[RFC PATCH bpf-next 0/2] bpf: bpf link iterator

[RFC PATCH bpf-next 0/2] bpf: bpf link iterator

[Dmitrii Dolgov]

Bpf links seem to be one of the important structures for which no
iterator is provided. Such iterator could be useful in those cases when
generic 'task/file' is not suitable or better performance is needed.

The implementation is mostly copied from prog iterator, and I would like
to get any high-level feedback about what needs to be different or have
to be taken into account. As a side note, I would also appreciate if
someone could point me out to some guide about writing selftests for bpf
subsystem -- for some unclear reason I couldn't compile the test from
this changeset, and was testing it only manually with a custom test
program.

Dmitrii Dolgov (2):
  bpf: Add bpf_link iterator
  selftests/bpf: Add bpf_link test

 include/linux/bpf.h                           |   1 +
 kernel/bpf/Makefile                           |   2 +-
 kernel/bpf/link_iter.c                        | 107 ++++++++++++++++++
 kernel/bpf/syscall.c                          |  19 ++++
 .../selftests/bpf/prog_tests/bpf_iter.c       |  15 +++
 .../selftests/bpf/progs/bpf_iter_bpf_link.c   |  18 +++
 6 files changed, 161 insertions(+), 1 deletion(-)
 create mode 100644 kernel/bpf/link_iter.c
 create mode 100644 tools/testing/selftests/bpf/progs/bpf_iter_bpf_link.c

-- 
2.32.0

[RFC PATCH bpf-next 1/2] bpf: Add bpf_link iterator

[Dmitrii Dolgov]

Implement bpf_link iterator to traverse links via bpf_seq_file
operations. The changeset is mostly shamelessly copied from
commit a228a64fc1e4 ("bpf: Add bpf_prog iterator")

Signed-off-by: Dmitrii Dolgov <9erthalion6@gmail.com>
---
 include/linux/bpf.h    |   1 +
 kernel/bpf/Makefile    |   2 +-
 kernel/bpf/link_iter.c | 107 +++++++++++++++++++++++++++++++++++++++++
 kernel/bpf/syscall.c   |  19 ++++++++
 4 files changed, 128 insertions(+), 1 deletion(-)
 create mode 100644 kernel/bpf/link_iter.c

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 7bf441563ffc..330e88fcc50e 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -1489,6 +1489,7 @@ void bpf_link_put(struct bpf_link *link);
 int bpf_link_new_fd(struct bpf_link *link);
 struct file *bpf_link_new_file(struct bpf_link *link, int *reserved_fd);
 struct bpf_link *bpf_link_get_from_fd(u32 ufd);
+struct bpf_link *bpf_link_get_curr_or_next(u32 *id);
 
 int bpf_obj_pin_user(u32 ufd, const char __user *pathname);
 int bpf_obj_get_user(const char __user *pathname, int flags);
diff --git a/kernel/bpf/Makefile b/kernel/bpf/Makefile
index c1a9be6a4b9f..057ba8e01e70 100644
--- a/kernel/bpf/Makefile
+++ b/kernel/bpf/Makefile
@@ -6,7 +6,7 @@ cflags-nogcse-$(CONFIG_X86)$(CONFIG_CC_IS_GCC) := -fno-gcse
 endif
 CFLAGS_core.o += $(call cc-disable-warning, override-init) $(cflags-nogcse-yy)
 
-obj-$(CONFIG_BPF_SYSCALL) += syscall.o verifier.o inode.o helpers.o tnum.o bpf_iter.o map_iter.o task_iter.o prog_iter.o
+obj-$(CONFIG_BPF_SYSCALL) += syscall.o verifier.o inode.o helpers.o tnum.o bpf_iter.o map_iter.o task_iter.o prog_iter.o link_iter.o
 obj-$(CONFIG_BPF_SYSCALL) += hashtab.o arraymap.o percpu_freelist.o bpf_lru_list.o lpm_trie.o map_in_map.o bloom_filter.o
 obj-$(CONFIG_BPF_SYSCALL) += local_storage.o queue_stack_maps.o ringbuf.o
 obj-$(CONFIG_BPF_SYSCALL) += bpf_local_storage.o bpf_task_storage.o
diff --git a/kernel/bpf/link_iter.c b/kernel/bpf/link_iter.c
new file mode 100644
index 000000000000..fde41d09f26b
--- /dev/null
+++ b/kernel/bpf/link_iter.c
@@ -0,0 +1,107 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/* Copyright (c) 2020 Facebook */
+#include <linux/bpf.h>
+#include <linux/fs.h>
+#include <linux/filter.h>
+#include <linux/kernel.h>
+#include <linux/btf_ids.h>
+
+struct bpf_iter_seq_link_info {
+	u32 link_id;
+};
+
+static void *bpf_link_seq_start(struct seq_file *seq, loff_t *pos)
+{
+	struct bpf_iter_seq_link_info *info = seq->private;
+	struct bpf_link *link;
+
+	link = bpf_link_get_curr_or_next(&info->link_id);
+	if (!link)
+		return NULL;
+
+	if (*pos == 0)
+		++*pos;
+	return link;
+}
+
+static void *bpf_link_seq_next(struct seq_file *seq, void *v, loff_t *pos)
+{
+	struct bpf_iter_seq_link_info *info = seq->private;
+
+	++*pos;
+	++info->link_id;
+	bpf_link_put((struct bpf_link *)v);
+	return bpf_link_get_curr_or_next(&info->link_id);
+}
+
+struct bpf_iter__bpf_link {
+	__bpf_md_ptr(struct bpf_iter_meta *, meta);
+	__bpf_md_ptr(struct bpf_link *, link);
+};
+
+DEFINE_BPF_ITER_FUNC(bpf_link, struct bpf_iter_meta *meta, struct bpf_link *link)
+
+static int __bpf_link_seq_show(struct seq_file *seq, void *v, bool in_stop)
+{
+	struct bpf_iter__bpf_link ctx;
+	struct bpf_iter_meta meta;
+	struct bpf_prog *prog;
+	int ret = 0;
+
+	ctx.meta = &meta;
+	ctx.link = v;
+	meta.seq = seq;
+	prog = bpf_iter_get_info(&meta, in_stop);
+	if (prog)
+		ret = bpf_iter_run_prog(prog, &ctx);
+
+	return ret;
+}
+
+static int bpf_link_seq_show(struct seq_file *seq, void *v)
+{
+	return __bpf_link_seq_show(seq, v, false);
+}
+
+static void bpf_link_seq_stop(struct seq_file *seq, void *v)
+{
+	if (!v)
+		(void)__bpf_link_seq_show(seq, v, true);
+	else
+		bpf_link_put((struct bpf_link *)v);
+}
+
+static const struct seq_operations bpf_link_seq_ops = {
+	.start	= bpf_link_seq_start,
+	.next	= bpf_link_seq_next,
+	.stop	= bpf_link_seq_stop,
+	.show	= bpf_link_seq_show,
+};
+
+BTF_ID_LIST(btf_bpf_link_id)
+BTF_ID(struct, bpf_link)
+
+static const struct bpf_iter_seq_info bpf_link_seq_info = {
+	.seq_ops		= &bpf_link_seq_ops,
+	.init_seq_private	= NULL,
+	.fini_seq_private	= NULL,
+	.seq_priv_size		= sizeof(struct bpf_iter_seq_link_info),
+};
+
+static struct bpf_iter_reg bpf_link_reg_info = {
+	.target			= "bpf_link",
+	.ctx_arg_info_size	= 1,
+	.ctx_arg_info		= {
+		{ offsetof(struct bpf_iter__bpf_link, link),
+		  PTR_TO_BTF_ID_OR_NULL },
+	},
+	.seq_info		= &bpf_link_seq_info,
+};
+
+static int __init bpf_link_iter_init(void)
+{
+	bpf_link_reg_info.ctx_arg_info[0].btf_id = *btf_bpf_link_id;
+	return bpf_iter_reg_target(&bpf_link_reg_info);
+}
+
+late_initcall(bpf_link_iter_init);
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index e9621cfa09f2..7d2d4cedea3f 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -4454,6 +4454,25 @@ struct bpf_link *bpf_link_by_id(u32 id)
 	return link;
 }
 
+struct bpf_link *bpf_link_get_curr_or_next(u32 *id)
+{
+	struct bpf_link *link;
+
+	spin_lock_bh(&link_idr_lock);
+again:
+	link = idr_get_next(&link_idr, id);
+	if (link) {
+		link = bpf_link_inc_not_zero(link);
+		if (IS_ERR(link)) {
+			(*id)++;
+			goto again;
+		}
+	}
+	spin_unlock_bh(&link_idr_lock);
+
+	return link;
+}
+
 #define BPF_LINK_GET_FD_BY_ID_LAST_FIELD link_id
 
 static int bpf_link_get_fd_by_id(const union bpf_attr *attr)
-- 
2.32.0

Re: [RFC PATCH bpf-next 1/2] bpf: Add bpf_link iterator

[Andrii Nakryiko]

On Fri, Apr 22, 2022 at 11:23 AM Dmitrii Dolgov <9erthalion6@gmail.com> wrote:
>
> Implement bpf_link iterator to traverse links via bpf_seq_file
> operations. The changeset is mostly shamelessly copied from
> commit a228a64fc1e4 ("bpf: Add bpf_prog iterator")
>
> Signed-off-by: Dmitrii Dolgov <9erthalion6@gmail.com>
> ---
>  include/linux/bpf.h    |   1 +
>  kernel/bpf/Makefile    |   2 +-
>  kernel/bpf/link_iter.c | 107 +++++++++++++++++++++++++++++++++++++++++
>  kernel/bpf/syscall.c   |  19 ++++++++
>  4 files changed, 128 insertions(+), 1 deletion(-)
>  create mode 100644 kernel/bpf/link_iter.c
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 7bf441563ffc..330e88fcc50e 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -1489,6 +1489,7 @@ void bpf_link_put(struct bpf_link *link);
>  int bpf_link_new_fd(struct bpf_link *link);
>  struct file *bpf_link_new_file(struct bpf_link *link, int *reserved_fd);
>  struct bpf_link *bpf_link_get_from_fd(u32 ufd);
> +struct bpf_link *bpf_link_get_curr_or_next(u32 *id);
>
>  int bpf_obj_pin_user(u32 ufd, const char __user *pathname);
>  int bpf_obj_get_user(const char __user *pathname, int flags);
> diff --git a/kernel/bpf/Makefile b/kernel/bpf/Makefile
> index c1a9be6a4b9f..057ba8e01e70 100644
> --- a/kernel/bpf/Makefile
> +++ b/kernel/bpf/Makefile
> @@ -6,7 +6,7 @@ cflags-nogcse-$(CONFIG_X86)$(CONFIG_CC_IS_GCC) := -fno-gcse
>  endif
>  CFLAGS_core.o += $(call cc-disable-warning, override-init) $(cflags-nogcse-yy)
>
> -obj-$(CONFIG_BPF_SYSCALL) += syscall.o verifier.o inode.o helpers.o tnum.o bpf_iter.o map_iter.o task_iter.o prog_iter.o
> +obj-$(CONFIG_BPF_SYSCALL) += syscall.o verifier.o inode.o helpers.o tnum.o bpf_iter.o map_iter.o task_iter.o prog_iter.o link_iter.o
>  obj-$(CONFIG_BPF_SYSCALL) += hashtab.o arraymap.o percpu_freelist.o bpf_lru_list.o lpm_trie.o map_in_map.o bloom_filter.o
>  obj-$(CONFIG_BPF_SYSCALL) += local_storage.o queue_stack_maps.o ringbuf.o
>  obj-$(CONFIG_BPF_SYSCALL) += bpf_local_storage.o bpf_task_storage.o
> diff --git a/kernel/bpf/link_iter.c b/kernel/bpf/link_iter.c
> new file mode 100644
> index 000000000000..fde41d09f26b
> --- /dev/null
> +++ b/kernel/bpf/link_iter.c
> @@ -0,0 +1,107 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/* Copyright (c) 2020 Facebook */

this probably needs a bit of update?

> +#include <linux/bpf.h>
> +#include <linux/fs.h>
> +#include <linux/filter.h>
> +#include <linux/kernel.h>
> +#include <linux/btf_ids.h>
> +
> +struct bpf_iter_seq_link_info {
> +       u32 link_id;
> +};
> +

[...]

Re: [RFC PATCH bpf-next 1/2] bpf: Add bpf_link iterator

[Yonghong Song]

On 4/22/22 11:22 AM, Dmitrii Dolgov wrote:
> Implement bpf_link iterator to traverse links via bpf_seq_file
> operations. The changeset is mostly shamelessly copied from
> commit a228a64fc1e4 ("bpf: Add bpf_prog iterator")

LGTM except one copyright issue mentioned by Andrii early.

Acked-by: Yonghong Song <yhs@fb.com>

> 
> Signed-off-by: Dmitrii Dolgov <9erthalion6@gmail.com>
> ---
>   include/linux/bpf.h    |   1 +
>   kernel/bpf/Makefile    |   2 +-
>   kernel/bpf/link_iter.c | 107 +++++++++++++++++++++++++++++++++++++++++
>   kernel/bpf/syscall.c   |  19 ++++++++
>   4 files changed, 128 insertions(+), 1 deletion(-)
>   create mode 100644 kernel/bpf/link_iter.c
> 
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 7bf441563ffc..330e88fcc50e 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -1489,6 +1489,7 @@ void bpf_link_put(struct bpf_link *link);
>   int bpf_link_new_fd(struct bpf_link *link);
>   struct file *bpf_link_new_file(struct bpf_link *link, int *reserved_fd);
>   struct bpf_link *bpf_link_get_from_fd(u32 ufd);
> +struct bpf_link *bpf_link_get_curr_or_next(u32 *id);
>   
>   int bpf_obj_pin_user(u32 ufd, const char __user *pathname);
>   int bpf_obj_get_user(const char __user *pathname, int flags);
> diff --git a/kernel/bpf/Makefile b/kernel/bpf/Makefile
> index c1a9be6a4b9f..057ba8e01e70 100644
> --- a/kernel/bpf/Makefile
> +++ b/kernel/bpf/Makefile
> @@ -6,7 +6,7 @@ cflags-nogcse-$(CONFIG_X86)$(CONFIG_CC_IS_GCC) := -fno-gcse
>   endif
>   CFLAGS_core.o += $(call cc-disable-warning, override-init) $(cflags-nogcse-yy)
>   
> -obj-$(CONFIG_BPF_SYSCALL) += syscall.o verifier.o inode.o helpers.o tnum.o bpf_iter.o map_iter.o task_iter.o prog_iter.o
> +obj-$(CONFIG_BPF_SYSCALL) += syscall.o verifier.o inode.o helpers.o tnum.o bpf_iter.o map_iter.o task_iter.o prog_iter.o link_iter.o
>   obj-$(CONFIG_BPF_SYSCALL) += hashtab.o arraymap.o percpu_freelist.o bpf_lru_list.o lpm_trie.o map_in_map.o bloom_filter.o
>   obj-$(CONFIG_BPF_SYSCALL) += local_storage.o queue_stack_maps.o ringbuf.o
>   obj-$(CONFIG_BPF_SYSCALL) += bpf_local_storage.o bpf_task_storage.o
> diff --git a/kernel/bpf/link_iter.c b/kernel/bpf/link_iter.c
> new file mode 100644
> index 000000000000..fde41d09f26b
> --- /dev/null
> +++ b/kernel/bpf/link_iter.c
> @@ -0,0 +1,107 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/* Copyright (c) 2020 Facebook */

Change to your own copyright.

> +#include <linux/bpf.h>
> +#include <linux/fs.h>
> +#include <linux/filter.h>
> +#include <linux/kernel.h>
> +#include <linux/btf_ids.h>
> +
[...]

Re: [RFC PATCH bpf-next 1/2] bpf: Add bpf_link iterator

[Yonghong Song]

On 4/28/22 10:07 AM, Dmitry Dolgov wrote:
> On Thu, Apr 28, 2022, 6:14 PM Yonghong Song <yhs@fb.com 
> <mailto:yhs@fb.com>> wrote:
> 
>  >> diff --git a/kernel/bpf/link_iter.c b/kernel/bpf/link_iter.c
>  >> new file mode 100644
>  >> index 000000000000..fde41d09f26b
>  >> --- /dev/null
>  >> +++ b/kernel/bpf/link_iter.c
>  >> @@ -0,0 +1,107 @@
>  >> +// SPDX-License-Identifier: GPL-2.0-only
>  >> +/* Copyright (c) 2020 Facebook */
> 
>  > Change to your own copyright.
> 
> Thanks for reviewing. I have to admit I'm not sure how should it look 
> like, is there anything like a guideline/best practices about what to 
> put into the copyright line?

You can do
   /* Copyright (c) 2022 <Your Company Name> */
or
   /* Copyright (c) 2022 <Your Name> */

[RFC PATCH bpf-next 2/2] selftests/bpf: Add bpf link iter test

[Dmitrii Dolgov]

Add a simple test for bpf link iterator

Signed-off-by: Dmitrii Dolgov <9erthalion6@gmail.com>
---
 .../selftests/bpf/prog_tests/bpf_iter.c        | 15 +++++++++++++++
 .../selftests/bpf/progs/bpf_iter_bpf_link.c    | 18 ++++++++++++++++++
 2 files changed, 33 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/progs/bpf_iter_bpf_link.c

diff --git a/tools/testing/selftests/bpf/prog_tests/bpf_iter.c b/tools/testing/selftests/bpf/prog_tests/bpf_iter.c
index 2c403ddc8076..e14a7a6d925c 100644
--- a/tools/testing/selftests/bpf/prog_tests/bpf_iter.c
+++ b/tools/testing/selftests/bpf/prog_tests/bpf_iter.c
@@ -26,6 +26,7 @@
 #include "bpf_iter_bpf_sk_storage_map.skel.h"
 #include "bpf_iter_test_kern5.skel.h"
 #include "bpf_iter_test_kern6.skel.h"
+#include "bpf_iter_bpf_link.skel.h"
 
 static int duration;
 
@@ -1172,6 +1173,20 @@ static void test_buf_neg_offset(void)
 		bpf_iter_test_kern6__destroy(skel);
 }
 
+static void test_link_iter(void)
+{
+	struct bpf_iter_bpf_link *skel;
+
+	skel = bpf_iter_bpf_link__open_and_load();
+	if (CHECK(skel, "bpf_iter_bpf_link__open_and_load",
+		  "skeleton open_and_load unexpected success\n"))
+		return;
+
+	do_dummy_read(skel->progs.dump_bpf_link);
+
+	bpf_iter_bpf_link__destroy(skel);
+}
+
 #define CMP_BUFFER_SIZE 1024
 static char task_vma_output[CMP_BUFFER_SIZE];
 static char proc_maps_output[CMP_BUFFER_SIZE];
diff --git a/tools/testing/selftests/bpf/progs/bpf_iter_bpf_link.c b/tools/testing/selftests/bpf/progs/bpf_iter_bpf_link.c
new file mode 100644
index 000000000000..a5041fa1cda9
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/bpf_iter_bpf_link.c
@@ -0,0 +1,18 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2020 Facebook */
+#include "bpf_iter.h"
+#include <bpf/bpf_helpers.h>
+
+char _license[] SEC("license") = "GPL";
+
+SEC("iter/bpf_link")
+int dump_bpf_link(struct bpf_iter__bpf_link *ctx)
+{
+	struct seq_file *seq = ctx->meta->seq;
+	struct bpf_link *link = ctx->link;
+	int link_id;
+
+	link_id = link->id;
+	bpf_seq_write(seq, &link_id, sizeof(link_id));
+	return 0;
+}
-- 
2.32.0

Re: [RFC PATCH bpf-next 2/2] selftests/bpf: Add bpf link iter test

[Andrii Nakryiko]

On Fri, Apr 22, 2022 at 11:23 AM Dmitrii Dolgov <9erthalion6@gmail.com> wrote:
>
> Add a simple test for bpf link iterator
>
> Signed-off-by: Dmitrii Dolgov <9erthalion6@gmail.com>
> ---
>  .../selftests/bpf/prog_tests/bpf_iter.c        | 15 +++++++++++++++
>  .../selftests/bpf/progs/bpf_iter_bpf_link.c    | 18 ++++++++++++++++++
>  2 files changed, 33 insertions(+)
>  create mode 100644 tools/testing/selftests/bpf/progs/bpf_iter_bpf_link.c
>
> diff --git a/tools/testing/selftests/bpf/prog_tests/bpf_iter.c b/tools/testing/selftests/bpf/prog_tests/bpf_iter.c
> index 2c403ddc8076..e14a7a6d925c 100644
> --- a/tools/testing/selftests/bpf/prog_tests/bpf_iter.c
> +++ b/tools/testing/selftests/bpf/prog_tests/bpf_iter.c
> @@ -26,6 +26,7 @@
>  #include "bpf_iter_bpf_sk_storage_map.skel.h"
>  #include "bpf_iter_test_kern5.skel.h"
>  #include "bpf_iter_test_kern6.skel.h"
> +#include "bpf_iter_bpf_link.skel.h"
>
>  static int duration;
>
> @@ -1172,6 +1173,20 @@ static void test_buf_neg_offset(void)
>                 bpf_iter_test_kern6__destroy(skel);
>  }
>
> +static void test_link_iter(void)
> +{
> +       struct bpf_iter_bpf_link *skel;
> +
> +       skel = bpf_iter_bpf_link__open_and_load();
> +       if (CHECK(skel, "bpf_iter_bpf_link__open_and_load",
> +                 "skeleton open_and_load unexpected success\n"))
> +               return;
> +

please don't use CHECK() for new tests, you need ASSERT_OK_PTR() here

> +       do_dummy_read(skel->progs.dump_bpf_link);
> +
> +       bpf_iter_bpf_link__destroy(skel);
> +}
> +
>  #define CMP_BUFFER_SIZE 1024
>  static char task_vma_output[CMP_BUFFER_SIZE];
>  static char proc_maps_output[CMP_BUFFER_SIZE];
> diff --git a/tools/testing/selftests/bpf/progs/bpf_iter_bpf_link.c b/tools/testing/selftests/bpf/progs/bpf_iter_bpf_link.c
> new file mode 100644
> index 000000000000..a5041fa1cda9
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/progs/bpf_iter_bpf_link.c
> @@ -0,0 +1,18 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/* Copyright (c) 2020 Facebook */
> +#include "bpf_iter.h"
> +#include <bpf/bpf_helpers.h>
> +
> +char _license[] SEC("license") = "GPL";
> +
> +SEC("iter/bpf_link")
> +int dump_bpf_link(struct bpf_iter__bpf_link *ctx)

see progs/bpf_iter.h, let's add bpf_iter__bpf_link there as well

> +{
> +       struct seq_file *seq = ctx->meta->seq;
> +       struct bpf_link *link = ctx->link;
> +       int link_id;
> +
> +       link_id = link->id;
> +       bpf_seq_write(seq, &link_id, sizeof(link_id));
> +       return 0;
> +}
> --
> 2.32.0
>

Re: [RFC PATCH bpf-next 2/2] selftests/bpf: Add bpf link iter test

[Yonghong Song]

On 4/22/22 11:22 AM, Dmitrii Dolgov wrote:
> Add a simple test for bpf link iterator
> 
> Signed-off-by: Dmitrii Dolgov <9erthalion6@gmail.com>
> ---
>   .../selftests/bpf/prog_tests/bpf_iter.c        | 15 +++++++++++++++
>   .../selftests/bpf/progs/bpf_iter_bpf_link.c    | 18 ++++++++++++++++++
>   2 files changed, 33 insertions(+)
>   create mode 100644 tools/testing/selftests/bpf/progs/bpf_iter_bpf_link.c
> 
> diff --git a/tools/testing/selftests/bpf/prog_tests/bpf_iter.c b/tools/testing/selftests/bpf/prog_tests/bpf_iter.c
> index 2c403ddc8076..e14a7a6d925c 100644
> --- a/tools/testing/selftests/bpf/prog_tests/bpf_iter.c
> +++ b/tools/testing/selftests/bpf/prog_tests/bpf_iter.c
> @@ -26,6 +26,7 @@
>   #include "bpf_iter_bpf_sk_storage_map.skel.h"
>   #include "bpf_iter_test_kern5.skel.h"
>   #include "bpf_iter_test_kern6.skel.h"
> +#include "bpf_iter_bpf_link.skel.h"
>   
>   static int duration;
>   
> @@ -1172,6 +1173,20 @@ static void test_buf_neg_offset(void)
>   		bpf_iter_test_kern6__destroy(skel);
>   }
>   
> +static void test_link_iter(void)

This function is used. Please add a proper subtest for this
in function test_bpf_iter().

> +{
> +	struct bpf_iter_bpf_link *skel;
> +
> +	skel = bpf_iter_bpf_link__open_and_load();
> +	if (CHECK(skel, "bpf_iter_bpf_link__open_and_load",
> +		  "skeleton open_and_load unexpected success\n"))
> +		return;

This is not correct. You should have CHECK(!skel, ...) to return
only if skel is NULL. The error message "skeleton open_and_load 
unexpected success\n" is not correct either. Probably a copy-paste
error.

Also, since you are working on this file, probably convert all
CHECK's in this file to ASSERT_*() macros as patch #2. Then
this patch itself can be patch #3 using ASSERT_*() as well.

> +
> +	do_dummy_read(skel->progs.dump_bpf_link);
> +
> +	bpf_iter_bpf_link__destroy(skel);
> +}
> +
>   #define CMP_BUFFER_SIZE 1024
>   static char task_vma_output[CMP_BUFFER_SIZE];
>   static char proc_maps_output[CMP_BUFFER_SIZE];
> diff --git a/tools/testing/selftests/bpf/progs/bpf_iter_bpf_link.c b/tools/testing/selftests/bpf/progs/bpf_iter_bpf_link.c
> new file mode 100644
> index 000000000000..a5041fa1cda9
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/progs/bpf_iter_bpf_link.c
> @@ -0,0 +1,18 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/* Copyright (c) 2020 Facebook */

copyright issue.

> +#include "bpf_iter.h"
> +#include <bpf/bpf_helpers.h>
> +
> +char _license[] SEC("license") = "GPL";
> +
> +SEC("iter/bpf_link")
> +int dump_bpf_link(struct bpf_iter__bpf_link *ctx)

Please put bpf_iter__bpf_link definition in bpf_iter.h so
the test can work with an old version of vmlinux.h.

> +{
> +	struct seq_file *seq = ctx->meta->seq;
> +	struct bpf_link *link = ctx->link;
> +	int link_id;

The 'link' pointer could be NULL as in previous patch
we have:

+	.ctx_arg_info		= {
+		{ offsetof(struct bpf_iter__bpf_link, link),
+		  PTR_TO_BTF_ID_OR_NULL },
+	},

So you need to add a check below.

	if (!link)
		return 0;

> +
> +	link_id = link->id;
> +	bpf_seq_write(seq, &link_id, sizeof(link_id));
> +	return 0;
> +}

Re: [RFC PATCH bpf-next 0/2] bpf: bpf link iterator

[Andrii Nakryiko]

On Fri, Apr 22, 2022 at 11:23 AM Dmitrii Dolgov <9erthalion6@gmail.com> wrote:
>
> Bpf links seem to be one of the important structures for which no
> iterator is provided. Such iterator could be useful in those cases when
> generic 'task/file' is not suitable or better performance is needed.
>

Overall looks good, I'll defer to Yonghong to validate kernel-side
iterator logic. Yonghong, can you please take a look? Thanks!

> The implementation is mostly copied from prog iterator, and I would like
> to get any high-level feedback about what needs to be different or have
> to be taken into account. As a side note, I would also appreciate if
> someone could point me out to some guide about writing selftests for bpf
> subsystem -- for some unclear reason I couldn't compile the test from
> this changeset, and was testing it only manually with a custom test
> program.
>

What was the error? Generally, you need very recent Clang (probably
built from sources), latest pahole built from sources, and you should
compile kernel before building selftests/bpf.


> Dmitrii Dolgov (2):
>   bpf: Add bpf_link iterator
>   selftests/bpf: Add bpf_link test
>
>  include/linux/bpf.h                           |   1 +
>  kernel/bpf/Makefile                           |   2 +-
>  kernel/bpf/link_iter.c                        | 107 ++++++++++++++++++
>  kernel/bpf/syscall.c                          |  19 ++++
>  .../selftests/bpf/prog_tests/bpf_iter.c       |  15 +++
>  .../selftests/bpf/progs/bpf_iter_bpf_link.c   |  18 +++
>  6 files changed, 161 insertions(+), 1 deletion(-)
>  create mode 100644 kernel/bpf/link_iter.c
>  create mode 100644 tools/testing/selftests/bpf/progs/bpf_iter_bpf_link.c
>
> --
> 2.32.0
>