp)
 }
 #endif
 
-static const struct file_operations bpf_link_fops = {
+/* Support opening pinned links */
+static int bpf_link_open(struct inode *inode, struct file *filp)
+{
+#ifdef CONFIG_BPF_JIT
+	struct bpf_link *link = inode->i_private;
+
+	if (link->type == BPF_LINK_TYPE_STRUCT_OPS)
+		return bpffs_struct_ops_link_open(inode, filp);
+#endif
+
+	return -EOPNOTSUPP;
+}
+
+const struct file_operations bpf_link_fops = {
+	.open = bpf_link_open,
 #ifdef CONFIG_PROC_FS
 	.show_fdinfo	= bpf_link_show_fdinfo,
 #endif
-- 
2.34.1

[PATCH bpf-next v2 2/2] selftests/bpf: open a pinned path of a struct_ops link.

[Kui-Feng Lee]

Ensure that a pinned path of a struct_ops link can be opened to obtain a
file descriptor, which applications can then utilize to update the link.

Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
---
 .../selftests/bpf/bpf_testmod/bpf_testmod.c   |  6 ++
 .../bpf/prog_tests/test_struct_ops_module.c   | 56 +++++++++++++++++++
 2 files changed, 62 insertions(+)

diff --git a/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c b/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c
index 39ad96a18123..c4acd4ec630c 100644
--- a/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c
+++ b/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c
@@ -579,6 +579,11 @@ static void bpf_dummy_unreg(void *kdata)
 {
 }
 
+static int bpf_dummy_update(void *kdata, void *old_kdata)
+{
+	return bpf_dummy_reg(kdata);
+}
+
 static int bpf_testmod_test_1(void)
 {
 	return 0;
@@ -606,6 +611,7 @@ struct bpf_struct_ops bpf_bpf_testmod_ops = {
 	.init_member = bpf_testmod_ops_init_member,
 	.reg = bpf_dummy_reg,
 	.unreg = bpf_dummy_unreg,
+	.update = bpf_dummy_update,
 	.cfi_stubs = &__bpf_testmod_ops,
 	.name = "bpf_testmod_ops",
 	.owner = THIS_MODULE,
diff --git a/tools/testing/selftests/bpf/prog_tests/test_struct_ops_module.c b/tools/testing/selftests/bpf/prog_tests/test_struct_ops_module.c
index 7cf2b9ddd3e1..47b965c4c3e1 100644
--- a/tools/testing/selftests/bpf/prog_tests/test_struct_ops_module.c
+++ b/tools/testing/selftests/bpf/prog_tests/test_struct_ops_module.c
@@ -160,6 +160,60 @@ static void test_struct_ops_incompatible(void)
 	struct_ops_module__destroy(skel);
 }
 
+/* Applications should be able to open a pinned path of a struct_ops link
+ * to get a file descriptor of the link and to update the link through the
+ * file descriptor.
+ */
+static void test_struct_ops_pinning_and_open(void)
+{
+	DECLARE_LIBBPF_OPTS(bpf_link_update_opts, opts);
+	struct struct_ops_module *skel;
+	int err, link_fd = -1, map_fd;
+	struct bpf_link *link;
+
+	/* Create and pin a struct_ops link */
+	skel = struct_ops_module__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "open_and_load"))
+		return;
+
+	link = bpf_map__attach_struct_ops(skel->maps.testmod_1);
+	if (!ASSERT_OK_PTR(link, "attach_struct_ops"))
+		goto cleanup;
+
+	err = bpf_link__pin(link, "/sys/fs/bpf/test_struct_ops_pinning");
+	if (!ASSERT_OK(err, "bpf_link__pin"))
+		goto cleanup;
+
+	/* Open the pinned path */
+	link_fd = open("/sys/fs/bpf/test_struct_ops_pinning", O_RDONLY);
+	bpf_link__unpin(link);
+	if (!ASSERT_GE(link_fd, 0, "open_pinned"))
+		goto cleanup;
+
+	skel->bss->test_1_result = 0;
+	skel->bss->test_2_result = 0;
+
+	map_fd = bpf_map__fd(skel->maps.testmod_1);
+	if (!ASSERT_GE(map_fd, 0, "map_fd"))
+		goto cleanup;
+
+	/* Update the link. test_1 and test_2 should be called again. */
+	err = bpf_link_update(link_fd, map_fd, &opts);
+	if (!ASSERT_OK(err, "bpf_link_update"))
+		goto cleanup;
+
+	/* Check if test_1 and test_2 have been called */
+	ASSERT_EQ(skel->bss->test_1_result, 0xdeadbeef,
+		  "bpf_link_update_test_1_result");
+	ASSERT_EQ(skel->bss->test_2_result, 5,
+		  "bpf_link_update_test_2_result");
+
+cleanup:
+	close(link_fd);
+	bpf_link__destroy(link);
+	struct_ops_module__destroy(skel);
+}
+
 void serial_test_struct_ops_module(void)
 {
 	if (test__start_subtest("test_struct_ops_load"))
@@ -168,5 +222,7 @@ void serial_test_struct_ops_module(void)
 		test_struct_ops_not_zeroed();
 	if (test__start_subtest("test_struct_ops_incompatible"))
 		test_struct_ops_incompatible();
+	if (test__start_subtest("test_struct_ops_pinning_and_open"))
+		test_struct_ops_pinning_and_open();
 }
 
-- 
2.34.1

Re: [PATCH bpf-next v2 0/2] Update a struct_ops link through a pinned path

[Eduard Zingerman]

On Thu, 2024-04-18 at 09:35 -0700, Kui-Feng Lee wrote:
> Applications already have the ability to update a struct_ops link with
> another struct_ops map. However, they were unable to open pinned paths
> of the links. This implies that updating a link through its pinned
> paths was not feasible. By allowing the "open" operator on pinned
> paths, applications can pin a struct_ops link and update the link
> through the pinned path later.
> 
> ---

Not an expert in how bpffs is tied together, but this patch-set seems to be fine.

Reviewed-by: Eduard Zingerman <eddyz87@gmail.com>

Re: [PATCH bpf-next v2 1/2] bpf: enable the "open" operator on a pinned path of a struct_osp link.

[Andrii Nakryiko]

On Thu, Apr 18, 2024 at 9:35 AM Kui-Feng Lee <thinker.li@gmail.com> wrote:
>
> Add the "open" operator for the inodes of BPF links to allow applications
> to obtain a file descriptor of a struct_ops link from a pinned path.
>
> Applications have the ability to update a struct_ops link with another
> struct_ops map. However, they were unable to open pinned paths of the links
> with this patch. This implies that updating a link through its pinned paths
> was not feasible.
>
> This patch adds the "open" operator to bpf_link_ops and uses bpf_link_ops
> as the i_fop for inodes of struct_ops links. "open" will be called to open
> the pinned path represented by an inode. Additionally, bpf_link_ops will be
> used as the f->f_ops of the opened "file" to provide operators for the
> "file".
>
> Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
> ---
>  include/linux/bpf.h         |  6 ++++++
>  kernel/bpf/bpf_struct_ops.c | 10 ++++++++++
>  kernel/bpf/inode.c          | 11 ++++++++---
>  kernel/bpf/syscall.c        | 16 +++++++++++++++-
>  4 files changed, 39 insertions(+), 4 deletions(-)
>

This is already supported, but you don't do it with open() syscall.
bpf() syscall provides BPF_OBJ_GET as a counterpart to BPF_OBJ_PIN. So
what you/your users want to do should be already supported through
libbpf's bpf_obj_get()/bpf_obj_get_ops() APIs. Have you tried that?

> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 5034c1b4ded7..a0c0234d754b 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -2160,6 +2160,12 @@ extern const struct super_operations bpf_super_ops;
>  extern const struct file_operations bpf_map_fops;
>  extern const struct file_operations bpf_prog_fops;
>  extern const struct file_operations bpf_iter_fops;
> +extern const struct file_operations bpf_link_fops;
> +

[...]