From: "Toke Høiland-Jørgensen" <toke@redhat.com>
To: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>,
Alexei Starovoitov <ast@fb.com>, bpf <bpf@vger.kernel.org>,
Networking <netdev@vger.kernel.org>
Subject: Re: [PATCH bpf-next] libbpf: Add bpf_object__rodata getter function
Date: Fri, 27 Mar 2020 13:24:37 +0100 [thread overview]
Message-ID: <87eetem1dm.fsf@toke.dk> (raw)
In-Reply-To: <CAEf4BzYxJjJygu_ZqJJB03n=ZetxhuUE7eLD9dsbkbvzQ5M08w@mail.gmail.com>
Andrii Nakryiko <andrii.nakryiko@gmail.com> writes:
> On Thu, Mar 26, 2020 at 8:18 AM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>>
>> This adds a new getter function to libbpf to get the rodata area of a bpf
>> object. This is useful if a program wants to modify the rodata before
>> loading the object. Any such modification needs to be done before loading,
>> since libbpf freezes the backing map after populating it (to allow the
>> kernel to do dead code elimination based on its contents).
>>
>> Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
>> ---
>> tools/lib/bpf/libbpf.c | 13 +++++++++++++
>> tools/lib/bpf/libbpf.h | 1 +
>> tools/lib/bpf/libbpf.map | 1 +
>> 3 files changed, 15 insertions(+)
>>
>> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
>> index 085e41f9b68e..d3e3bbe12f78 100644
>> --- a/tools/lib/bpf/libbpf.c
>> +++ b/tools/lib/bpf/libbpf.c
>> @@ -1352,6 +1352,19 @@ bpf_object__init_internal_map(struct bpf_object *obj, enum libbpf_map_type type,
>> return 0;
>> }
>>
>> +void *bpf_object__rodata(const struct bpf_object *obj, size_t *size)
>
> We probably don't want to expose this API. It just doesn't scale,
> especially if/when we add support for custom sections names for global
> variables.
Right. I was not aware of any such plans, but OK.
> Also checking for map->mmaped is too restrictive. See how BPF skeleton
> solves this problem and still allows .rodata initialization even on
> kernels that don't support memory-mapping global variables.
Not sure what you mean here? As far as I can tell, the map->mmaped
pointer has nothing to do with the kernel support for mmaping the map
contents. It's just what libbpf does to store the data of any
internal_maps?
I mean, bpf_object__open_skeleton() just does this:
if (mmaped && (*map)->libbpf_type != LIBBPF_MAP_KCONFIG)
*mmaped = (*map)->mmaped;
which amounts to the same as I'm doing in this patch?
> But basically, why can't you use BPF skeleton?
Couple of reasons:
- I don't need any of the other features of the skeleton
- I don't want to depend on bpftool in the build process
- I don't want to embed the BPF bytecode into the C object
> Also, application can already find that map by looking at name.
Yes, it can find the map, but it can't access the data. But I guess I
could just add a getter for that. Just figured this was easier to
consume; but I can see why it might impose restrictions on future
changes, so I'll send a v2 with such a map-level getter instead.
-Toke
next prev parent reply other threads:[~2020-03-27 12:24 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-26 15:17 [PATCH bpf-next] libbpf: Add bpf_object__rodata getter function Toke Høiland-Jørgensen
2020-03-26 19:20 ` Andrii Nakryiko
2020-03-27 12:24 ` Toke Høiland-Jørgensen [this message]
2020-03-27 17:49 ` Andrii Nakryiko
2020-03-27 18:39 ` Toke Høiland-Jørgensen
2020-03-27 12:58 ` [PATCH bpf-next v2] libbpf: Add getter for pointer to data area for internal maps Toke Høiland-Jørgensen
2020-03-27 19:17 ` Andrii Nakryiko
2020-03-27 22:26 ` Toke Høiland-Jørgensen
2020-03-27 22:28 ` Alexei Starovoitov
2020-03-28 0:08 ` Toke Høiland-Jørgensen
2020-03-28 18:28 ` [PATCH v3 1/2] libbpf: Add setter for initial value " Toke Høiland-Jørgensen
2020-03-28 20:01 ` Andrii Nakryiko
2020-03-29 12:17 ` Toke Høiland-Jørgensen
2020-03-29 13:22 ` [PATCH v4 " Toke Høiland-Jørgensen
2020-03-29 20:06 ` Andrii Nakryiko
2020-03-29 23:46 ` Daniel Borkmann
2020-03-29 13:22 ` [PATCH v4 2/2] selftests: Add test for overriding global data value before load Toke Høiland-Jørgensen
2020-03-29 20:13 ` Andrii Nakryiko
2020-03-28 18:28 ` [PATCH v3 " Toke Høiland-Jørgensen
2020-03-28 20:06 ` Andrii Nakryiko
2020-03-29 13:10 ` Toke Høiland-Jørgensen
2020-03-29 20:08 ` Andrii Nakryiko
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