bpf: Permit size-0 datasec

llvm patch https://reviews.llvm.org/D84002 permitted
to emit empty rodata datasec if the elf .rodata section
contains read-only data from local variables. These
local variables will be not emitted as BTF_KIND_VARs
since llvm converted these local variables as
static variables with private linkage without debuginfo
types. Such an empty rodata datasec will make
skeleton code generation easy since for skeleton
a rodata struct will be generated if there is a
.rodata elf section. The existence of a rodata
btf datasec is also consistent with the existence
of a rodata map created by libbpf.

The btf with such an empty rodata datasec will fail
in the kernel though as kernel will reject a datasec
with zero vlen and zero size. For example, for the below code,
    int sys_enter(void *ctx)
    {
       int fmt[6] = {1, 2, 3, 4, 5, 6};
       int dst[6];

       bpf_probe_read(dst, sizeof(dst), fmt);
       return 0;
    }
We got the below btf (bpftool btf dump ./test.o):
    [1] PTR '(anon)' type_id=0
    [2] FUNC_PROTO '(anon)' ret_type_id=3 vlen=1
            'ctx' type_id=1
    [3] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
    [4] FUNC 'sys_enter' type_id=2 linkage=global
    [5] INT 'char' size=1 bits_offset=0 nr_bits=8 encoding=SIGNED
    [6] ARRAY '(anon)' type_id=5 index_type_id=7 nr_elems=4
    [7] INT '__ARRAY_SIZE_TYPE__' size=4 bits_offset=0 nr_bits=32 encoding=(none)
    [8] VAR '_license' type_id=6, linkage=global-alloc
    [9] DATASEC '.rodata' size=0 vlen=0
    [10] DATASEC 'license' size=0 vlen=1
            type_id=8 offset=0 size=4
When loading the ./test.o to the kernel with bpftool,
we see the following error:
    libbpf: Error loading BTF: Invalid argument(22)
    libbpf: magic: 0xeb9f
    ...
    [6] ARRAY (anon) type_id=5 index_type_id=7 nr_elems=4
    [7] INT __ARRAY_SIZE_TYPE__ size=4 bits_offset=0 nr_bits=32 encoding=(none)
    [8] VAR _license type_id=6 linkage=1
    [9] DATASEC .rodata size=24 vlen=0 vlen == 0
    libbpf: Error loading .BTF into kernel: -22. BTF is optional, ignoring.

Basically, libbpf changed .rodata datasec size to 24 since elf .rodata
section size is 24. The kernel then rejected the BTF since vlen = 0.
Note that the above kernel verifier failure can be worked around with
changing local variable "fmt" to a static or global, optionally const, variable.

This patch permits a datasec with vlen = 0 in kernel.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210119153519.3901963-1-yhs@fb.com

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