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

kernel/bpf/btf.c

@@ -3540,11 +3540,6 @@ static s32 btf_datasec_check_meta(struct btf_verifier_env *env,
 		return -EINVAL;
 	}
 
-	if (!btf_type_vlen(t)) {
-		btf_verifier_log_type(env, t, "vlen == 0");
-		return -EINVAL;
-	}
-
 	if (!t->size) {
 		btf_verifier_log_type(env, t, "size == 0");
 		return -EINVAL;

tools/testing/selftests/bpf/prog_tests/btf.c

@@ -3509,6 +3509,27 @@ static struct btf_raw_test raw_tests[] = {
 	.value_type_id = 3 /* arr_t */,
 	.max_entries = 4,
 },
+/*
+ * elf .rodata section size 4 and btf .rodata section vlen 0.
+ */
+{
+	.descr = "datasec: vlen == 0",
+	.raw_types = {
+		/* int */
+		BTF_TYPE_INT_ENC(0, BTF_INT_SIGNED, 0, 32, 4),	/* [1] */
+		/* .rodata section */
+		BTF_TYPE_ENC(NAME_NTH(1), BTF_INFO_ENC(BTF_KIND_DATASEC, 0, 0), 4),
+								 /* [2] */
+		BTF_END_RAW,
+	},
+	BTF_STR_SEC("\0.rodata"),
+	.map_type = BPF_MAP_TYPE_ARRAY,
+	.key_size = sizeof(int),
+	.value_size = sizeof(int),
+	.key_type_id = 1,
+	.value_type_id = 1,
+	.max_entries = 1,
+},
 
 }; /* struct btf_raw_test raw_tests[] */