libbpf: pinning multiple progs from the same section

libbpf: pinning multiple progs from the same section

[Gilad Reti]

Hello there,

Last year, libbpf added support for attaching multiple handlers to the
same event by allowing defining multiple progs in the same elf
section. However, the pin path for a bpf program is obtained via the
__bpf_program__pin_name function, which uses an escaped version of the
elf section name as the pin_name. Thus, trying to pin multiple bpf
programs defined on the same section fails with "File exists" error,
since libbpf tries to pin more than one program on a single path.
Does adding the actual program name to the pin path makes sense?

Thanks.

Re: libbpf: pinning multiple progs from the same section

[Gilad Reti]

Also, is there a way to set the pin path to all maps/programs at once?
For example, bpf_object__pin_maps pins all maps at a specific path,
but as far as I was able to find there is no similar function to set
the pin path for all maps only (without pinning) so that at loading
time libbpf will try to reuse all maps. The only way to achieve a
complete reuse of all maps that I could find is to "reverse engineer"
libbpf's pin path generation algorithm (i.e. <path>/<map_name>) and
set the pin path on each map before load.

On Sun, Feb 7, 2021 at 1:35 PM Gilad Reti <gilad.reti@gmail.com> wrote:
>
> Hello there,
>
> Last year, libbpf added support for attaching multiple handlers to the
> same event by allowing defining multiple progs in the same elf
> section. However, the pin path for a bpf program is obtained via the
> __bpf_program__pin_name function, which uses an escaped version of the
> elf section name as the pin_name. Thus, trying to pin multiple bpf
> programs defined on the same section fails with "File exists" error,
> since libbpf tries to pin more than one program on a single path.
> Does adding the actual program name to the pin path makes sense?
>
> Thanks.

Re: libbpf: pinning multiple progs from the same section

[Toke Høiland-Jørgensen]

Gilad Reti <gilad.reti@gmail.com> writes:

> Also, is there a way to set the pin path to all maps/programs at once?
> For example, bpf_object__pin_maps pins all maps at a specific path,
> but as far as I was able to find there is no similar function to set
> the pin path for all maps only (without pinning) so that at loading
> time libbpf will try to reuse all maps. The only way to achieve a
> complete reuse of all maps that I could find is to "reverse engineer"
> libbpf's pin path generation algorithm (i.e. <path>/<map_name>) and
> set the pin path on each map before load.

You can set the 'pinning' attribute in the map definition - add
'__uint(pinning, LIBBPF_PIN_BY_NAME);' to the map struct. By default
this will pin beneath /sys/fs/bpf, but you can customise that by setting
the pin_root_path attribute in bpf_object_open_opts.

-Toke

Re: libbpf: pinning multiple progs from the same section

[Gilad Reti]

On Mon, Feb 8, 2021 at 1:42 PM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>
> Gilad Reti <gilad.reti@gmail.com> writes:
>
> > Also, is there a way to set the pin path to all maps/programs at once?
> > For example, bpf_object__pin_maps pins all maps at a specific path,
> > but as far as I was able to find there is no similar function to set
> > the pin path for all maps only (without pinning) so that at loading
> > time libbpf will try to reuse all maps. The only way to achieve a
> > complete reuse of all maps that I could find is to "reverse engineer"
> > libbpf's pin path generation algorithm (i.e. <path>/<map_name>) and
> > set the pin path on each map before load.
>
> You can set the 'pinning' attribute in the map definition - add
> '__uint(pinning, LIBBPF_PIN_BY_NAME);' to the map struct. By default
> this will pin beneath /sys/fs/bpf, but you can customise that by setting
> the pin_root_path attribute in bpf_object_open_opts.

Yes, I am familiar with that feature, but it has some downsides:
1. I need to set it manually on every map (and in cases that I have
only the compiled object file that would be hard).
2. It only works for bpf maps and not bpf programs.
3. It only works for bpf maps that are defined explicitly in the bpf
code and not for implicit (inner) bpf maps (bss, rodata, etc).

>
> -Toke
>

Re: libbpf: pinning multiple progs from the same section

[Toke Høiland-Jørgensen]

Gilad Reti <gilad.reti@gmail.com> writes:

> On Mon, Feb 8, 2021 at 1:42 PM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>>
>> Gilad Reti <gilad.reti@gmail.com> writes:
>>
>> > Also, is there a way to set the pin path to all maps/programs at once?
>> > For example, bpf_object__pin_maps pins all maps at a specific path,
>> > but as far as I was able to find there is no similar function to set
>> > the pin path for all maps only (without pinning) so that at loading
>> > time libbpf will try to reuse all maps. The only way to achieve a
>> > complete reuse of all maps that I could find is to "reverse engineer"
>> > libbpf's pin path generation algorithm (i.e. <path>/<map_name>) and
>> > set the pin path on each map before load.
>>
>> You can set the 'pinning' attribute in the map definition - add
>> '__uint(pinning, LIBBPF_PIN_BY_NAME);' to the map struct. By default
>> this will pin beneath /sys/fs/bpf, but you can customise that by setting
>> the pin_root_path attribute in bpf_object_open_opts.
>
> Yes, I am familiar with that feature, but it has some downsides:
> 1. I need to set it manually on every map (and in cases that I have
> only the compiled object file that would be hard).
> 2. It only works for bpf maps and not bpf programs.
> 3. It only works for bpf maps that are defined explicitly in the bpf
> code and not for implicit (inner) bpf maps (bss, rodata, etc).

Ah, right. Well, other than that I don't think there's a way to set pin
paths in bulk, other than by manually iterating and setting them one at
a time. But, erm, can't you just do that? :)

-Toke

Re: libbpf: pinning multiple progs from the same section

[Gilad Reti]

On Mon, Feb 8, 2021 at 4:28 PM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>
> Gilad Reti <gilad.reti@gmail.com> writes:
>
> > On Mon, Feb 8, 2021 at 1:42 PM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
> >>
> >> Gilad Reti <gilad.reti@gmail.com> writes:
> >>
> >> > Also, is there a way to set the pin path to all maps/programs at once?
> >> > For example, bpf_object__pin_maps pins all maps at a specific path,
> >> > but as far as I was able to find there is no similar function to set
> >> > the pin path for all maps only (without pinning) so that at loading
> >> > time libbpf will try to reuse all maps. The only way to achieve a
> >> > complete reuse of all maps that I could find is to "reverse engineer"
> >> > libbpf's pin path generation algorithm (i.e. <path>/<map_name>) and
> >> > set the pin path on each map before load.
> >>
> >> You can set the 'pinning' attribute in the map definition - add
> >> '__uint(pinning, LIBBPF_PIN_BY_NAME);' to the map struct. By default
> >> this will pin beneath /sys/fs/bpf, but you can customise that by setting
> >> the pin_root_path attribute in bpf_object_open_opts.
> >
> > Yes, I am familiar with that feature, but it has some downsides:
> > 1. I need to set it manually on every map (and in cases that I have
> > only the compiled object file that would be hard).
> > 2. It only works for bpf maps and not bpf programs.
> > 3. It only works for bpf maps that are defined explicitly in the bpf
> > code and not for implicit (inner) bpf maps (bss, rodata, etc).
>
> Ah, right. Well, other than that I don't think there's a way to set pin
> paths in bulk, other than by manually iterating and setting them one at
> a time. But, erm, can't you just do that? :)
>

Sure, I can, but I think we should avoid that. As I said this forces
the user to know libbpf's
pin path naming algorithm, which is not part of the libbpf api afaik.
I think that if we have
a method to pin all maps at a specific path there should also be a
method for reusing them
all from this path, either by exposing the function that builds the
pin path, or a function that
sets all the paths from a root path.

> -Toke
>

Re: libbpf: pinning multiple progs from the same section

[Toke Høiland-Jørgensen]

Gilad Reti <gilad.reti@gmail.com> writes:

> On Mon, Feb 8, 2021 at 4:28 PM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>>
>> Gilad Reti <gilad.reti@gmail.com> writes:
>>
>> > On Mon, Feb 8, 2021 at 1:42 PM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>> >>
>> >> Gilad Reti <gilad.reti@gmail.com> writes:
>> >>
>> >> > Also, is there a way to set the pin path to all maps/programs at once?
>> >> > For example, bpf_object__pin_maps pins all maps at a specific path,
>> >> > but as far as I was able to find there is no similar function to set
>> >> > the pin path for all maps only (without pinning) so that at loading
>> >> > time libbpf will try to reuse all maps. The only way to achieve a
>> >> > complete reuse of all maps that I could find is to "reverse engineer"
>> >> > libbpf's pin path generation algorithm (i.e. <path>/<map_name>) and
>> >> > set the pin path on each map before load.
>> >>
>> >> You can set the 'pinning' attribute in the map definition - add
>> >> '__uint(pinning, LIBBPF_PIN_BY_NAME);' to the map struct. By default
>> >> this will pin beneath /sys/fs/bpf, but you can customise that by setting
>> >> the pin_root_path attribute in bpf_object_open_opts.
>> >
>> > Yes, I am familiar with that feature, but it has some downsides:
>> > 1. I need to set it manually on every map (and in cases that I have
>> > only the compiled object file that would be hard).
>> > 2. It only works for bpf maps and not bpf programs.
>> > 3. It only works for bpf maps that are defined explicitly in the bpf
>> > code and not for implicit (inner) bpf maps (bss, rodata, etc).
>>
>> Ah, right. Well, other than that I don't think there's a way to set pin
>> paths in bulk, other than by manually iterating and setting them one at
>> a time. But, erm, can't you just do that? :)
>>
>
> Sure, I can, but I think we should avoid that. As I said this forces
> the user to know libbpf's pin path naming algorithm, which is not part
> of the libbpf api afaik.

Why? If you set the pin path from your application, libbpf will also try
to reuse the map from that path. So you don't need to know libbpf's
algorithm if you just override it with your own paths?

> I think that if we have a method to pin all maps at a specific path
> there should also be a method for reusing them all from this path,
> either by exposing the function that builds the pin path, or a
> function that sets all the paths from a root path.

What you're asking for is basically a function
bpf_object__set_all_pin_paths(obj, path)

instead of having to do

bpf_object__for_each_map(map, obj) {
  sprintf(path, "path/%s", bpf_map__name(map));
  bpf_map__set_pin_path(map, path);
}

or? Is that really needed?

-Toke

Re: libbpf: pinning multiple progs from the same section

[Gilad Reti]

On Mon, Feb 8, 2021 at 5:09 PM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>
> Gilad Reti <gilad.reti@gmail.com> writes:
>
> > On Mon, Feb 8, 2021 at 4:28 PM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
> >>
> >> Gilad Reti <gilad.reti@gmail.com> writes:
> >>
> >> > On Mon, Feb 8, 2021 at 1:42 PM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
> >> >>
> >> >> Gilad Reti <gilad.reti@gmail.com> writes:
> >> >>
> >> >> > Also, is there a way to set the pin path to all maps/programs at once?
> >> >> > For example, bpf_object__pin_maps pins all maps at a specific path,
> >> >> > but as far as I was able to find there is no similar function to set
> >> >> > the pin path for all maps only (without pinning) so that at loading
> >> >> > time libbpf will try to reuse all maps. The only way to achieve a
> >> >> > complete reuse of all maps that I could find is to "reverse engineer"
> >> >> > libbpf's pin path generation algorithm (i.e. <path>/<map_name>) and
> >> >> > set the pin path on each map before load.
> >> >>
> >> >> You can set the 'pinning' attribute in the map definition - add
> >> >> '__uint(pinning, LIBBPF_PIN_BY_NAME);' to the map struct. By default
> >> >> this will pin beneath /sys/fs/bpf, but you can customise that by setting
> >> >> the pin_root_path attribute in bpf_object_open_opts.
> >> >
> >> > Yes, I am familiar with that feature, but it has some downsides:
> >> > 1. I need to set it manually on every map (and in cases that I have
> >> > only the compiled object file that would be hard).
> >> > 2. It only works for bpf maps and not bpf programs.
> >> > 3. It only works for bpf maps that are defined explicitly in the bpf
> >> > code and not for implicit (inner) bpf maps (bss, rodata, etc).
> >>
> >> Ah, right. Well, other than that I don't think there's a way to set pin
> >> paths in bulk, other than by manually iterating and setting them one at
> >> a time. But, erm, can't you just do that? :)
> >>
> >
> > Sure, I can, but I think we should avoid that. As I said this forces
> > the user to know libbpf's pin path naming algorithm, which is not part
> > of the libbpf api afaik.
>
> Why? If you set the pin path from your application, libbpf will also try
> to reuse the map from that path. So you don't need to know libbpf's
> algorithm if you just override it with your own paths?
>

If I do bpf_object__pin_maps then libbpf decides where it wants to pin them.
I can set each path by my own, but then why do we need this function? All
It does is pinning all the maps at paths that are not part of the api. In this
libbpf version it is here, in the next it is there, and user code will need to
change accordingly. For example, libbpf today uses <path>/<map_name> as
the pin path, but it is also doing sanitize_pin_pathon each path. This means
that after if use bpf_object__pin_maps I also need to know how libbpf sanitizes
its paths and mimic that behavior on my side.

> > I think that if we have a method to pin all maps at a specific path
> > there should also be a method for reusing them all from this path,
> > either by exposing the function that builds the pin path, or a
> > function that sets all the paths from a root path.
>
> What you're asking for is basically a function
> bpf_object__set_all_pin_paths(obj, path)
>
> instead of having to do
>
> bpf_object__for_each_map(map, obj) {
>   sprintf(path, "path/%s", bpf_map__name(map));
>   bpf_map__set_pin_path(map, path);
> }
>
> or? Is that really needed?
>

Yes, that is what I am asking for. Either that or a
bpf_map__build_pin_path(path, map)
That will return a pin path that is compatible with libbpf's one, and
then I can iterate
over all maps.

> -Toke
>

Re: libbpf: pinning multiple progs from the same section

[Toke Høiland-Jørgensen]

Gilad Reti <gilad.reti@gmail.com> writes:

> On Mon, Feb 8, 2021 at 5:09 PM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>>
>> Gilad Reti <gilad.reti@gmail.com> writes:
>>
>> > On Mon, Feb 8, 2021 at 4:28 PM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>> >>
>> >> Gilad Reti <gilad.reti@gmail.com> writes:
>> >>
>> >> > On Mon, Feb 8, 2021 at 1:42 PM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>> >> >>
>> >> >> Gilad Reti <gilad.reti@gmail.com> writes:
>> >> >>
>> >> >> > Also, is there a way to set the pin path to all maps/programs at once?
>> >> >> > For example, bpf_object__pin_maps pins all maps at a specific path,
>> >> >> > but as far as I was able to find there is no similar function to set
>> >> >> > the pin path for all maps only (without pinning) so that at loading
>> >> >> > time libbpf will try to reuse all maps. The only way to achieve a
>> >> >> > complete reuse of all maps that I could find is to "reverse engineer"
>> >> >> > libbpf's pin path generation algorithm (i.e. <path>/<map_name>) and
>> >> >> > set the pin path on each map before load.
>> >> >>
>> >> >> You can set the 'pinning' attribute in the map definition - add
>> >> >> '__uint(pinning, LIBBPF_PIN_BY_NAME);' to the map struct. By default
>> >> >> this will pin beneath /sys/fs/bpf, but you can customise that by setting
>> >> >> the pin_root_path attribute in bpf_object_open_opts.
>> >> >
>> >> > Yes, I am familiar with that feature, but it has some downsides:
>> >> > 1. I need to set it manually on every map (and in cases that I have
>> >> > only the compiled object file that would be hard).
>> >> > 2. It only works for bpf maps and not bpf programs.
>> >> > 3. It only works for bpf maps that are defined explicitly in the bpf
>> >> > code and not for implicit (inner) bpf maps (bss, rodata, etc).
>> >>
>> >> Ah, right. Well, other than that I don't think there's a way to set pin
>> >> paths in bulk, other than by manually iterating and setting them one at
>> >> a time. But, erm, can't you just do that? :)
>> >>
>> >
>> > Sure, I can, but I think we should avoid that. As I said this forces
>> > the user to know libbpf's pin path naming algorithm, which is not part
>> > of the libbpf api afaik.
>>
>> Why? If you set the pin path from your application, libbpf will also try
>> to reuse the map from that path. So you don't need to know libbpf's
>> algorithm if you just override it with your own paths?
>>
>
> If I do bpf_object__pin_maps then libbpf decides where it wants to pin
> them. I can set each path by my own, but then why do we need this
> function?

Erm, what do you mean, "libbpf decides". bpf_object__pin_maps(obj, path)
does exactly what you're asking for: If you supply the path, all maps
are going to be pinned by name underneath that directory...

> All It does is pinning all the maps at paths that are not part of the
> api. In this libbpf version it is here, in the next it is there, and
> user code will need to change accordingly.

Why would you assume the paths would change? That would be an API break?

> For example, libbpf today uses <path>/<map_name> as the pin path, but
> it is also doing sanitize_pin_path on each path. This means that after
> if use bpf_object__pin_maps I also need to know how libbpf sanitizes
> its paths and mimic that behavior on my side.

The paths are sanitised so the kernel will accept them. If you're using
invalid paths your pinning is not going to work at all. If you just want
the paths that the maps are pinned under, use bpf_map__get_pin_path().

>> > I think that if we have a method to pin all maps at a specific path
>> > there should also be a method for reusing them all from this path,
>> > either by exposing the function that builds the pin path, or a
>> > function that sets all the paths from a root path.
>>
>> What you're asking for is basically a function
>> bpf_object__set_all_pin_paths(obj, path)
>>
>> instead of having to do
>>
>> bpf_object__for_each_map(map, obj) {
>>   sprintf(path, "path/%s", bpf_map__name(map));
>>   bpf_map__set_pin_path(map, path);
>> }
>>
>> or? Is that really needed?
>>
>
> Yes, that is what I am asking for. Either that or a
> bpf_map__build_pin_path(path, map) That will return a pin path that is
> compatible with libbpf's one, and then I can iterate over all maps.

See above; this is what bpf_object__pin_maps() does today?

-Toke

Re: libbpf: pinning multiple progs from the same section

[Gilad Reti]

On Mon, Feb 8, 2021 at 7:55 PM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>
> Gilad Reti <gilad.reti@gmail.com> writes:
>
> > On Mon, Feb 8, 2021 at 5:09 PM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
> >>
> >> Gilad Reti <gilad.reti@gmail.com> writes:
> >>
> >> > On Mon, Feb 8, 2021 at 4:28 PM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
> >> >>
> >> >> Gilad Reti <gilad.reti@gmail.com> writes:
> >> >>
> >> >> > On Mon, Feb 8, 2021 at 1:42 PM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
> >> >> >>
> >> >> >> Gilad Reti <gilad.reti@gmail.com> writes:
> >> >> >>
> >> >> >> > Also, is there a way to set the pin path to all maps/programs at once?
> >> >> >> > For example, bpf_object__pin_maps pins all maps at a specific path,
> >> >> >> > but as far as I was able to find there is no similar function to set
> >> >> >> > the pin path for all maps only (without pinning) so that at loading
> >> >> >> > time libbpf will try to reuse all maps. The only way to achieve a
> >> >> >> > complete reuse of all maps that I could find is to "reverse engineer"
> >> >> >> > libbpf's pin path generation algorithm (i.e. <path>/<map_name>) and
> >> >> >> > set the pin path on each map before load.
> >> >> >>
> >> >> >> You can set the 'pinning' attribute in the map definition - add
> >> >> >> '__uint(pinning, LIBBPF_PIN_BY_NAME);' to the map struct. By default
> >> >> >> this will pin beneath /sys/fs/bpf, but you can customise that by setting
> >> >> >> the pin_root_path attribute in bpf_object_open_opts.
> >> >> >
> >> >> > Yes, I am familiar with that feature, but it has some downsides:
> >> >> > 1. I need to set it manually on every map (and in cases that I have
> >> >> > only the compiled object file that would be hard).
> >> >> > 2. It only works for bpf maps and not bpf programs.
> >> >> > 3. It only works for bpf maps that are defined explicitly in the bpf
> >> >> > code and not for implicit (inner) bpf maps (bss, rodata, etc).
> >> >>
> >> >> Ah, right. Well, other than that I don't think there's a way to set pin
> >> >> paths in bulk, other than by manually iterating and setting them one at
> >> >> a time. But, erm, can't you just do that? :)
> >> >>
> >> >
> >> > Sure, I can, but I think we should avoid that. As I said this forces
> >> > the user to know libbpf's pin path naming algorithm, which is not part
> >> > of the libbpf api afaik.
> >>
> >> Why? If you set the pin path from your application, libbpf will also try
> >> to reuse the map from that path. So you don't need to know libbpf's
> >> algorithm if you just override it with your own paths?
> >>
> >
> > If I do bpf_object__pin_maps then libbpf decides where it wants to pin
> > them. I can set each path by my own, but then why do we need this
> > function?
>
> Erm, what do you mean, "libbpf decides". bpf_object__pin_maps(obj, path)
> does exactly what you're asking for: If you supply the path, all maps
> are going to be pinned by name underneath that directory...

They are pinned under this directory, but with which filename? Today
libbpf builds the filename by taking the map name and escaping it, but
what will happen if this will have to change?
For example, libbpf pins programs by thier section name, but as I
mentioned in the first message in this thread this causes failures
when multiple programs reside in one section, so we may need to change
the naming and this can break user code.

>
> > All It does is pinning all the maps at paths that are not part of the
> > api. In this libbpf version it is here, in the next it is there, and
> > user code will need to change accordingly.
>
> Why would you assume the paths would change? That would be an API break?

The point that I am trying to make is that this naming convention is
libbpf internal, so either it should be defined as a part of the api
and each user will need to implement the same naming logic on his side
(escaping etc), relying on libbpf implementation to remain consistent
(this may be the case right now) but for in cases like the program
pinning for example api will have to break, or just expose the pinning
path as another api and then libbpf will need to make no guarantees
about his naming convention.

>
> > For example, libbpf today uses <path>/<map_name> as the pin path, but
> > it is also doing sanitize_pin_path on each path. This means that after
> > if use bpf_object__pin_maps I also need to know how libbpf sanitizes
> > its paths and mimic that behavior on my side.
>
> The paths are sanitised so the kernel will accept them. If you're using
> invalid paths your pinning is not going to work at all. If you just want
> the paths that the maps are pinned under, use bpf_map__get_pin_path().
>

I am not saying that sanitization is redundant, but rather that it
needs to be properly defined (i.e. all dots will always be replaced
with underscores), so either expose it in the api or document it so
that users don't have to look after the specific implementation.
Of course I can retrieve that paths after pinning, but this is sounds
like sort of a workaround to me, though I am okay with staying this
way. The problem that I have with this approach is that if other
processes want to reuse the same map they will have to share the
pinning path somehow instead of each one retreiving it from the map
directly.

> >> > I think that if we have a method to pin all maps at a specific path
> >> > there should also be a method for reusing them all from this path,
> >> > either by exposing the function that builds the pin path, or a
> >> > function that sets all the paths from a root path.
> >>
> >> What you're asking for is basically a function
> >> bpf_object__set_all_pin_paths(obj, path)
> >>
> >> instead of having to do
> >>
> >> bpf_object__for_each_map(map, obj) {
> >>   sprintf(path, "path/%s", bpf_map__name(map));
> >>   bpf_map__set_pin_path(map, path);
> >> }
> >>
> >> or? Is that really needed?
> >>
> >
> > Yes, that is what I am asking for. Either that or a
> > bpf_map__build_pin_path(path, map) That will return a pin path that is
> > compatible with libbpf's one, and then I can iterate over all maps.
>
> See above; this is what bpf_object__pin_maps() does today?

I didn't get this last comment. What I meant is that I want something
like the bpf_object__pin_maps but that doesn't pin the maps, just
exposing its naming part.

>
> -Toke
>

Re: libbpf: pinning multiple progs from the same section

[Toke Høiland-Jørgensen]

Gilad Reti <gilad.reti@gmail.com> writes:

> On Mon, Feb 8, 2021 at 7:55 PM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>>
>> Gilad Reti <gilad.reti@gmail.com> writes:
>>
>> > On Mon, Feb 8, 2021 at 5:09 PM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>> >>
>> >> Gilad Reti <gilad.reti@gmail.com> writes:
>> >>
>> >> > On Mon, Feb 8, 2021 at 4:28 PM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>> >> >>
>> >> >> Gilad Reti <gilad.reti@gmail.com> writes:
>> >> >>
>> >> >> > On Mon, Feb 8, 2021 at 1:42 PM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>> >> >> >>
>> >> >> >> Gilad Reti <gilad.reti@gmail.com> writes:
>> >> >> >>
>> >> >> >> > Also, is there a way to set the pin path to all maps/programs at once?
>> >> >> >> > For example, bpf_object__pin_maps pins all maps at a specific path,
>> >> >> >> > but as far as I was able to find there is no similar function to set
>> >> >> >> > the pin path for all maps only (without pinning) so that at loading
>> >> >> >> > time libbpf will try to reuse all maps. The only way to achieve a
>> >> >> >> > complete reuse of all maps that I could find is to "reverse engineer"
>> >> >> >> > libbpf's pin path generation algorithm (i.e. <path>/<map_name>) and
>> >> >> >> > set the pin path on each map before load.
>> >> >> >>
>> >> >> >> You can set the 'pinning' attribute in the map definition - add
>> >> >> >> '__uint(pinning, LIBBPF_PIN_BY_NAME);' to the map struct. By default
>> >> >> >> this will pin beneath /sys/fs/bpf, but you can customise that by setting
>> >> >> >> the pin_root_path attribute in bpf_object_open_opts.
>> >> >> >
>> >> >> > Yes, I am familiar with that feature, but it has some downsides:
>> >> >> > 1. I need to set it manually on every map (and in cases that I have
>> >> >> > only the compiled object file that would be hard).
>> >> >> > 2. It only works for bpf maps and not bpf programs.
>> >> >> > 3. It only works for bpf maps that are defined explicitly in the bpf
>> >> >> > code and not for implicit (inner) bpf maps (bss, rodata, etc).
>> >> >>
>> >> >> Ah, right. Well, other than that I don't think there's a way to set pin
>> >> >> paths in bulk, other than by manually iterating and setting them one at
>> >> >> a time. But, erm, can't you just do that? :)
>> >> >>
>> >> >
>> >> > Sure, I can, but I think we should avoid that. As I said this forces
>> >> > the user to know libbpf's pin path naming algorithm, which is not part
>> >> > of the libbpf api afaik.
>> >>
>> >> Why? If you set the pin path from your application, libbpf will also try
>> >> to reuse the map from that path. So you don't need to know libbpf's
>> >> algorithm if you just override it with your own paths?
>> >>
>> >
>> > If I do bpf_object__pin_maps then libbpf decides where it wants to pin
>> > them. I can set each path by my own, but then why do we need this
>> > function?
>>
>> Erm, what do you mean, "libbpf decides". bpf_object__pin_maps(obj, path)
>> does exactly what you're asking for: If you supply the path, all maps
>> are going to be pinned by name underneath that directory...
>
> They are pinned under this directory, but with which filename? Today
> libbpf builds the filename by taking the map name and escaping it, but
> what will happen if this will have to change?

Then that would have to be done in a way that was backwards-compatible
so as not to break user code :)

>> > For example, libbpf today uses <path>/<map_name> as the pin path, but
>> > it is also doing sanitize_pin_path on each path. This means that after
>> > if use bpf_object__pin_maps I also need to know how libbpf sanitizes
>> > its paths and mimic that behavior on my side.
>>
>> The paths are sanitised so the kernel will accept them. If you're using
>> invalid paths your pinning is not going to work at all. If you just want
>> the paths that the maps are pinned under, use bpf_map__get_pin_path().
>>
>
> I am not saying that sanitization is redundant, but rather that it
> needs to be properly defined (i.e. all dots will always be replaced
> with underscores), so either expose it in the api or document it so
> that users don't have to look after the specific implementation.

Lack of documentation is a perennial problem, and patches are always
welcome. But it is API: libbpf does things a certain way today, and if
it changes in a way that will break user programs, that is an API break.

>> >> > I think that if we have a method to pin all maps at a specific path
>> >> > there should also be a method for reusing them all from this path,
>> >> > either by exposing the function that builds the pin path, or a
>> >> > function that sets all the paths from a root path.
>> >>
>> >> What you're asking for is basically a function
>> >> bpf_object__set_all_pin_paths(obj, path)
>> >>
>> >> instead of having to do
>> >>
>> >> bpf_object__for_each_map(map, obj) {
>> >>   sprintf(path, "path/%s", bpf_map__name(map));
>> >>   bpf_map__set_pin_path(map, path);
>> >> }
>> >>
>> >> or? Is that really needed?
>> >>
>> >
>> > Yes, that is what I am asking for. Either that or a
>> > bpf_map__build_pin_path(path, map) That will return a pin path that is
>> > compatible with libbpf's one, and then I can iterate over all maps.
>>
>> See above; this is what bpf_object__pin_maps() does today?
>
> I didn't get this last comment. What I meant is that I want something
> like the bpf_object__pin_maps but that doesn't pin the maps, just
> exposing its naming part.

Right, OK. Why, though? I can kinda see how it could be convenient to
(basically )make libbpf behave as if all maps has the 'pinning'
attribute set, for map reuse. But I'm not sure I can think any concrete
use cases where this would be needed. What's yours?

-Toke

Re: libbpf: pinning multiple progs from the same section

[Andrii Nakryiko]

On Sun, Feb 7, 2021 at 3:36 AM Gilad Reti <gilad.reti@gmail.com> wrote:
>
> Hello there,
>
> Last year, libbpf added support for attaching multiple handlers to the
> same event by allowing defining multiple progs in the same elf
> section. However, the pin path for a bpf program is obtained via the
> __bpf_program__pin_name function, which uses an escaped version of the
> elf section name as the pin_name. Thus, trying to pin multiple bpf
> programs defined on the same section fails with "File exists" error,
> since libbpf tries to pin more than one program on a single path.
> Does adding the actual program name to the pin path makes sense?

Yes it does, but as Toke mentioned, that would be API-breaking change,
so I'm holding that off to major version bump or at least until we
agree that it's OK to change this behavior from the current
nonsensical one.

>
> Thanks.

Re: libbpf: pinning multiple progs from the same section

[Gilad Reti]

On Mon, Feb 8, 2021 at 9:16 PM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>
> Gilad Reti <gilad.reti@gmail.com> writes:
>
> > On Mon, Feb 8, 2021 at 7:55 PM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
> >>
> >> Gilad Reti <gilad.reti@gmail.com> writes:
> >>
> >> > On Mon, Feb 8, 2021 at 5:09 PM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
> >> >>
> >> >> Gilad Reti <gilad.reti@gmail.com> writes:
> >> >>
> >> >> > On Mon, Feb 8, 2021 at 4:28 PM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
> >> >> >>
> >> >> >> Gilad Reti <gilad.reti@gmail.com> writes:
> >> >> >>
> >> >> >> > On Mon, Feb 8, 2021 at 1:42 PM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
> >> >> >> >>
> >> >> >> >> Gilad Reti <gilad.reti@gmail.com> writes:
> >> >> >> >>
> >> >> >> >> > Also, is there a way to set the pin path to all maps/programs at once?
> >> >> >> >> > For example, bpf_object__pin_maps pins all maps at a specific path,
> >> >> >> >> > but as far as I was able to find there is no similar function to set
> >> >> >> >> > the pin path for all maps only (without pinning) so that at loading
> >> >> >> >> > time libbpf will try to reuse all maps. The only way to achieve a
> >> >> >> >> > complete reuse of all maps that I could find is to "reverse engineer"
> >> >> >> >> > libbpf's pin path generation algorithm (i.e. <path>/<map_name>) and
> >> >> >> >> > set the pin path on each map before load.
> >> >> >> >>
> >> >> >> >> You can set the 'pinning' attribute in the map definition - add
> >> >> >> >> '__uint(pinning, LIBBPF_PIN_BY_NAME);' to the map struct. By default
> >> >> >> >> this will pin beneath /sys/fs/bpf, but you can customise that by setting
> >> >> >> >> the pin_root_path attribute in bpf_object_open_opts.
> >> >> >> >
> >> >> >> > Yes, I am familiar with that feature, but it has some downsides:
> >> >> >> > 1. I need to set it manually on every map (and in cases that I have
> >> >> >> > only the compiled object file that would be hard).
> >> >> >> > 2. It only works for bpf maps and not bpf programs.
> >> >> >> > 3. It only works for bpf maps that are defined explicitly in the bpf
> >> >> >> > code and not for implicit (inner) bpf maps (bss, rodata, etc).
> >> >> >>
> >> >> >> Ah, right. Well, other than that I don't think there's a way to set pin
> >> >> >> paths in bulk, other than by manually iterating and setting them one at
> >> >> >> a time. But, erm, can't you just do that? :)
> >> >> >>
> >> >> >
> >> >> > Sure, I can, but I think we should avoid that. As I said this forces
> >> >> > the user to know libbpf's pin path naming algorithm, which is not part
> >> >> > of the libbpf api afaik.
> >> >>
> >> >> Why? If you set the pin path from your application, libbpf will also try
> >> >> to reuse the map from that path. So you don't need to know libbpf's
> >> >> algorithm if you just override it with your own paths?
> >> >>
> >> >
> >> > If I do bpf_object__pin_maps then libbpf decides where it wants to pin
> >> > them. I can set each path by my own, but then why do we need this
> >> > function?
> >>
> >> Erm, what do you mean, "libbpf decides". bpf_object__pin_maps(obj, path)
> >> does exactly what you're asking for: If you supply the path, all maps
> >> are going to be pinned by name underneath that directory...
> >
> > They are pinned under this directory, but with which filename? Today
> > libbpf builds the filename by taking the map name and escaping it, but
> > what will happen if this will have to change?
>
> Then that would have to be done in a way that was backwards-compatible
> so as not to break user code :)
>
> >> > For example, libbpf today uses <path>/<map_name> as the pin path, but
> >> > it is also doing sanitize_pin_path on each path. This means that after
> >> > if use bpf_object__pin_maps I also need to know how libbpf sanitizes
> >> > its paths and mimic that behavior on my side.
> >>
> >> The paths are sanitised so the kernel will accept them. If you're using
> >> invalid paths your pinning is not going to work at all. If you just want
> >> the paths that the maps are pinned under, use bpf_map__get_pin_path().
> >>
> >
> > I am not saying that sanitization is redundant, but rather that it
> > needs to be properly defined (i.e. all dots will always be replaced
> > with underscores), so either expose it in the api or document it so
> > that users don't have to look after the specific implementation.
>
> Lack of documentation is a perennial problem, and patches are always
> welcome. But it is API: libbpf does things a certain way today, and if
> it changes in a way that will break user programs, that is an API break.
>

Okay than, you got me convinced. If we agree that sanitization etc is
all part of the api then I am fine with the current state. I am still
somewhat uncomfortable with forcing the user to read libbpf's source
code just to find out the implementation details, but it may be that
we only need more documentation.

> >> >> > I think that if we have a method to pin all maps at a specific path
> >> >> > there should also be a method for reusing them all from this path,
> >> >> > either by exposing the function that builds the pin path, or a
> >> >> > function that sets all the paths from a root path.
> >> >>
> >> >> What you're asking for is basically a function
> >> >> bpf_object__set_all_pin_paths(obj, path)
> >> >>
> >> >> instead of having to do
> >> >>
> >> >> bpf_object__for_each_map(map, obj) {
> >> >>   sprintf(path, "path/%s", bpf_map__name(map));
> >> >>   bpf_map__set_pin_path(map, path);
> >> >> }
> >> >>
> >> >> or? Is that really needed?
> >> >>
> >> >
> >> > Yes, that is what I am asking for. Either that or a
> >> > bpf_map__build_pin_path(path, map) That will return a pin path that is
> >> > compatible with libbpf's one, and then I can iterate over all maps.
> >>
> >> See above; this is what bpf_object__pin_maps() does today?
> >
> > I didn't get this last comment. What I meant is that I want something
> > like the bpf_object__pin_maps but that doesn't pin the maps, just
> > exposing its naming part.
>
> Right, OK. Why, though? I can kinda see how it could be convenient to
> (basically )make libbpf behave as if all maps has the 'pinning'
> attribute set, for map reuse. But I'm not sure I can think any concrete
> use cases where this would be needed. What's yours?
>

I am using the same bpf objects (more specifically, the new skeleton
feature) in two different processes that need access to the same
maps/programs (for example, they both need access to shared maps).
Thus, I want to reuse the entire object in both. Since we already have
a way to pin an entire bpf object, I thought it would be convenient to
have a way of reusing it entirely (though I am fine with pinning and
reusing each one manually).
(I cannot set the __uint(pinning, LIBBPF_PIN_BY_NAME) on each since I
want to share the bss map too)

> -Toke
>

Re: libbpf: pinning multiple progs from the same section

[Toke Høiland-Jørgensen]

Gilad Reti <gilad.reti@gmail.com> writes:

>> > I didn't get this last comment. What I meant is that I want something
>> > like the bpf_object__pin_maps but that doesn't pin the maps, just
>> > exposing its naming part.
>>
>> Right, OK. Why, though? I can kinda see how it could be convenient to
>> (basically )make libbpf behave as if all maps has the 'pinning'
>> attribute set, for map reuse. But I'm not sure I can think any concrete
>> use cases where this would be needed. What's yours?
>>
>
> I am using the same bpf objects (more specifically, the new skeleton
> feature) in two different processes that need access to the same
> maps/programs (for example, they both need access to shared maps).
> Thus, I want to reuse the entire object in both. Since we already have
> a way to pin an entire bpf object, I thought it would be convenient to
> have a way of reusing it entirely (though I am fine with pinning and
> reusing each one manually).
> (I cannot set the __uint(pinning, LIBBPF_PIN_BY_NAME) on each since I
> want to share the bss map too)

Ah, see, now *this* could go under the "missing API" header: having a
way to make libbpf pin (and reuse) the auto-generated maps, like you can
do with the 'pinning' attribute.

Andrii, WDYT?

-Toke

Re: libbpf: pinning multiple progs from the same section

[Andrii Nakryiko]

On Tue, Feb 9, 2021 at 3:03 AM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>
> Gilad Reti <gilad.reti@gmail.com> writes:
>
> >> > I didn't get this last comment. What I meant is that I want something
> >> > like the bpf_object__pin_maps but that doesn't pin the maps, just
> >> > exposing its naming part.
> >>
> >> Right, OK. Why, though? I can kinda see how it could be convenient to
> >> (basically )make libbpf behave as if all maps has the 'pinning'
> >> attribute set, for map reuse. But I'm not sure I can think any concrete
> >> use cases where this would be needed. What's yours?
> >>
> >
> > I am using the same bpf objects (more specifically, the new skeleton
> > feature) in two different processes that need access to the same
> > maps/programs (for example, they both need access to shared maps).
> > Thus, I want to reuse the entire object in both. Since we already have
> > a way to pin an entire bpf object, I thought it would be convenient to
> > have a way of reusing it entirely (though I am fine with pinning and
> > reusing each one manually).
> > (I cannot set the __uint(pinning, LIBBPF_PIN_BY_NAME) on each since I
> > want to share the bss map too)
>
> Ah, see, now *this* could go under the "missing API" header: having a
> way to make libbpf pin (and reuse) the auto-generated maps, like you can
> do with the 'pinning' attribute.
>
> Andrii, WDYT?

I think that the whole pinning handling in libbpf feels a bit ad-hoc.
It would be good for someone to sit and think through this end-to-end.
Unfortunately I never had a need for pinning, so I'm not the best
person to do this.

In this case, you can still do bpf_map__set_pin_path() on internal
maps (.bss, .data, etc), but you'll need to specify pin path
explicitly, which is different from what you get with
LIBBPF_PIN_BY_NAME.

So I think pinning support is not complete, but I'd also like to avoid
adding new APIs in an ad-hoc manner without a holistic view of how
pinning should work with libbpf.

>
> -Toke
>