Date:   Sat, 2 Mar 2019 00:23:13 +0000
From:   Yonghong Song <yhs@...com>
To:     Daniel Borkmann <daniel@...earbox.net>,
        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 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.

> 
> Thanks,
> Daniel
> 

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