Date:   Sat, 2 Mar 2019 01:27:44 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Yonghong Song <yhs@...com>, Stanislav Fomichev <sdf@...ichev.me>
Cc:     Alexei Starovoitov <ast@...com>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "joe@...d.net.nz" <joe@...d.net.nz>,
        "john.fastabend@...il.com" <john.fastabend@...il.com>,
        "tgraf@...g.ch" <tgraf@...g.ch>, Andrii Nakryiko <andriin@...com>,
        "jakub.kicinski@...ronome.com" <jakub.kicinski@...ronome.com>,
        "lmb@...udflare.com" <lmb@...udflare.com>
Subject: Re: [PATCH bpf-next v2 5/7] bpf, libbpf: support global
 data/bss/rodata sections

On 03/02/2019 01:23 AM, Yonghong Song wrote:
> On 2/28/19 4:19 PM, Daniel Borkmann wrote:
>> On 03/01/2019 12:41 AM, Stanislav Fomichev wrote:
>>> On 03/01, Daniel Borkmann wrote:
>>>> This work adds BPF loader support for global data sections
>>>> to libbpf. This allows to write BPF programs in more natural
>>>> C-like way by being able to define global variables and const
>>>> data.
>>>>
>>>> Back at LPC 2018 [0] we presented a first prototype which
>>>> implemented support for global data sections by extending BPF
>>>> syscall where union bpf_attr would get additional memory/size
>>>> pair for each section passed during prog load in order to later
>>>> add this base address into the ldimm64 instruction along with
>>>> the user provided offset when accessing a variable. Consensus
>>>> from LPC was that for proper upstream support, it would be
>>>> more desirable to use maps instead of bpf_attr extension as
>>>> this would allow for introspection of these sections as well
>>>> as potential life updates of their content. This work follows
>>>> this path by taking the following steps from loader side:
>>>>
>>>>   1) In bpf_object__elf_collect() step we pick up ".data",
>>>>      ".rodata", and ".bss" section information.
>>>>
>>>>   2) If present, in bpf_object__init_global_maps() we create
>>>>      a map that corresponds to each of the present sections.
>>>>      Given section size and access properties can differ, a
>>>>      single entry array map is created with value size that
>>>>      is corresponding to the ELF section size of .data, .bss
>>>>      or .rodata. In the latter case, the map is created as
>>>>      read-only from program side such that verifier rejects
>>>>      any write attempts into .rodata. In a subsequent step,
>>>>      for .data and .rodata sections, the section content is
>>>>      copied into the map through bpf_map_update_elem(). For
>>>>      .bss this is not necessary since array map is already
>>>>      zero-initialized by default.
>>>>
>>>>   3) In bpf_program__collect_reloc() step, we record the
>>>>      corresponding map, insn index, and relocation type for
>>>>      the global data.
>>>>
>>>>   4) And last but not least in the actual relocation step in
>>>>      bpf_program__relocate(), we mark the ldimm64 instruction
>>>>      with src_reg = BPF_PSEUDO_MAP_VALUE where in the first
>>>>      imm field the map's file descriptor is stored as similarly
>>>>      done as in BPF_PSEUDO_MAP_FD, and in the second imm field
>>>>      (as ldimm64 is 2-insn wide) we store the access offset
>>>>      into the section.
>>>>
>>>>   5) On kernel side, this special marked BPF_PSEUDO_MAP_VALUE
>>>>      load will then store the actual target address in order
>>>>      to have a 'map-lookup'-free access. That is, the actual
>>>>      map value base address + offset. The destination register
>>>>      in the verifier will then be marked as PTR_TO_MAP_VALUE,
>>>>      containing the fixed offset as reg->off and backing BPF
>>>>      map as reg->map_ptr. Meaning, it's treated as any other
>>>>      normal map value from verification side, only with
>>>>      efficient, direct value access instead of actual call to
>>>>      map lookup helper as in the typical case.
>>>>
>>>> Simple example dump of program using globals vars in each
>>>> section:
>>>>
>>>>    # readelf -a test_global_data.o
>>>>    [...]
>>>>    [ 6] .bss              NOBITS           0000000000000000  00000328
>>>>         0000000000000010  0000000000000000  WA       0     0     8
>>>>    [ 7] .data             PROGBITS         0000000000000000  00000328
>>>>         0000000000000010  0000000000000000  WA       0     0     8
>>>>    [ 8] .rodata           PROGBITS         0000000000000000  00000338
>>>>         0000000000000018  0000000000000000   A       0     0     8
>>>>    [...]
>>>>      95: 0000000000000000     8 OBJECT  LOCAL  DEFAULT    6 static_bss
>>>>      96: 0000000000000008     8 OBJECT  LOCAL  DEFAULT    6 static_bss2
>>>>      97: 0000000000000000     8 OBJECT  LOCAL  DEFAULT    7 static_data
>>>>      98: 0000000000000008     8 OBJECT  LOCAL  DEFAULT    7 static_data2
>>>>      99: 0000000000000000     8 OBJECT  LOCAL  DEFAULT    8 static_rodata
>>>>     100: 0000000000000008     8 OBJECT  LOCAL  DEFAULT    8 static_rodata2
>>>>     101: 0000000000000010     8 OBJECT  LOCAL  DEFAULT    8 static_rodata3
>>>>    [...]
>>>>
>>>>    # bpftool prog
>>>>    103: sched_cls  name load_static_dat  tag 37a8b6822fc39a29  gpl
>>>>         loaded_at 2019-02-28T02:02:35+0000  uid 0
>>>>         xlated 712B  jited 426B  memlock 4096B  map_ids 63,64,65,66
>>>>    # bpftool map show id 63
>>>>    63: array  name .bss  flags 0x0                      <-- .bss area, rw
>>> Can we use <main prog>.bss/data/rodata names? If we load more than one
>>> prog with global data that should make it easier to find which one is which.
>>
>> Yeah that's fine, we can change it. They could potentially also be shared,
>> so <main prog>.bss/data/rodata might be misleading, but <obj>.bss/data/rodata
>> could be.
> 
> Note the map_name field only 16 bytes (excluding ending '\0', only 15 
> bytes). If <obj> file has a long name like test_verifier.o, you may have
> to shorten the <obj> part of the name.

Yes, it needs to be ensured that (bss/)data/rodata part is still visible
to the user, so <obj> part would need to be truncated accordingly.

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