[bpf-next,v10,6/8] libbpf: Update a bpf_link with another struct_ops.

Commit Message

Kui-Feng Lee March 21, 2023, 11:28 p.m. UTC

Introduce bpf_link__update_map(), which allows to atomically update
underlying struct_ops implementation for given struct_ops BPF link

Signed-off-by: Kui-Feng Lee <[e-mail]>
---
 tools/lib/bpf/bpf.h      |  5 ++++-
 tools/lib/bpf/libbpf.c   | 35 +++++++++++++++++++++++++++++++++++
 tools/lib/bpf/libbpf.h   |  1 +
 tools/lib/bpf/libbpf.map |  1 +
 4 files changed, 41 insertions(+), 1 deletion(-)

Comments

Andrii Nakryiko March 22, 2023, 11:42 p.m. UTC | #1

On Tue, Mar 21, 2023 at 4:28 PM Kui-Feng Lee <[e-mail]> wrote:
>
> Introduce bpf_link__update_map(), which allows to atomically update
> underlying struct_ops implementation for given struct_ops BPF link
>
> Signed-off-by: Kui-Feng Lee <[e-mail]>
> ---
>  tools/lib/bpf/bpf.h      |  5 ++++-
>  tools/lib/bpf/libbpf.c   | 35 +++++++++++++++++++++++++++++++++++
>  tools/lib/bpf/libbpf.h   |  1 +
>  tools/lib/bpf/libbpf.map |  1 +
>  4 files changed, 41 insertions(+), 1 deletion(-)
>
> diff --git a/tools/lib/bpf/bpf.h b/tools/lib/bpf/bpf.h
> index f0f786373238..4fae4e698a8e 100644
> --- a/tools/lib/bpf/bpf.h
> +++ b/tools/lib/bpf/bpf.h
> @@ -335,7 +335,10 @@ LIBBPF_API int bpf_link_detach(int link_fd);
>  struct bpf_link_update_opts {
>         size_t sz; /* size of this struct for forward/backward compatibility */
>         __u32 flags;       /* extra flags */
> -       __u32 old_prog_fd; /* expected old program FD */
> +       union {
> +               __u32 old_prog_fd; /* expected old program FD */
> +               __u32 old_map_fd;  /* expected old map FD */
> +       };

so for these low-level wrappers in libbpf with OPTS we've been trying
to avoid unnecessary unions. If you look at bpf_link_create and
bpf_link_create_ops some fields that are in a union in kernel UAPI are
actually listed as separate fields, and libbpf makes sure that both
fields are not specified at the same time (like iter_info_len and
target_btf_id, for instance).

So let's do the same here, instead of making a union, let's have

__u32 old_prog_fd;
__u32 old_map_fd;

and then in bpf_link_update() implementation make sure that both can't
be set at the same time.

The rest of the patch looks good to me, thanks.


>  };
>  #define bpf_link_update_opts__last_field old_prog_fd
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 3b257d5170cb..935a7da501d7 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -11670,6 +11670,41 @@ struct bpf_link *bpf_map__attach_struct_ops(const struct bpf_map *map)
>         return &link->link;
>  }
>
> +/*
> + * Swap the back struct_ops of a link with a new struct_ops map.
> + */
> +int bpf_link__update_map(struct bpf_link *link, const struct bpf_map *map)
> +{
> +       struct bpf_link_struct_ops *st_ops_link;
> +       __u32 zero = 0;
> +       int err;
> +
> +       if (!bpf_map__is_struct_ops(map) || map->fd < 0)
> +               return -EINVAL;
> +
> +       st_ops_link = container_of(link, struct bpf_link_struct_ops, link);
> +       /* Ensure the type of a link is correct */
> +       if (st_ops_link->map_fd < 0)
> +               return -EINVAL;
> +
> +       err = bpf_map_update_elem(map->fd, &zero, map->st_ops->kern_vdata, 0);
> +       /* It can be EBUSY if the map has been used to create or
> +        * update a link before.  We don't allow updating the value of
> +        * a struct_ops once it is set.  That ensures that the value
> +        * never changed.  So, it is safe to skip EBUSY.
> +        */
> +       if (err && err != -EBUSY)
> +               return err;
> +
> +       err = bpf_link_update(link->fd, map->fd, NULL);
> +       if (err < 0)
> +               return err;
> +
> +       st_ops_link->map_fd = map->fd;
> +
> +       return 0;
> +}
> +
>  typedef enum bpf_perf_event_ret (*bpf_perf_event_print_t)(struct perf_event_header *hdr,
>                                                           void *private_data);
>
> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> index db4992a036f8..1615e55e2e79 100644
> --- a/tools/lib/bpf/libbpf.h
> +++ b/tools/lib/bpf/libbpf.h
> @@ -719,6 +719,7 @@ bpf_program__attach_freplace(const struct bpf_program *prog,
>  struct bpf_map;
>
>  LIBBPF_API struct bpf_link *bpf_map__attach_struct_ops(const struct bpf_map *map);
> +LIBBPF_API int bpf_link__update_map(struct bpf_link *link, const struct bpf_map *map);
>
>  struct bpf_iter_attach_opts {
>         size_t sz; /* size of this struct for forward/backward compatibility */
> diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
> index 50dde1f6521e..a5aa3a383d69 100644
> --- a/tools/lib/bpf/libbpf.map
> +++ b/tools/lib/bpf/libbpf.map
> @@ -386,6 +386,7 @@ LIBBPF_1.1.0 {
>  LIBBPF_1.2.0 {
>         global:
>                 bpf_btf_get_info_by_fd;
> +               bpf_link__update_map;
>                 bpf_link_get_info_by_fd;
>                 bpf_map_get_info_by_fd;
>                 bpf_prog_get_info_by_fd;
> --
> 2.34.1
>

Kuifeng Lee March 22, 2023, 11:58 p.m. UTC | #2

On 3/22/23 16:42, Andrii Nakryiko wrote:
> On Tue, Mar 21, 2023 at 4:28 PM Kui-Feng Lee <[e-mail]> wrote:
>>
>> Introduce bpf_link__update_map(), which allows to atomically update
>> underlying struct_ops implementation for given struct_ops BPF link
>>
>> Signed-off-by: Kui-Feng Lee <[e-mail]>
>> ---
>>   tools/lib/bpf/bpf.h      |  5 ++++-
>>   tools/lib/bpf/libbpf.c   | 35 +++++++++++++++++++++++++++++++++++
>>   tools/lib/bpf/libbpf.h   |  1 +
>>   tools/lib/bpf/libbpf.map |  1 +
>>   4 files changed, 41 insertions(+), 1 deletion(-)
>>
>> diff --git a/tools/lib/bpf/bpf.h b/tools/lib/bpf/bpf.h
>> index f0f786373238..4fae4e698a8e 100644
>> --- a/tools/lib/bpf/bpf.h
>> +++ b/tools/lib/bpf/bpf.h
>> @@ -335,7 +335,10 @@ LIBBPF_API int bpf_link_detach(int link_fd);
>>   struct bpf_link_update_opts {
>>          size_t sz; /* size of this struct for forward/backward compatibility */
>>          __u32 flags;       /* extra flags */
>> -       __u32 old_prog_fd; /* expected old program FD */
>> +       union {
>> +               __u32 old_prog_fd; /* expected old program FD */
>> +               __u32 old_map_fd;  /* expected old map FD */
>> +       };
> 
> so for these low-level wrappers in libbpf with OPTS we've been trying
> to avoid unnecessary unions. If you look at bpf_link_create and
> bpf_link_create_ops some fields that are in a union in kernel UAPI are
> actually listed as separate fields, and libbpf makes sure that both
> fields are not specified at the same time (like iter_info_len and
> target_btf_id, for instance).
> 
> So let's do the same here, instead of making a union, let's have
> 
> __u32 old_prog_fd;
> __u32 old_map_fd;
> 
> and then in bpf_link_update() implementation make sure that both can't
> be set at the same time.

Got it! Thanks!

> 
> The rest of the patch looks good to me, thanks....

Patch

diff --git a/tools/lib/bpf/bpf.h b/tools/lib/bpf/bpf.h
index f0f786373238..4fae4e698a8e 100644
--- a/tools/lib/bpf/bpf.h
+++ b/tools/lib/bpf/bpf.h
@@ -335,7 +335,10 @@  LIBBPF_API int bpf_link_detach(int link_fd);
 struct bpf_link_update_opts {
 	size_t sz; /* size of this struct for forward/backward compatibility */
 	__u32 flags;	   /* extra flags */
-	__u32 old_prog_fd; /* expected old program FD */
+	union {
+		__u32 old_prog_fd; /* expected old program FD */
+		__u32 old_map_fd;  /* expected old map FD */
+	};
 };
 #define bpf_link_update_opts__last_field old_prog_fd
 
diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 3b257d5170cb..935a7da501d7 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -11670,6 +11670,41 @@  struct bpf_link *bpf_map__attach_struct_ops(const struct bpf_map *map)
 	return &link->link;
 }
 
+/*
+ * Swap the back struct_ops of a link with a new struct_ops map.
+ */
+int bpf_link__update_map(struct bpf_link *link, const struct bpf_map *map)
+{
+	struct bpf_link_struct_ops *st_ops_link;
+	__u32 zero = 0;
+	int err;
+
+	if (!bpf_map__is_struct_ops(map) || map->fd < 0)
+		return -EINVAL;
+
+	st_ops_link = container_of(link, struct bpf_link_struct_ops, link);
+	/* Ensure the type of a link is correct */
+	if (st_ops_link->map_fd < 0)
+		return -EINVAL;
+
+	err = bpf_map_update_elem(map->fd, &zero, map->st_ops->kern_vdata, 0);
+	/* It can be EBUSY if the map has been used to create or
+	 * update a link before.  We don't allow updating the value of
+	 * a struct_ops once it is set.  That ensures that the value
+	 * never changed.  So, it is safe to skip EBUSY.
+	 */
+	if (err && err != -EBUSY)
+		return err;
+
+	err = bpf_link_update(link->fd, map->fd, NULL);
+	if (err < 0)
+		return err;
+
+	st_ops_link->map_fd = map->fd;
+
+	return 0;
+}
+
 typedef enum bpf_perf_event_ret (*bpf_perf_event_print_t)(struct perf_event_header *hdr,
 							  void *private_data);
 
diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
index db4992a036f8..1615e55e2e79 100644
--- a/tools/lib/bpf/libbpf.h
+++ b/tools/lib/bpf/libbpf.h
@@ -719,6 +719,7 @@  bpf_program__attach_freplace(const struct bpf_program *prog,
 struct bpf_map;
 
 LIBBPF_API struct bpf_link *bpf_map__attach_struct_ops(const struct bpf_map *map);
+LIBBPF_API int bpf_link__update_map(struct bpf_link *link, const struct bpf_map *map);
 
 struct bpf_iter_attach_opts {
 	size_t sz; /* size of this struct for forward/backward compatibility */
diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
index 50dde1f6521e..a5aa3a383d69 100644
--- a/tools/lib/bpf/libbpf.map
+++ b/tools/lib/bpf/libbpf.map
@@ -386,6 +386,7 @@  LIBBPF_1.1.0 {
 LIBBPF_1.2.0 {
 	global:
 		bpf_btf_get_info_by_fd;
+		bpf_link__update_map;
 		bpf_link_get_info_by_fd;
 		bpf_map_get_info_by_fd;
 		bpf_prog_get_info_by_fd;