bpf: Silence a warning in btf_type_id_size()

In the Linux kernel, the following vulnerability has been resolved:

bpf: Silence a warning in btf_type_id_size()

syzbot reported a warning in [1] with the following stacktrace:

WARNING: CPU: 0 PID: 5005 at kernel/bpf/btf.c:1988 btf_type_id_size+0x2d9/0x9d0 kernel/bpf/btf.c:1988

...

RIP: 0010:btf_type_id_size+0x2d9/0x9d0 kernel/bpf/btf.c:1988

...

Call Trace:

<TASK>

map_check_btf kernel/bpf/syscall.c:1024 [inline]

map_create+0x1157/0x1860 kernel/bpf/syscall.c:1198

__sys_bpf+0x127f/0x5420 kernel/bpf/syscall.c:5040

__do_sys_bpf kernel/bpf/syscall.c:5162 [inline]

__se_sys_bpf kernel/bpf/syscall.c:5160 [inline]

__x64_sys_bpf+0x79/0xc0 kernel/bpf/syscall.c:5160

do_syscall_x64 arch/x86/entry/common.c:50 [inline]

do_syscall_64+0x39/0xb0 arch/x86/entry/common.c:80

entry_SYSCALL_64_after_hwframe+0x63/0xcd

With the following btf

[1] DECL_TAG 'a' type_id=4 component_idx=-1

[2] PTR '(anon)' type_id=0

[3] TYPE_TAG 'a' type_id=2

[4] VAR 'a' type_id=3, linkage=static

and when the bpf_attr.btf_key_type_id = 1 (DECL_TAG),

the following WARN_ON_ONCE in btf_type_id_size() is triggered:

if (WARN_ON_ONCE(!btf_type_is_modifier(size_type) &&

!btf_type_is_var(size_type)))

return NULL;

Note that 'return NULL' is the correct behavior as we don't want

a DECL_TAG type to be used as a btf_{key,value}_type_id even

for the case like 'DECL_TAG -> STRUCT'. So there

is no correctness issue here, we just want to silence warning.

To silence the warning, I added DECL_TAG as one of kinds in

btf_type_nosize() which will cause btf_type_id_size() returning

NULL earlier without the warning.

[1] https://lore.kernel.org/bpf/[email protected]/