This patch implements code-gen for BPF_ALU | BPF_ARSH | BPF_*.

Cc: Naveen N. Rao <naveen.n.rao@linux.ibm.com>
Cc: Sandipan Das <sandipan@linux.ibm.com>
Signed-off-by: Jiong Wang <jiong.wang@netronome.com>
Reviewed-by: Sandipan Das <sandipan@linux.ibm.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Jitting of BPF_K is supported already, but not BPF_X. This patch complete
the support for the latter on both MIPS and microMIPS.

Cc: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@vger.kernel.org
Acked-by: Paul Burton <paul.burton@mips.com>
Signed-off-by: Jiong Wang <jiong.wang@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

For micro-mips, srlv inside POOL32A encoding space should use 0x50
sub-opcode, NOT 0x90.

Some early version ISA doc describes the encoding as 0x90 for both srlv and
srav, this looks to me was a typo. I checked Binutils libopcode
implementation which is using 0x50 for srlv and 0x90 for srav.

v1->v2:
  - Keep mm_srlv32_op sorted by value.

Fixes: f31318fd ("MIPS: uasm: Add srlv uasm instruction")
Cc: Markos Chandras <markos.chandras@imgtec.com>
Cc: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@vger.kernel.org
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Jiong Wang <jiong.wang@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Martin KaFai Lau says:

====================
The patchset has a few improvements on bpf_func_info:
1. Improvements on the behaviors of info.func_info, info.func_info_cnt
   and info.func_info_rec_size.
2. Name change: s/insn_offset/insn_off/

Please see individual patch for details.
====================
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Similar to info.jited_*, info.func_info could be 0 if
bpf_dump_raw_ok() == false.

This patch makes changes to test_btf and bpftool to expect info.func_info
could be 0.

This patch also makes the needed changes for s/insn_offset/insn_off/.
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

This patch sync the name changes in bpf_func_info to
the tools/.
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

The later patch will introduce "struct bpf_line_info" which
has member "line_off" and "file_off" referring back to the
string section in btf.  The line_"off" and file_"off"
are more consistent to the naming convention in btf.h that
means "offset" (e.g. name_off in "struct btf_type").

The to-be-added "struct bpf_line_info" also has another
member, "insn_off" which is the same as the "insn_offset"
in "struct bpf_func_info".  Hence, this patch renames "insn_offset"
to "insn_off" for "struct bpf_func_info".
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1) When bpf_dump_raw_ok() == false and the kernel can provide >=1
   func_info to the userspace, the current behavior is setting
   the info.func_info_cnt to 0 instead of setting info.func_info
   to 0.

   It is different from the behavior in jited_func_lens/nr_jited_func_lens,
   jited_ksyms/nr_jited_ksyms...etc.

   This patch fixes it. (i.e. set func_info to 0 instead of
   func_info_cnt to 0 when bpf_dump_raw_ok() == false).

2) When the userspace passed in info.func_info_cnt == 0, the kernel
   will set the expected func_info size back to the
   info.func_info_rec_size.  It is a way for the userspace to learn
   the kernel expected func_info_rec_size introduced in
   commit 838e9690 ("bpf: Introduce bpf_func_info").

   An exception is the kernel expected size is not set when
   func_info is not available for a bpf_prog.  This makes the
   returned info.func_info_rec_size has different values
   depending on the returned value of info.func_info_cnt.

   This patch sets the kernel expected size to info.func_info_rec_size
   independent of the info.func_info_cnt.

3) The current logic only rejects invalid func_info_rec_size if
   func_info_cnt is non zero.  This patch also rejects invalid
   nonzero info.func_info_rec_size and not equal to the kernel
   expected size.

4) Set info.btf_id as long as prog->aux->btf != NULL.  That will
   setup the later copy_to_user() codes look the same as others
   which then easier to understand and maintain.

   prog->aux->btf is not NULL only if prog->aux->func_info_cnt > 0.

   Breaking up info.btf_id from prog->aux->func_info_cnt is needed
   for the later line info patch anyway.

   A similar change is made to bpf_get_prog_name().

Fixes: 838e9690 ("bpf: Introduce bpf_func_info")
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

BPF programs can use the bpf_trace_printk() helper to print debug
information into the trace pipe. Add a subcommand
"bpftool prog tracelog" to simply dump this pipe to the console.

This is for a good part copied from iproute2, where the feature is
available with "tc exec bpf dbg". Changes include dumping pipe content
to stdout instead of stderr and adding JSON support (content is dumped
as an array of strings, one per line read from the pipe). This version
is dual-licensed, with Daniel's permission.

Cc: Daniel Borkmann <daniel@iogearbox.net>
Suggested-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

Ard Biesheuvel says:

====================
On arm64, modules are allocated from a 128 MB window which is close to
the core kernel, so that relative direct branches are guaranteed to be
in range (except in some KASLR configurations). Also, module_alloc()
is in charge of allocating KASAN shadow memory when running with KASAN
enabled.

This means that the way BPF reuses module_alloc()/module_memfree() is
undesirable on arm64 (and potentially other architectures as well),
and so this series refactors BPF's use of those functions to permit
architectures to change this behavior.

Patch #1 breaks out the module_alloc() and module_memfree() calls into
__weak functions so they can be overridden.

Patch #2 implements the new alloc/free overrides for arm64

Changes since v3:
- drop 'const' modifier for free() hook void* argument
- move the dedicated BPF region to before the module region, putting it
  within 4GB of the module and kernel regions on non-KASLR kernels

Changes since v2:
- properly build time and runtime tested this time (log after the diffstat)
- create a dedicated 128 MB region at the top of the vmalloc space for BPF
  programs, ensuring that the programs will be in branching range of each
  other (which we currently rely upon) but at an arbitrary distance from
  the kernel and modules (which we don't care about)

Changes since v1:
- Drop misguided attempt to 'fix' and refactor the free path. Instead,
  just add another __weak wrapper for the invocation of module_memfree()
====================

Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Rick Edgecombe <rick.p.edgecombe@intel.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Jann Horn <jannh@google.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Jessica Yu <jeyu@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Cc: netdev@vger.kernel.org
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

The arm64 module region is a 128 MB region that is kept close to
the core kernel, in order to ensure that relative branches are
always in range. So using the same region for programs that do
not have this restriction is wasteful, and preferably avoided.

Now that the core BPF JIT code permits the alloc/free routines to
be overridden, implement them by vmalloc()/vfree() calls from a
dedicated 128 MB region set aside for BPF programs. This ensures
that BPF programs are still in branching range of each other, which
is something the JIT currently depends upon (and is not guaranteed
when using module_alloc() on KASLR kernels like we do currently).
It also ensures that placement of BPF programs does not correlate
with the placement of the core kernel or modules, making it less
likely that leaking the former will reveal the latter.

This also solves an issue under KASAN, where shadow memory is
needlessly allocated for all BPF programs (which don't require KASAN
shadow pages since they are not KASAN instrumented)
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

By default, BPF uses module_alloc() to allocate executable memory,
but this is not necessary on all arches and potentially undesirable
on some of them.

So break out the module_alloc() and module_memfree() calls into __weak
functions to allow them to be overridden in arch code.
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

Lorenz Bauer says:

====================
Right now, there is no safe way to use BPF_PROG_TEST_RUN with data_out.
This is because bpf_test_finish copies the output buffer to user space
without checking its size. This can lead to the kernel overwriting
data in user space after the buffer if xdp_adjust_head and friends are
in play.

Thanks to everyone for their advice and patience with this patch set!

Changes in v5:
* Fix up libbpf.map

Changes in v4:
* Document bpf_prog_test_run and bpf_prog_test_run_xattr
* Use struct bpf_prog_test_run_attr for return values

Changes in v3:
* Introduce bpf_prog_test_run_xattr instead of modifying the existing
  function

Changes in v2:
* Make the syscall return ENOSPC if data_size_out is too small
* Make bpf_prog_test_run return EINVAL if size_out is missing
* Document the new behaviour of data_size_out
====================
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Make sure that bpf_prog_test_run_xattr returns the correct length
and that the kernel respects the output size hint. Also check
that errno indicates ENOSPC if there is a short output buffer given.
Signed-off-by: Lorenz Bauer <lmb@cloudflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Add a new function, which encourages safe usage of the test interface.
bpf_prog_test_run continues to work as before, but should be considered
unsafe.
Signed-off-by: Lorenz Bauer <lmb@cloudflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Pull changes from "bpf: respect size hint to BPF_PROG_TEST_RUN if present".
Signed-off-by: Lorenz Bauer <lmb@cloudflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Use data_size_out as a size hint when copying test output to user space.
ENOSPC is returned if the output buffer is too small.
Callers which so far did not set data_size_out are not affected.
Signed-off-by: Lorenz Bauer <lmb@cloudflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

When NULL pointer accidentally passed to write_kprobe_events,
due to strlen(NULL), segmentation fault happens.
Changed code returns -1 to deal with this situation.

Bug issued with Smatch, static analysis.
Signed-off-by: Daniel T. Lee <danieltimlee@gmail.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

The missing indentation on the "Return" sections for bpf_map_pop_elem()
and bpf_map_peek_elem() helpers break RST and man pages generation. This
patch fixes them, and moves the description of those two helpers towards
the end of the list (even though they are somehow related to the three
first helpers for maps, the man page explicitly states that the helpers
are sorted in chronological order).

While at it, bring other minor formatting edits for eBPF helpers
documentation: mostly blank lines removal, RST formatting, or other
small nits for consistency.
Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

The pkt_len field in qdisc_skb_cb stores the skb length as it will
appear on the wire after segmentation. For byte accounting, this value
is more accurate than skb->len. It is computed on entry to the TC
layer, so only valid there.

Allow read access to this field from BPF tc classifier and action
programs. The implementation is analogous to tc_classid, aside from
restricting to read access.

To distinguish it from skb->len and self-describe export as wire_len.

Changes v1->v2
  - Rename pkt_len to wire_len
Signed-off-by: Petar Penkov <ppenkov@google.com>
Signed-off-by: Vlad Dumitrescu <vladum@google.com>
Signed-off-by: Willem de Bruijn <willemb@google.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

The whole libbpf is licensed as (LGPL-2.1 OR BSD-2-Clause). I missed it
while adding README.rst. Fix it and use same license as all other files
in libbpf do. Since I'm the only author of README.rst so far, no others'
permissions should be needed.

Fixes: 76d1b894 ("libbpf: Document API and ABI conventions")
Signed-off-by: Andrey Ignatov <rdna@fb.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

The aux->func_info and aux->btf are leaked in the error out cases
during bpf_prog_load().  This patch fixes it.

Fixes: ba64e7d8 ("bpf: btf: support proper non-jit func info")
Cc: Yonghong Song <yhs@fb.com>
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Matteo Croce says:

====================
Small improvements to improve the readability and easiness
to use of the xdp1 sample.
====================
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Find the ifindex with if_nametoindex() instead of requiring the
numeric ifindex.
Signed-off-by: Matteo Croce <mcroce@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Store only the total packet count for every protocol, instead of the
whole per-cpu array.
Use bpf_map_get_next_key() to iterate the map, instead of looking up
all the protocols.
Signed-off-by: Matteo Croce <mcroce@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

David Miller says:

====================
On sparc64 a ton of test cases in test_verifier.c fail because
the memory accesses in the test case are unaligned (or cannot
be proven to be aligned by the verifier).

Perhaps we can eventually try to (carefully) modify each test case
which has this problem to not use unaligned accesses but:

1) That is delicate work.

2) The changes might not fully respect the original
   intention of the testcase.

3) In some cases, such a transformation might not even
   be feasible at all.

So add an "any alignment" flag to tell the verifier to forcefully
disable it's alignment checks completely.

test_verifier.c is then annotated to use this flag when necessary.

The presence of the flag in each test case is good documentation to
anyone who wants to actually tackle the job of eliminating the
unaligned memory accesses in the test cases.

I've also seen several weird things in test cases, like trying to
access __skb->mark in a packet buffer.

This gets rid of 104 test_verifier.c failures on sparc64.

Changes since v1:

1) Explain the new BPF_PROG_LOAD flag in easier to understand terms.
   Suggested by Alexei.

2) Make bpf_verify_program() just take a __u32 prog_flags instead of
   just accumulating boolean arguments over and over.  Also suggested
   by Alexei.

Changes since RFC:

1) Only the admin can allow the relaxation of alignment restrictions
   on inefficient unaligned access architectures.

2) Use F_NEEDS_EFFICIENT_UNALIGNED_ACCESS instead of making a new
   flag.

3) Annotate in the output, when we have a test case that the verifier
   accepted but we did not try to execute because we are on an
   inefficient unaligned access platform.  Maybe with some arch
   machinery we can avoid this in the future.
Signed-off-by: David S. Miller <davem@davemloft.net>
====================
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

If a testcase has alignment problems but is expected to be ACCEPT,
verify it using F_NEEDS_EFFICIENT_UNALIGNED_ACCESS too.

Maybe in the future if we add some architecture specific code to elide
the unaligned memory access warnings during the test, we can execute
these as well.
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Use F_NEEDS_EFFICIENT_UNALIGNED_ACCESS in more tests where the
expected result is REJECT.
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Make it set the flag argument to bpf_verify_program() which will relax
the alignment restrictions.

Now all such test cases will go properly through the verifier even on
inefficient unaligned access architectures.

On inefficient unaligned access architectures do not try to run such
programs, instead mark the test case as passing but annotate the
result similarly to how it is done now in the presence of this flag.

So, we get complete full coverage for all REJECT test cases, and at
least verifier level coverage for ACCEPT test cases.
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Often we want to write tests cases that check things like bad context
offset accesses.  And one way to do this is to use an odd offset on,
for example, a 32-bit load.

This unfortunately triggers the alignment checks first on platforms
that do not set CONFIG_EFFICIENT_UNALIGNED_ACCESS.  So the test
case see the alignment failure rather than what it was testing for.

It is often not completely possible to respect the original intention
of the test, or even test the same exact thing, while solving the
alignment issue.

Another option could have been to check the alignment after the
context and other validations are performed by the verifier, but
that is a non-trivial change to the verifier.
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

The message got changed a lot time ago.

This was responsible for 36 test case failures on sparc64.

Fixes: f1174f77 ("bpf/verifier: rework value tracking")
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Quentin Monnet says:

====================
Hi,
Several items for bpftool are included in this set: the first three patches
are fixes for bpftool itself and bash completion, while the last two
slightly improve the information obtained when dumping programs or maps, on
Daniel's suggestion. Please refer to individual commit logs for more
details.
====================
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

For prog array maps, the type of the owner program, and the JIT-ed state
of that program, are available from the file descriptor information
under /proc. Add them to "bpftool map show" output. Example output:

    # bpftool map show
    158225: prog_array  name jmp_table  flags 0x0
        key 4B  value 4B  max_entries 8  memlock 4096B
        owner_prog_type flow_dissector  owner jited
    # bpftool --json --pretty map show
    [{
            "id": 1337,
            "type": "prog_array",
            "name": "jmp_table",
            "flags": 0,
            "bytes_key": 4,
            "bytes_value": 4,
            "max_entries": 8,
            "bytes_memlock": 4096,
            "owner_prog_type": "flow_dissector",
            "owner_jited": true
        }
    ]

As we move the table used for associating names to program types,
complete it with the missing types (lwt_seg6local and sk_reuseport).
Also add missing types to the help message for "bpftool prog"
(sk_reuseport and flow_dissector).
Suggested-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

In bpftool (plain) output for "bpftool prog show" or "bpftool map show",
an offloaded BPF object is simply denoted with "dev ifname", which is
not really explicit. Change it with something that clearly shows the
program is offloaded.

While at it also add an additional space, as done between other
information fields.

Example output, before:

    # bpftool prog show
    1337: xdp  tag a04f5eef06a7f555 dev foo
            loaded_at 2018-10-19T16:40:36+0100  uid 0
            xlated 16B  not jited  memlock 4096B

After:

    # bpftool prog show
    1337: xdp  tag a04f5eef06a7f555  offloaded_to foo
            loaded_at 2018-10-19T16:40:36+0100  uid 0
            xlated 16B  not jited  memlock 4096B
Suggested-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Commit 197c2dac ("bpf: Add BPF_MAP_TYPE_QUEUE and BPF_MAP_TYPE_STACK
to bpftool-map") added support for queue and stack eBPF map types in
bpftool map handling. Let's update the bash completion accordingly.

Fixes: 197c2dac ("bpf: Add BPF_MAP_TYPE_QUEUE and BPF_MAP_TYPE_STACK to bpftool-map")
Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Fix bash completion for "bpftool prog (attach|detach) PROG TYPE MAP" so
that the list of indices proposed for MAP are map indices, and not PROG
indices. Also use variables for map and prog reference types ("id",
"pinned", and "tag" for programs).

Fixes: b7d3826c ("bpf: bpftool, add support for attaching programs to maps")
Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

The getpid() function is called in a couple of places in bpftool to
craft links of the shape "/proc/<pid>/...". Instead, it is possible to
use the "/proc/self/" shortcut, which makes things a bit easier, in
particular in jit_disasm.c.

Do the replacement, and remove the includes of <sys/types.h> from the
relevant files, now we do not use getpid() anymore.
Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

On arm64, all executable code is guaranteed to reside in the vmalloc
space (or the module space), and so jump targets will only use 48
bits at most, and the remaining bits are guaranteed to be 0x1.

This means we can generate an immediate jump address using a sequence
of one MOVN (move wide negated) and two MOVK instructions, where the
first one sets the lower 16 bits but also sets all top bits to 0x1.
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: Will Deacon <will.deacon@arm.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

Daniel Borkmann says:

====================
bpf-next 2018-11-30

The following pull-request contains BPF updates for your *net-next* tree.

(Getting out bit earlier this time to pull in a dependency from bpf.)

The main changes are:

1) Add libbpf ABI versioning and document API naming conventions
   as well as ABI versioning process, from Andrey.

2) Add a new sk_msg_pop_data() helper for sk_msg based BPF
   programs that is used in conjunction with sk_msg_push_data()
   for adding / removing meta data to the msg data, from John.

3) Optimize convert_bpf_ld_abs() for 0 offset and fix various
   lib and testsuite build failures on 32 bit, from David.

4) Make BPF prog dump for !JIT identical to how we dump subprogs
   when JIT is in use, from Yonghong.

5) Rename btf_get_from_id() to make it more conform with libbpf
   API naming conventions, from Martin.

6) Add a missing BPF kselftest config item, from Naresh.
====================
Signed-off-by: David S. Miller <davem@davemloft.net>

During porting libbpf to bcc, I got some warnings like below:
  ...
  [  2%] Building C object src/cc/CMakeFiles/bpf-shared.dir/libbpf/src/libbpf.c.o
  /home/yhs/work/bcc2/src/cc/libbpf/src/libbpf.c:12:0:
  warning: "_GNU_SOURCE" redefined [enabled by default]
   #define _GNU_SOURCE
  ...
  [  3%] Building C object src/cc/CMakeFiles/bpf-shared.dir/libbpf/src/libbpf_errno.c.o
  /home/yhs/work/bcc2/src/cc/libbpf/src/libbpf_errno.c: In function ‘libbpf_strerror’:
  /home/yhs/work/bcc2/src/cc/libbpf/src/libbpf_errno.c:45:7:
  warning: assignment makes integer from pointer without a cast [enabled by default]
     ret = strerror_r(err, buf, size);
  ...

bcc is built with _GNU_SOURCE defined and this caused the above warning.
This patch intends to make libpf _GNU_SOURCE friendly by
  . define _GNU_SOURCE in libbpf.c unless it is not defined
  . undefine _GNU_SOURCE as non-gnu version of strerror_r is expected.
Signed-off-by: Yonghong Song <yhs@fb.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>