Re: [PATCH bpf-next 1/6] libbpf: Sanitize BTF_KIND_FUNC linkage

Yonghong Song
2020-01-08T18:57:34Z

On 1/7/20 11:25 PM, Alexei Starovoitov wrote:
> In case kernel doesn't support static/global/extern liknage of BTF_KIND_FUNC
> sanitize BTF produced by llvm.
>
> Signed-off-by: Alexei Starovoitov <ast@kernel.org>
> ---
> tools/include/uapi/linux/btf.h | 6 ++++++
> tools/lib/bpf/libbpf.c | 35 +++++++++++++++++++++++++++++++++-
> 2 files changed, 40 insertions(+), 1 deletion(-)
>
> diff --git a/tools/include/uapi/linux/btf.h b/tools/include/uapi/linux/btf.h
> index 1a2898c482ee..5a667107ad2c 100644
> --- a/tools/include/uapi/linux/btf.h
> +++ b/tools/include/uapi/linux/btf.h
> @@ -146,6 +146,12 @@ enum {
> BTF_VAR_GLOBAL_EXTERN = 2,
> };
>
> +enum btf_func_linkage {
> + BTF_FUNC_STATIC = 0,
> + BTF_FUNC_GLOBAL = 1,
> + BTF_FUNC_EXTERN = 2,
> +};
> +
> /* BTF_KIND_VAR is followed by a single "struct btf_var" to describe
> * additional information related to the variable such as its linkage.
> */
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 7513165b104f..f72b3ed6c34b 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -166,6 +166,8 @@ struct bpf_capabilities {
> __u32 btf_datasec:1;
> /* BPF_F_MMAPABLE is supported for arrays */
> __u32 array_mmap:1;
> + /* static/global/extern is supported for BTF_KIND_FUNC */
> + __u32 btf_func_linkage:1;
> };
>
> enum reloc_type {
> @@ -1817,13 +1819,14 @@ static bool section_have_execinstr(struct bpf_object *obj, int idx)
>
> static void bpf_object__sanitize_btf(struct bpf_object *obj)
> {
> + bool has_func_linkage = obj->caps.btf_func_linkage;
> bool has_datasec = obj->caps.btf_datasec;
> bool has_func = obj->caps.btf_func;
> struct btf *btf = obj->btf;
> struct btf_type *t;
> int i, j, vlen;
>
> - if (!obj->btf || (has_func && has_datasec))
> + if (!obj->btf || (has_func && has_datasec && has_func_linkage))
> return;
>
> for (i = 1; i <= btf__get_nr_types(btf); i++) {
> @@ -1871,6 +1874,9 @@ static void bpf_object__sanitize_btf(struct bpf_object *obj)
> } else if (!has_func && btf_is_func(t)) {
> /* replace FUNC with TYPEDEF */
> t->info = BTF_INFO_ENC(BTF_KIND_TYPEDEF, 0, 0);
> + } else if (!has_func_linkage && btf_is_func(t)) {
> + /* replace BTF_FUNC_GLOBAL with BTF_FUNC_STATIC */
> + t->info = BTF_INFO_ENC(BTF_KIND_FUNC, 0, 0);

The comment says we only sanitize BTF_FUNC_GLOBAL here.
Actually, it also sanitize BTF_FUNC_EXTERN.

Currently, in kernel/bpf/btf.c, we have
static int btf_check_all_types(struct btf_verifier_env *env)
{
...
if (btf_type_is_func(t)) {
err = btf_func_check(env, t);
if (err)
return err;
}
...
}

btf_func_check() will ensure func btf_type->type is a func_proto
and all arguments of func_proto has a name except void which is
considered as varg.

For extern function, the argument name is lost in llvm/clang.

-bash-4.4$ cat test.c

extern int foo(int a);
int test() { return foo(5); }
-bash-4.4$
-bash-4.4$ clang -target bpf -O2 -g -S -emit-llvm test.c

!2 = !{}
!4 = !DISubprogram(name: "foo", scope: !1, file: !1, line: 1, type: !5,
flags: DIFlagPrototyped, spFlags: DISPFlagOptimized, retainedNodes: !2)
!5 = !DISubroutineType(types: !6)
!6 = !{!7, !7}
!7 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)

To avoid kernel complaints, we need to sanitize in a different way.
For example extern BTF_KIND_FUNC could be rewritten to a
BTF_KIND_PTR to void.

> }
> }
> }
> @@ -2804,6 +2810,32 @@ static int bpf_object__probe_btf_func(struct bpf_object *obj)
> return 0;
> }
>
> +static int bpf_object__probe_btf_func_linkage(struct bpf_object *obj)
> +{
> + static const char strs[] = "\0int\0x\0a";
> + /* static void x(int a) {} */
> + __u32 types[] = {
> + /* int */
> + BTF_TYPE_INT_ENC(1, BTF_INT_SIGNED, 0, 32, 4), /* [1] */
> + /* FUNC_PROTO */ /* [2] */
> + BTF_TYPE_ENC(0, BTF_INFO_ENC(BTF_KIND_FUNC_PROTO, 0, 1), 0),
> + BTF_PARAM_ENC(7, 1),
> + /* FUNC x BTF_FUNC_GLOBAL */ /* [3] */
> + BTF_TYPE_ENC(5, BTF_INFO_ENC(BTF_KIND_FUNC, 0, 1), 2),
> + };
> + int btf_fd;
> +
> + btf_fd = libbpf__load_raw_btf((char *)types, sizeof(types),
> + strs, sizeof(strs));
> + if (btf_fd >= 0) {
> + obj->caps.btf_func_linkage = 1;
> + close(btf_fd);
> + return 1;
> + }
> +
> + return 0;
> +}
> +
> static int bpf_object__probe_btf_datasec(struct bpf_object *obj)
> {
> static const char strs[] = "\0x\0.data";
> @@ -2859,6 +2891,7 @@ bpf_object__probe_caps(struct bpf_object *obj)
> bpf_object__probe_name,
> bpf_object__probe_global_data,
> bpf_object__probe_btf_func,
> + bpf_object__probe_btf_func_linkage,
> bpf_object__probe_btf_datasec,
> bpf_object__probe_array_mmap,
> };
>