[1/1] libbpf: Convert ELF notes into read-only maps

Commit Message

Blaise Boscaccy Feb. 5, 2025, 7:06 p.m. UTC

Add a flexible mechanism, using existing ELF constructs, to attach
additional metadata to BPF programs for possible use by BPF
gatekeepers and skeletons.

During object file parsing, note sections are no longer skipped and
now treated as read-only data. During libbpf-based loading or skeleton
generation, those sections are then transformed into read-only maps
which are subsequently passed into the kernel.

Signed-off-by: Blaise Boscaccy <[e-mail]>
---
 tools/bpf/bpftool/gen.c | 4 ++--
 tools/lib/bpf/libbpf.c  | 6 ++++++
 2 files changed, 8 insertions(+), 2 deletions(-)

Comments

Andrii Nakryiko Feb. 5, 2025, 9:22 p.m. UTC | #1

On Wed, Feb 5, 2025 at 11:09 AM Blaise Boscaccy
<[e-mail]> wrote:
>
> Add a flexible mechanism, using existing ELF constructs, to attach
> additional metadata to BPF programs for possible use by BPF
> gatekeepers and skeletons.
>
> During object file parsing, note sections are no longer skipped and
> now treated as read-only data. During libbpf-based loading or skeleton
> generation, those sections are then transformed into read-only maps
> which are subsequently passed into the kernel.

We already have this mechanism, it's .rodata (and
.rodata.<customname>) section(s). Adding .note sections as BPF maps
make no sense to me. Just piggy-back on .rodata for storing any
necessary metadata.

pw-bot: cr

>
> Signed-off-by: Blaise Boscaccy <[e-mail]>
> ---
>  tools/bpf/bpftool/gen.c | 4 ++--
>  tools/lib/bpf/libbpf.c  | 6 ++++++
>  2 files changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/tools/bpf/bpftool/gen.c b/tools/bpf/bpftool/gen.c
> index 5a4d3240689ed..311d6a3f1c4bb 100644
> --- a/tools/bpf/bpftool/gen.c
> +++ b/tools/bpf/bpftool/gen.c
> @@ -92,7 +92,7 @@ static void get_header_guard(char *guard, const char *obj_name, const char *suff
>
>  static bool get_map_ident(const struct bpf_map *map, char *buf, size_t buf_sz)
>  {
> -       static const char *sfxs[] = { ".data", ".rodata", ".bss", ".kconfig" };
> +       static const char *sfxs[] = { ".data", ".rodata", ".bss", ".kconfig", ".note" };
>         const char *name = bpf_map__name(map);
>         int i, n;
>
> @@ -117,7 +117,7 @@ static bool get_map_ident(const struct bpf_map *map, char *buf, size_t buf_sz)
>
>  static bool get_datasec_ident(const char *sec_name, char *buf, size_t buf_sz)
>  {
> -       static const char *pfxs[] = { ".data", ".rodata", ".bss", ".kconfig" };
> +       static const char *pfxs[] = { ".data", ".rodata", ".bss", ".kconfig", ".note" };
>         int i, n;
>
>         /* recognize hard coded LLVM section name */
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 194809da51725..be6af0fece040 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -523,6 +523,7 @@ struct bpf_struct_ops {
>  #define STRUCT_OPS_SEC ".struct_ops"
>  #define STRUCT_OPS_LINK_SEC ".struct_ops.link"
>  #define ARENA_SEC ".addr_space.1"
> +#define NOTE_SEC ".note"
>
>  enum libbpf_map_type {
>         LIBBPF_MAP_UNSPEC,
> @@ -3977,6 +3978,11 @@ static int bpf_object__elf_collect(struct bpf_object *obj)
>                         sec_desc->sec_type = SEC_BSS;
>                         sec_desc->shdr = sh;
>                         sec_desc->data = data;
> +               } else if (sh->sh_type == SHT_NOTE && (strcmp(name, NOTE_SEC) == 0 ||
> +                                                      str_has_pfx(name, NOTE_SEC "."))) {
> +                       sec_desc->sec_type = SEC_RODATA;
> +                       sec_desc->shdr = sh;
> +                       sec_desc->data = data;
>                 } else {
>                         pr_info("elf: skipping section(%d) %s (size %zu)\n", idx, name,
>                                 (size_t)sh->sh_size);
> --
> 2.48.1
>

Patch

diff --git a/tools/bpf/bpftool/gen.c b/tools/bpf/bpftool/gen.c
index 5a4d3240689ed..311d6a3f1c4bb 100644
--- a/tools/bpf/bpftool/gen.c
+++ b/tools/bpf/bpftool/gen.c
@@ -92,7 +92,7 @@  static void get_header_guard(char *guard, const char *obj_name, const char *suff
 
 static bool get_map_ident(const struct bpf_map *map, char *buf, size_t buf_sz)
 {
-	static const char *sfxs[] = { ".data", ".rodata", ".bss", ".kconfig" };
+	static const char *sfxs[] = { ".data", ".rodata", ".bss", ".kconfig", ".note" };
 	const char *name = bpf_map__name(map);
 	int i, n;
 
@@ -117,7 +117,7 @@  static bool get_map_ident(const struct bpf_map *map, char *buf, size_t buf_sz)
 
 static bool get_datasec_ident(const char *sec_name, char *buf, size_t buf_sz)
 {
-	static const char *pfxs[] = { ".data", ".rodata", ".bss", ".kconfig" };
+	static const char *pfxs[] = { ".data", ".rodata", ".bss", ".kconfig", ".note" };
 	int i, n;
 
 	/* recognize hard coded LLVM section name */
diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 194809da51725..be6af0fece040 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -523,6 +523,7 @@  struct bpf_struct_ops {
 #define STRUCT_OPS_SEC ".struct_ops"
 #define STRUCT_OPS_LINK_SEC ".struct_ops.link"
 #define ARENA_SEC ".addr_space.1"
+#define NOTE_SEC ".note"
 
 enum libbpf_map_type {
 	LIBBPF_MAP_UNSPEC,
@@ -3977,6 +3978,11 @@  static int bpf_object__elf_collect(struct bpf_object *obj)
 			sec_desc->sec_type = SEC_BSS;
 			sec_desc->shdr = sh;
 			sec_desc->data = data;
+		} else if (sh->sh_type == SHT_NOTE && (strcmp(name, NOTE_SEC) == 0 ||
+						       str_has_pfx(name, NOTE_SEC "."))) {
+			sec_desc->sec_type = SEC_RODATA;
+			sec_desc->shdr = sh;
+			sec_desc->data = data;
 		} else {
 			pr_info("elf: skipping section(%d) %s (size %zu)\n", idx, name,
 				(size_t)sh->sh_size);