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Date: Thu, 25 Jul 2019 00:00:06 +0000
From: Song Liu <songliubraving@...com>
To: Andrii Nakryiko <andriin@...com>
CC: bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
"Alexei Starovoitov" <ast@...com>,
Daniel Borkmann <daniel@...earbox.net>,
"Yonghong Song" <yhs@...com>,
"andrii.nakryiko@...il.com" <andrii.nakryiko@...il.com>,
Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH bpf-next 01/10] libbpf: add .BTF.ext offset relocation
section loading
> On Jul 24, 2019, at 12:27 PM, Andrii Nakryiko <andriin@...com> wrote:
>
> Add support for BPF CO-RE offset relocations. Add section/record
> iteration macros for .BTF.ext. These macro are useful for iterating over
> each .BTF.ext record, either for dumping out contents or later for BPF
> CO-RE relocation handling.
>
> To enable other parts of libbpf to work with .BTF.ext contents, moved
> a bunch of type definitions into libbpf_internal.h.
>
> Signed-off-by: Andrii Nakryiko <andriin@...com>
> ---
> tools/lib/bpf/btf.c | 64 +++++++++--------------
> tools/lib/bpf/btf.h | 4 ++
> tools/lib/bpf/libbpf_internal.h | 91 +++++++++++++++++++++++++++++++++
> 3 files changed, 118 insertions(+), 41 deletions(-)
>
> diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
> index 467224feb43b..4a36bc783848 100644
> --- a/tools/lib/bpf/btf.c
> +++ b/tools/lib/bpf/btf.c
> @@ -42,47 +42,6 @@ struct btf {
> int fd;
> };
>
> -struct btf_ext_info {
> - /*
> - * info points to the individual info section (e.g. func_info and
> - * line_info) from the .BTF.ext. It does not include the __u32 rec_size.
> - */
> - void *info;
> - __u32 rec_size;
> - __u32 len;
> -};
> -
> -struct btf_ext {
> - union {
> - struct btf_ext_header *hdr;
> - void *data;
> - };
> - struct btf_ext_info func_info;
> - struct btf_ext_info line_info;
> - __u32 data_size;
> -};
> -
> -struct btf_ext_info_sec {
> - __u32 sec_name_off;
> - __u32 num_info;
> - /* Followed by num_info * record_size number of bytes */
> - __u8 data[0];
> -};
> -
> -/* The minimum bpf_func_info checked by the loader */
> -struct bpf_func_info_min {
> - __u32 insn_off;
> - __u32 type_id;
> -};
> -
> -/* The minimum bpf_line_info checked by the loader */
> -struct bpf_line_info_min {
> - __u32 insn_off;
> - __u32 file_name_off;
> - __u32 line_off;
> - __u32 line_col;
> -};
> -
> static inline __u64 ptr_to_u64(const void *ptr)
> {
> return (__u64) (unsigned long) ptr;
> @@ -831,6 +790,9 @@ static int btf_ext_setup_info(struct btf_ext *btf_ext,
> /* The start of the info sec (including the __u32 record_size). */
> void *info;
>
> + if (ext_sec->len == 0)
> + return 0;
> +
> if (ext_sec->off & 0x03) {
> pr_debug(".BTF.ext %s section is not aligned to 4 bytes\n",
> ext_sec->desc);
> @@ -934,6 +896,19 @@ static int btf_ext_setup_line_info(struct btf_ext *btf_ext)
> return btf_ext_setup_info(btf_ext, ¶m);
> }
>
> +static int btf_ext_setup_offset_reloc(struct btf_ext *btf_ext)
> +{
> + struct btf_ext_sec_setup_param param = {
> + .off = btf_ext->hdr->offset_reloc_off,
> + .len = btf_ext->hdr->offset_reloc_len,
> + .min_rec_size = sizeof(struct bpf_offset_reloc),
> + .ext_info = &btf_ext->offset_reloc_info,
> + .desc = "offset_reloc",
> + };
> +
> + return btf_ext_setup_info(btf_ext, ¶m);
> +}
> +
> static int btf_ext_parse_hdr(__u8 *data, __u32 data_size)
> {
> const struct btf_ext_header *hdr = (struct btf_ext_header *)data;
> @@ -1004,6 +979,13 @@ struct btf_ext *btf_ext__new(__u8 *data, __u32 size)
> if (err)
> goto done;
>
> + /* check if there is offset_reloc_off/offset_reloc_len fields */
> + if (btf_ext->hdr->hdr_len < sizeof(struct btf_ext_header))
This check will break when we add more optional sections to btf_ext_header.
Maybe use offsetof() instead?
> + goto done;
> + err = btf_ext_setup_offset_reloc(btf_ext);
> + if (err)
> + goto done;
> +
> done:
> if (err) {
> btf_ext__free(btf_ext);
> diff --git a/tools/lib/bpf/btf.h b/tools/lib/bpf/btf.h
> index 88a52ae56fc6..287361ee1f6b 100644
> --- a/tools/lib/bpf/btf.h
> +++ b/tools/lib/bpf/btf.h
> @@ -57,6 +57,10 @@ struct btf_ext_header {
> __u32 func_info_len;
> __u32 line_info_off;
> __u32 line_info_len;
> +
> + /* optional part of .BTF.ext header */
> + __u32 offset_reloc_off;
> + __u32 offset_reloc_len;
> };
>
> LIBBPF_API void btf__free(struct btf *btf);
> diff --git a/tools/lib/bpf/libbpf_internal.h b/tools/lib/bpf/libbpf_internal.h
> index 2ac29bd36226..087ff512282f 100644
> --- a/tools/lib/bpf/libbpf_internal.h
> +++ b/tools/lib/bpf/libbpf_internal.h
> @@ -46,4 +46,95 @@ do { \
> int libbpf__load_raw_btf(const char *raw_types, size_t types_len,
> const char *str_sec, size_t str_len);
>
> +struct btf_ext_info {
> + /*
> + * info points to the individual info section (e.g. func_info and
> + * line_info) from the .BTF.ext. It does not include the __u32 rec_size.
> + */
> + void *info;
> + __u32 rec_size;
> + __u32 len;
> +};
> +
> +#define for_each_btf_ext_sec(seg, sec) \
> + for (sec = (seg)->info; \
> + (void *)sec < (seg)->info + (seg)->len; \
> + sec = (void *)sec + sizeof(struct btf_ext_info_sec) + \
> + (seg)->rec_size * sec->num_info)
> +
> +#define for_each_btf_ext_rec(seg, sec, i, rec) \
> + for (i = 0, rec = (void *)&(sec)->data; \
> + i < (sec)->num_info; \
> + i++, rec = (void *)rec + (seg)->rec_size)
> +
> +struct btf_ext {
> + union {
> + struct btf_ext_header *hdr;
> + void *data;
> + };
> + struct btf_ext_info func_info;
> + struct btf_ext_info line_info;
> + struct btf_ext_info offset_reloc_info;
> + __u32 data_size;
> +};
> +
> +struct btf_ext_info_sec {
> + __u32 sec_name_off;
> + __u32 num_info;
> + /* Followed by num_info * record_size number of bytes */
> + __u8 data[0];
> +};
> +
> +/* The minimum bpf_func_info checked by the loader */
> +struct bpf_func_info_min {
> + __u32 insn_off;
> + __u32 type_id;
> +};
> +
> +/* The minimum bpf_line_info checked by the loader */
> +struct bpf_line_info_min {
> + __u32 insn_off;
> + __u32 file_name_off;
> + __u32 line_off;
> + __u32 line_col;
> +};
> +
> +/* The minimum bpf_offset_reloc checked by the loader
> + *
> + * Offset relocation captures the following data:
> + * - insn_off - instruction offset (in bytes) within a BPF program that needs
> + * its insn->imm field to be relocated with actual offset;
> + * - type_id - BTF type ID of the "root" (containing) entity of a relocatable
> + * offset;
> + * - access_str_off - offset into corresponding .BTF string section. String
> + * itself encodes an accessed field using a sequence of field and array
> + * indicies, separated by colon (:). It's conceptually very close to LLVM's
> + * getelementptr ([0]) instruction's arguments for identifying offset to
> + * a field.
> + *
> + * Example to provide a better feel.
> + *
> + * struct sample {
> + * int a;
> + * struct {
> + * int b[10];
> + * };
> + * };
> + *
> + * struct sample *s = ...;
> + * int x = &s->a; // encoded as "0:0" (a is field #0)
> + * int y = &s->b[5]; // encoded as "0:1:5" (b is field #1, arr elem #5)
> + * int z = &s[10]->b; // encoded as "10:1" (ptr is used as an array)
> + *
> + * type_id for all relocs in this example will capture BTF type id of
> + * `struct sample`.
> + *
> + * [0] https://llvm.org/docs/LangRef.html#getelementptr-instruction
> + */
> +struct bpf_offset_reloc {
> + __u32 insn_off;
> + __u32 type_id;
> + __u32 access_str_off;
> +};
> +
> #endif /* __LIBBPF_LIBBPF_INTERNAL_H */
> --
> 2.17.1
>
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