From 13ca51d5eb358edcb673afccb48c3440b9fda21b Mon Sep 17 00:00:00 2001 From: Yonghong Song Date: Tue, 19 Jan 2021 07:35:18 -0800 Subject: 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 Signed-off-by: Alexei Starovoitov Link: https://lore.kernel.org/bpf/20210119153519.3901963-1-yhs@fb.com --- kernel/bpf/btf.c | 5 ----- tools/testing/selftests/bpf/prog_tests/btf.c | 21 +++++++++++++++++++++ 2 files changed, 21 insertions(+), 5 deletions(-) diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c index 8962f988514f..756a93f534b6 100644 --- a/kernel/bpf/btf.c +++ b/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; diff --git a/tools/testing/selftests/bpf/prog_tests/btf.c b/tools/testing/selftests/bpf/prog_tests/btf.c index 8ae97e2a4b9d..055d2c0486ed 100644 --- a/tools/testing/selftests/bpf/prog_tests/btf.c +++ b/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[] */ -- cgit v1.2.3