linux-toradex.git/include/linux/btf_ids.h, branch v5.19-rc8 Linux kernel for Apalis and Colibri modules bpf: Add bpf_skc_to_mptcp_sock_proto 2022-05-20T22:29:00+00:00 Geliang Tang geliang.tang@suse.com 2022-05-19T23:30:10+00:00 3bc253c2e652cf5f12cd8c00d80d8ec55d67d1a7 This patch implements a new struct bpf_func_proto, named bpf_skc_to_mptcp_sock_proto. Define a new bpf_id BTF_SOCK_TYPE_MPTCP, and a new helper bpf_skc_to_mptcp_sock(), which invokes another new helper bpf_mptcp_sock_from_subflow() in net/mptcp/bpf.c to get struct mptcp_sock from a given subflow socket. v2: Emit BTF type, add func_id checks in verifier.c and bpf_trace.c, remove build check for CONFIG_BPF_JIT v5: Drop EXPORT_SYMBOL (Martin) Co-developed-by: Nicolas Rybowski <nicolas.rybowski@tessares.net> Co-developed-by: Matthieu Baerts <matthieu.baerts@tessares.net> Signed-off-by: Nicolas Rybowski <nicolas.rybowski@tessares.net> Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net> Signed-off-by: Geliang Tang <geliang.tang@suse.com> Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com> Signed-off-by: Andrii Nakryiko <andrii@kernel.org> Link: https://lore.kernel.org/bpf/20220519233016.105670-2-mathew.j.martineau@linux.intel.com 
This patch implements a new struct bpf_func_proto, named
bpf_skc_to_mptcp_sock_proto. Define a new bpf_id BTF_SOCK_TYPE_MPTCP,
and a new helper bpf_skc_to_mptcp_sock(), which invokes another new
helper bpf_mptcp_sock_from_subflow() in net/mptcp/bpf.c to get struct
mptcp_sock from a given subflow socket.

v2: Emit BTF type, add func_id checks in verifier.c and bpf_trace.c,
remove build check for CONFIG_BPF_JIT
v5: Drop EXPORT_SYMBOL (Martin)

Co-developed-by: Nicolas Rybowski <nicolas.rybowski@tessares.net>
Co-developed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Nicolas Rybowski <nicolas.rybowski@tessares.net>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220519233016.105670-2-mathew.j.martineau@linux.intel.com
bpf: Populate kfunc BTF ID sets in struct btf 2022-01-18T22:26:41+00:00 Kumar Kartikeya Dwivedi memxor@gmail.com 2022-01-14T16:39:45+00:00 dee872e124e8d5de22b68c58f6f6c3f5e8889160 This patch prepares the kernel to support putting all kinds of kfunc BTF ID sets in the struct btf itself. The various kernel subsystems will make register_btf_kfunc_id_set call in the initcalls (for built-in code and modules). The 'hook' is one of the many program types, e.g. XDP and TC/SCHED_CLS, STRUCT_OPS, and 'types' are check (allowed or not), acquire, release, and ret_null (with PTR_TO_BTF_ID_OR_NULL return type). A maximum of BTF_KFUNC_SET_MAX_CNT (32) kfunc BTF IDs are permitted in a set of certain hook and type for vmlinux sets, since they are allocated on demand, and otherwise set as NULL. Module sets can only be registered once per hook and type, hence they are directly assigned. A new btf_kfunc_id_set_contains function is exposed for use in verifier, this new method is faster than the existing list searching method, and is also automatic. It also lets other code not care whether the set is unallocated or not. Note that module code can only do single register_btf_kfunc_id_set call per hook. This is why sorting is only done for in-kernel vmlinux sets, because there might be multiple sets for the same hook and type that must be concatenated, hence sorting them is required to ensure bsearch in btf_id_set_contains continues to work correctly. Next commit will update the kernel users to make use of this infrastructure. Finally, add __maybe_unused annotation for BTF ID macros for the !CONFIG_DEBUG_INFO_BTF case, so that they don't produce warnings during build time. The previous patch is also needed to provide synchronization against initialization for module BTF's kfunc_set_tab introduced here, as described below: The kfunc_set_tab pointer in struct btf is write-once (if we consider the registration phase (comprised of multiple register_btf_kfunc_id_set calls) as a single operation). In this sense, once it has been fully prepared, it isn't modified, only used for lookup (from the verifier context). For btf_vmlinux, it is initialized fully during the do_initcalls phase, which happens fairly early in the boot process, before any processes are present. This also eliminates the possibility of bpf_check being called at that point, thus relieving us of ensuring any synchronization between the registration and lookup function (btf_kfunc_id_set_contains). However, the case for module BTF is a bit tricky. The BTF is parsed, prepared, and published from the MODULE_STATE_COMING notifier callback. After this, the module initcalls are invoked, where our registration function will be called to populate the kfunc_set_tab for module BTF. At this point, BTF may be available to userspace while its corresponding module is still intializing. A BTF fd can then be passed to verifier using bpf syscall (e.g. for kfunc call insn). Hence, there is a race window where verifier may concurrently try to lookup the kfunc_set_tab. To prevent this race, we must ensure the operations are serialized, or waiting for the __init functions to complete. In the earlier registration API, this race was alleviated as verifier bpf_check_mod_kfunc_call didn't find the kfunc BTF ID until it was added by the registration function (called usually at the end of module __init function after all module resources have been initialized). If the verifier made the check_kfunc_call before kfunc BTF ID was added to the list, it would fail verification (saying call isn't allowed). The access to list was protected using a mutex. Now, it would still fail verification, but for a different reason (returning ENXIO due to the failed btf_try_get_module call in add_kfunc_call), because if the __init call is in progress the module will be in the middle of MODULE_STATE_COMING -> MODULE_STATE_LIVE transition, and the BTF_MODULE_LIVE flag for btf_module instance will not be set, so the btf_try_get_module call will fail. Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com> Link: https://lore.kernel.org/r/20220114163953.1455836-3-memxor@gmail.com Signed-off-by: Alexei Starovoitov <ast@kernel.org> 
This patch prepares the kernel to support putting all kinds of kfunc BTF
ID sets in the struct btf itself. The various kernel subsystems will
make register_btf_kfunc_id_set call in the initcalls (for built-in code
and modules).

The 'hook' is one of the many program types, e.g. XDP and TC/SCHED_CLS,
STRUCT_OPS, and 'types' are check (allowed or not), acquire, release,
and ret_null (with PTR_TO_BTF_ID_OR_NULL return type).

A maximum of BTF_KFUNC_SET_MAX_CNT (32) kfunc BTF IDs are permitted in a
set of certain hook and type for vmlinux sets, since they are allocated
on demand, and otherwise set as NULL. Module sets can only be registered
once per hook and type, hence they are directly assigned.

A new btf_kfunc_id_set_contains function is exposed for use in verifier,
this new method is faster than the existing list searching method, and
is also automatic. It also lets other code not care whether the set is
unallocated or not.

Note that module code can only do single register_btf_kfunc_id_set call
per hook. This is why sorting is only done for in-kernel vmlinux sets,
because there might be multiple sets for the same hook and type that
must be concatenated, hence sorting them is required to ensure bsearch
in btf_id_set_contains continues to work correctly.

Next commit will update the kernel users to make use of this
infrastructure.

Finally, add __maybe_unused annotation for BTF ID macros for the
!CONFIG_DEBUG_INFO_BTF case, so that they don't produce warnings during
build time.

The previous patch is also needed to provide synchronization against
initialization for module BTF's kfunc_set_tab introduced here, as
described below:

  The kfunc_set_tab pointer in struct btf is write-once (if we consider
  the registration phase (comprised of multiple register_btf_kfunc_id_set
  calls) as a single operation). In this sense, once it has been fully
  prepared, it isn't modified, only used for lookup (from the verifier
  context).

  For btf_vmlinux, it is initialized fully during the do_initcalls phase,
  which happens fairly early in the boot process, before any processes are
  present. This also eliminates the possibility of bpf_check being called
  at that point, thus relieving us of ensuring any synchronization between
  the registration and lookup function (btf_kfunc_id_set_contains).

  However, the case for module BTF is a bit tricky. The BTF is parsed,
  prepared, and published from the MODULE_STATE_COMING notifier callback.
  After this, the module initcalls are invoked, where our registration
  function will be called to populate the kfunc_set_tab for module BTF.

  At this point, BTF may be available to userspace while its corresponding
  module is still intializing. A BTF fd can then be passed to verifier
  using bpf syscall (e.g. for kfunc call insn).

  Hence, there is a race window where verifier may concurrently try to
  lookup the kfunc_set_tab. To prevent this race, we must ensure the
  operations are serialized, or waiting for the __init functions to
  complete.

  In the earlier registration API, this race was alleviated as verifier
  bpf_check_mod_kfunc_call didn't find the kfunc BTF ID until it was added
  by the registration function (called usually at the end of module __init
  function after all module resources have been initialized). If the
  verifier made the check_kfunc_call before kfunc BTF ID was added to the
  list, it would fail verification (saying call isn't allowed). The
  access to list was protected using a mutex.

  Now, it would still fail verification, but for a different reason
  (returning ENXIO due to the failed btf_try_get_module call in
  add_kfunc_call), because if the __init call is in progress the module
  will be in the middle of MODULE_STATE_COMING -> MODULE_STATE_LIVE
  transition, and the BTF_MODULE_LIVE flag for btf_module instance will
  not be set, so the btf_try_get_module call will fail.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20220114163953.1455836-3-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
bpf: Introduce btf_tracing_ids 2021-11-12T18:19:09+00:00 Song Liu songliubraving@fb.com 2021-11-12T15:02:43+00:00 d19ddb476a539fd78ad1028ae13bb38506286931 Similar to btf_sock_ids, btf_tracing_ids provides btf ID for task_struct, file, and vm_area_struct via easy to understand format like btf_tracing_ids[BTF_TRACING_TYPE_[TASK|file|VMA]]. Suggested-by: Alexei Starovoitov <ast@kernel.org> Signed-off-by: Song Liu <songliubraving@fb.com> Signed-off-by: Alexei Starovoitov <ast@kernel.org> Acked-by: Yonghong Song <yhs@fb.com> Link: https://lore.kernel.org/bpf/20211112150243.1270987-3-songliubraving@fb.com 
Similar to btf_sock_ids, btf_tracing_ids provides btf ID for task_struct,
file, and vm_area_struct via easy to understand format like
btf_tracing_ids[BTF_TRACING_TYPE_[TASK|file|VMA]].

Suggested-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20211112150243.1270987-3-songliubraving@fb.com
bpf: Extend BTF_ID_LIST_GLOBAL with parameter for number of IDs 2021-11-12T18:19:09+00:00 Song Liu songliubraving@fb.com 2021-11-12T15:02:42+00:00 9e2ad638ae3632ef916ceb39f70e3104bf8fdc97 syzbot reported the following BUG w/o CONFIG_DEBUG_INFO_BTF BUG: KASAN: global-out-of-bounds in task_iter_init+0x212/0x2e7 kernel/bpf/task_iter.c:661 Read of size 4 at addr ffffffff90297404 by task swapper/0/1 CPU: 1 PID: 1 Comm: swapper/0 Not tainted 5.15.0-syzkaller #0 Hardware name: ... Google Compute Engine, BIOS Google 01/01/2011 Call Trace: <TASK> __dump_stack lib/dump_stack.c:88 [inline] dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:106 print_address_description.constprop.0.cold+0xf/0x309 mm/kasan/report.c:256 __kasan_report mm/kasan/report.c:442 [inline] kasan_report.cold+0x83/0xdf mm/kasan/report.c:459 task_iter_init+0x212/0x2e7 kernel/bpf/task_iter.c:661 do_one_initcall+0x103/0x650 init/main.c:1295 do_initcall_level init/main.c:1368 [inline] do_initcalls init/main.c:1384 [inline] do_basic_setup init/main.c:1403 [inline] kernel_init_freeable+0x6b1/0x73a init/main.c:1606 kernel_init+0x1a/0x1d0 init/main.c:1497 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295 </TASK> This is caused by hard-coded name[1] in BTF_ID_LIST_GLOBAL (w/o CONFIG_DEBUG_INFO_BTF). Fix this by adding a parameter n to BTF_ID_LIST_GLOBAL. This avoids ifdef CONFIG_DEBUG_INFO_BTF in btf.c and filter.c. Fixes: 7c7e3d31e785 ("bpf: Introduce helper bpf_find_vma") Reported-by: syzbot+e0d81ec552a21d9071aa@syzkaller.appspotmail.com Reported-by: Eric Dumazet <edumazet@google.com> Suggested-by: Eric Dumazet <edumazet@google.com> Signed-off-by: Song Liu <songliubraving@fb.com> Signed-off-by: Alexei Starovoitov <ast@kernel.org> Acked-by: Yonghong Song <yhs@fb.com> Link: https://lore.kernel.org/bpf/20211112150243.1270987-2-songliubraving@fb.com 
syzbot reported the following BUG w/o CONFIG_DEBUG_INFO_BTF

BUG: KASAN: global-out-of-bounds in task_iter_init+0x212/0x2e7 kernel/bpf/task_iter.c:661
Read of size 4 at addr ffffffff90297404 by task swapper/0/1

CPU: 1 PID: 1 Comm: swapper/0 Not tainted 5.15.0-syzkaller #0
Hardware name: ... Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
<TASK>
__dump_stack lib/dump_stack.c:88 [inline]
dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:106
print_address_description.constprop.0.cold+0xf/0x309 mm/kasan/report.c:256
__kasan_report mm/kasan/report.c:442 [inline]
kasan_report.cold+0x83/0xdf mm/kasan/report.c:459
task_iter_init+0x212/0x2e7 kernel/bpf/task_iter.c:661
do_one_initcall+0x103/0x650 init/main.c:1295
do_initcall_level init/main.c:1368 [inline]
do_initcalls init/main.c:1384 [inline]
do_basic_setup init/main.c:1403 [inline]
kernel_init_freeable+0x6b1/0x73a init/main.c:1606
kernel_init+0x1a/0x1d0 init/main.c:1497
ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295
</TASK>

This is caused by hard-coded name[1] in BTF_ID_LIST_GLOBAL (w/o
CONFIG_DEBUG_INFO_BTF). Fix this by adding a parameter n to
BTF_ID_LIST_GLOBAL. This avoids ifdef CONFIG_DEBUG_INFO_BTF in btf.c and
filter.c.

Fixes: 7c7e3d31e785 ("bpf: Introduce helper bpf_find_vma")
Reported-by: syzbot+e0d81ec552a21d9071aa@syzkaller.appspotmail.com
Reported-by: Eric Dumazet <edumazet@google.com>
Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20211112150243.1270987-2-songliubraving@fb.com
bpf: Fix bpf-next builds without CONFIG_BPF_EVENTS 2021-08-26T02:41:39+00:00 Daniel Xu dxu@dxuuu.xyz 2021-08-26T01:48:31+00:00 eb529c5b10b9401a0f2d1f469e82c6a0ba98082c This commit fixes linker errors along the lines of: s390-linux-ld: task_iter.c:(.init.text+0xa4): undefined reference to `btf_task_struct_ids'` Fix by defining btf_task_struct_ids unconditionally in kernel/bpf/btf.c since there exists code that unconditionally uses btf_task_struct_ids. Reported-by: kernel test robot <lkp@intel.com> Signed-off-by: Daniel Xu <dxu@dxuuu.xyz> Signed-off-by: Alexei Starovoitov <ast@kernel.org> Link: https://lore.kernel.org/bpf/05d94748d9f4b3eecedc4fddd6875418a396e23c.1629942444.git.dxu@dxuuu.xyz 
This commit fixes linker errors along the lines of:

    s390-linux-ld: task_iter.c:(.init.text+0xa4): undefined reference to `btf_task_struct_ids'`

Fix by defining btf_task_struct_ids unconditionally in kernel/bpf/btf.c
since there exists code that unconditionally uses btf_task_struct_ids.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/05d94748d9f4b3eecedc4fddd6875418a396e23c.1629942444.git.dxu@dxuuu.xyz
bpf: Consolidate task_struct BTF_ID declarations 2021-08-25T17:37:05+00:00 Daniel Xu dxu@dxuuu.xyz 2021-08-24T02:43:47+00:00 33c5cb36015ac1034b50b823fae367e908d05147 No need to have it defined 5 times. Once is enough. Signed-off-by: Daniel Xu <dxu@dxuuu.xyz> Signed-off-by: Alexei Starovoitov <ast@kernel.org> Link: https://lore.kernel.org/bpf/6dcefa5bed26fe1226f26683f36819bb53ec19a2.1629772842.git.dxu@dxuuu.xyz 
No need to have it defined 5 times. Once is enough.

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/6dcefa5bed26fe1226f26683f36819bb53ec19a2.1629772842.git.dxu@dxuuu.xyz
bpf: Add BTF_ID_LIST_GLOBAL_SINGLE macro 2021-08-25T17:37:05+00:00 Daniel Xu dxu@dxuuu.xyz 2021-08-24T02:43:46+00:00 1b07d00a15d6a96d1a36b6a284c4fd5f2e2fa383 Same as BTF_ID_LIST_SINGLE macro except defines a global ID. Signed-off-by: Daniel Xu <dxu@dxuuu.xyz> Signed-off-by: Alexei Starovoitov <ast@kernel.org> Link: https://lore.kernel.org/bpf/a867a97517df42fd3953eeb5454402b57e74538f.1629772842.git.dxu@dxuuu.xyz 
Same as BTF_ID_LIST_SINGLE macro except defines a global ID.

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/a867a97517df42fd3953eeb5454402b57e74538f.1629772842.git.dxu@dxuuu.xyz
bpf: af_unix: Implement BPF iterator for UNIX domain socket. 2021-08-15T07:13:32+00:00 Kuniyuki Iwashima kuniyu@amazon.co.jp 2021-08-14T01:57:15+00:00 2c860a43dd77f969bb959336a2f743d7103a8f63 This patch implements the BPF iterator for the UNIX domain socket. Currently, the batch optimisation introduced for the TCP iterator in the commit 04c7820b776f ("bpf: tcp: Bpf iter batching and lock_sock") is not used for the UNIX domain socket. It will require replacing the big lock for the hash table with small locks for each hash list not to block other processes. Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.co.jp> Signed-off-by: Andrii Nakryiko <andrii@kernel.org> Acked-by: Yonghong Song <yhs@fb.com> Link: https://lore.kernel.org/bpf/20210814015718.42704-2-kuniyu@amazon.co.jp 
This patch implements the BPF iterator for the UNIX domain socket.

Currently, the batch optimisation introduced for the TCP iterator in the
commit 04c7820b776f ("bpf: tcp: Bpf iter batching and lock_sock") is not
used for the UNIX domain socket.  It will require replacing the big lock
for the hash table with small locks for each hash list not to block other
processes.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.co.jp>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210814015718.42704-2-kuniyu@amazon.co.jp
btf: Add BTF_ID_LIST_SINGLE macro 2020-09-21T22:00:40+00:00 Lorenz Bauer lmb@cloudflare.com 2020-09-21T12:12:19+00:00 27774b7073b5d520c80f1fcb8e9993fc139f21bd Add a convenience macro that allows defining a BTF ID list with a single item. This lets us cut down on repetitive macros. Suggested-by: Andrii Nakryiko <andriin@fb.com> Signed-off-by: Lorenz Bauer <lmb@cloudflare.com> Signed-off-by: Alexei Starovoitov <ast@kernel.org> Acked-by: Martin KaFai Lau <kafai@fb.com> Link: https://lore.kernel.org/bpf/20200921121227.255763-4-lmb@cloudflare.com 
Add a convenience macro that allows defining a BTF ID list with
a single item. This lets us cut down on repetitive macros.

Suggested-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Lorenz Bauer <lmb@cloudflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20200921121227.255763-4-lmb@cloudflare.com
bpf: Add BTF_SET_START/END macros 2020-08-25T22:37:41+00:00 Jiri Olsa jolsa@kernel.org 2020-08-25T19:21:19+00:00 eae2e83e62633a2659e3bc690facba1c2fc9c45b Adding support to define sorted set of BTF ID values. Following defines sorted set of BTF ID values: BTF_SET_START(btf_allowlist_d_path) BTF_ID(func, vfs_truncate) BTF_ID(func, vfs_fallocate) BTF_ID(func, dentry_open) BTF_ID(func, vfs_getattr) BTF_ID(func, filp_close) BTF_SET_END(btf_allowlist_d_path) It defines following 'struct btf_id_set' variable to access values and count: struct btf_id_set btf_allowlist_d_path; Adding 'allowed' callback to struct bpf_func_proto, to allow verifier the check on allowed callers. Adding btf_id_set_contains function, which will be used by allowed callbacks to verify the caller's BTF ID value is within allowed set. Also removing extra '\' in __BTF_ID_LIST macro. Added BTF_SET_START_GLOBAL macro for global sets. Signed-off-by: Jiri Olsa <jolsa@kernel.org> Signed-off-by: Alexei Starovoitov <ast@kernel.org> Acked-by: Andrii Nakryiko <andriin@fb.com> Link: https://lore.kernel.org/bpf/20200825192124.710397-10-jolsa@kernel.org 
Adding support to define sorted set of BTF ID values.

Following defines sorted set of BTF ID values:

  BTF_SET_START(btf_allowlist_d_path)
  BTF_ID(func, vfs_truncate)
  BTF_ID(func, vfs_fallocate)
  BTF_ID(func, dentry_open)
  BTF_ID(func, vfs_getattr)
  BTF_ID(func, filp_close)
  BTF_SET_END(btf_allowlist_d_path)

It defines following 'struct btf_id_set' variable to access
values and count:

  struct btf_id_set btf_allowlist_d_path;

Adding 'allowed' callback to struct bpf_func_proto, to allow
verifier the check on allowed callers.

Adding btf_id_set_contains function, which will be used by
allowed callbacks to verify the caller's BTF ID value is
within allowed set.

Also removing extra '\' in __BTF_ID_LIST macro.

Added BTF_SET_START_GLOBAL macro for global sets.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200825192124.710397-10-jolsa@kernel.org