[PATCH v2 bpf-next 0/3] Bpf skeleton helper method

[PATCH v2 bpf-next 0/3] Bpf skeleton helper method

[Matt Smith]

This patch series changes the type of bpf_object_skeleton->data
to const void * and provides a helper method X__elf_bytes(size_t *sz)
for accessing the raw binary data of the compiled embedded BPF object.

The type change enforces the previously implied behavior of immutability
for this field while casting it to (void *) before assignment allows
for compiling with previous versions of the libbpf headers without
compiler warnings.

The helper method allows easier access to the BPF binary object data
and is leveraged to populate the skeleton field.  The inclusion of
this helper method will allow users to get access to the data without
needing to populate an entire skeleton first.

Checks are added in the third patch to validate the behavior of the
added method

Matt Smith (3):
  libbpf: Change bpf_object_skelecton data field to const void*
  bpftool: Provide a helper method for accessing bpf binary data
  selftests/bpf: Add checks for X__elf_bytes skeleton helper

 tools/bpf/bpftool/gen.c                       | 39 ++++++++++++-------
 tools/lib/bpf/libbpf.h                        |  2 +-
 .../selftests/bpf/prog_tests/skeleton.c       |  7 ++++
 3 files changed, 32 insertions(+), 16 deletions(-)

-- 
2.30.2

[PATCH v2 bpf-next 1/3] libbpf: Change bpf_object_skelecton data field to const void*

[Matt Smith]

This change was necessary to enforce the implied contract
that bpf_object_skeleton->data should not be mutated.  The data
will be cast to `void *` during assignment to handle the case
where a user is compiling with older libbpf headers to avoid
a compiler warning of `const void *` data being cast to `void *`

Signed-off-by: Matt Smith <alastorze@fb.com>
---
 tools/lib/bpf/libbpf.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
index f177d897c5f7..2f6f0e15d1e7 100644
--- a/tools/lib/bpf/libbpf.h
+++ b/tools/lib/bpf/libbpf.h
@@ -854,7 +854,7 @@ struct bpf_object_skeleton {
 	size_t sz; /* size of this struct, for forward/backward compatibility */
 
 	const char *name;
-	void *data;
+	const void *data;
 	size_t data_sz;
 
 	struct bpf_object **obj;
-- 
2.30.2

[PATCH v2 bpf-next 2/3] bpftool: Provide a helper method for accessing bpf binary data

[Matt Smith]

This adds a binary method X_elf_bytes() which returns the binary data of
the compiled bpf object.  It additionally sets the size of the return
data to the provided size_t pointer argument.

The assignment to s->data is cast to void * to ensure no warning is
issued if compiled with a previous version of libbpf where the
bpf_object_skeleton field is void * instead of const void *

Signed-off-by: Matt Smith <alastorze@fb.com>
---
 tools/bpf/bpftool/gen.c | 39 ++++++++++++++++++++++++---------------
 1 file changed, 24 insertions(+), 15 deletions(-)

diff --git a/tools/bpf/bpftool/gen.c b/tools/bpf/bpftool/gen.c
index d40d92bbf0e4..f54dc4792fce 100644
--- a/tools/bpf/bpftool/gen.c
+++ b/tools/bpf/bpftool/gen.c
@@ -238,8 +238,8 @@ static void codegen(const char *template, ...)
 		} else if (c == '\n') {
 			break;
 		} else {
-			p_err("unrecognized character at pos %td in template '%s'",
-			      src - template - 1, template);
+			p_err("unrecognized character at pos %td in template '%s': '%c'",
+			      src - template - 1, template, c);
 			free(s);
 			exit(-1);
 		}
@@ -406,7 +406,7 @@ static void codegen_destroy(struct bpf_object *obj, const char *obj_name)
 	}
 
 	bpf_object__for_each_map(map, obj) {
-		const char * ident;
+		const char *ident;
 
 		ident = get_map_ident(map);
 		if (!ident)
@@ -787,6 +787,9 @@ static int do_skeleton(int argc, char **argv)
 			free(obj);					    \n\
 		}							    \n\
 									    \n\
+		static inline const void *		\n\
+		%1$s__elf_bytes(size_t *sz);	\n\
+										\n\
 		static inline int					    \n\
 		%1$s__create_skeleton(struct %1$s *obj);		    \n\
 									    \n\
@@ -943,25 +946,31 @@ static int do_skeleton(int argc, char **argv)
 	codegen("\
 		\n\
 									    \n\
-			s->data_sz = %d;				    \n\
-			s->data = (void *)\"\\				    \n\
-		",
-		file_sz);
-
-	/* embed contents of BPF object file */
-	print_hex(obj_data, file_sz);
-
-	codegen("\
+			s->data = (void *)%2$s__elf_bytes(&s->data_sz);			    \n\
 		\n\
-		\";							    \n\
 									    \n\
 			return 0;					    \n\
 		err:							    \n\
 			bpf_object__destroy_skeleton(s);		    \n\
 			return -ENOMEM;					    \n\
 		}							    \n\
-									    \n\
-		#endif /* %s */						    \n\
+										\n\
+		static inline const void *		\n\
+		%2$s__elf_bytes(size_t *sz)		\n\
+		{								\n\
+			*sz = %1$d;					\n\
+			return (const void *)\"\\	\n\
+		"
+		, file_sz, obj_name);
+
+	/* embed contents of BPF object file */
+	print_hex(obj_data, file_sz);
+
+	codegen("\
+		\n\
+		\";								\n\
+		}								\n\
+		#endif /* %s */					\n\
 		",
 		header_guard);
 	err = 0;
-- 
2.30.2

[PATCH v2 bpf-next 3/3] selftests/bpf: Add checks for X__elf_bytes skeleton helper

[Matt Smith]

This patch adds two checks for the X__elf_bytes bpf skeleton helper
method.  The first asserts that the pointer returned from the helper
method is valid, the second asserts that the provided size pointer is
set.

Signed-off-by: Matt Smith <alastorze@fb.com>
---
 tools/testing/selftests/bpf/prog_tests/skeleton.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/tools/testing/selftests/bpf/prog_tests/skeleton.c b/tools/testing/selftests/bpf/prog_tests/skeleton.c
index f6f130c99b8c..963a4f5f596d 100644
--- a/tools/testing/selftests/bpf/prog_tests/skeleton.c
+++ b/tools/testing/selftests/bpf/prog_tests/skeleton.c
@@ -18,6 +18,9 @@ void test_skeleton(void)
 	struct test_skeleton__data *data;
 	struct test_skeleton__rodata *rodata;
 	struct test_skeleton__kconfig *kcfg;
+	const void *eb;
+	size_t ebs_val = 0;
+	size_t *ebs = &ebs_val;
 
 	skel = test_skeleton__open();
 	if (CHECK(!skel, "skel_open", "failed to open skeleton\n"))
@@ -91,6 +94,10 @@ void test_skeleton(void)
 	CHECK(bss->kern_ver != kcfg->LINUX_KERNEL_VERSION, "ext2",
 	      "got %d != exp %d\n", bss->kern_ver, kcfg->LINUX_KERNEL_VERSION);
 
+	eb = test_skeleton__elf_bytes(ebs);
+	ASSERT_OK_PTR(eb, "elf_bytes_not_null");
+	ASSERT_NEQ(*ebs, 0, "elf_bytes_size_not_zero");
+
 cleanup:
 	test_skeleton__destroy(skel);
 }
-- 
2.30.2

Re: [PATCH v2 bpf-next 0/3] Bpf skeleton helper method

[Andrii Nakryiko]

On Wed, Sep 1, 2021 at 1:25 PM Matt Smith <alastorze@fb.com> wrote:
>
> This patch series changes the type of bpf_object_skeleton->data
> to const void * and provides a helper method X__elf_bytes(size_t *sz)
> for accessing the raw binary data of the compiled embedded BPF object.
>
> The type change enforces the previously implied behavior of immutability
> for this field while casting it to (void *) before assignment allows
> for compiling with previous versions of the libbpf headers without
> compiler warnings.
>
> The helper method allows easier access to the BPF binary object data
> and is leveraged to populate the skeleton field.  The inclusion of
> this helper method will allow users to get access to the data without
> needing to populate an entire skeleton first.
>
> Checks are added in the third patch to validate the behavior of the
> added method
>
> Matt Smith (3):
>   libbpf: Change bpf_object_skelecton data field to const void*
>   bpftool: Provide a helper method for accessing bpf binary data
>   selftests/bpf: Add checks for X__elf_bytes skeleton helper
>
>  tools/bpf/bpftool/gen.c                       | 39 ++++++++++++-------
>  tools/lib/bpf/libbpf.h                        |  2 +-
>  .../selftests/bpf/prog_tests/skeleton.c       |  7 ++++
>  3 files changed, 32 insertions(+), 16 deletions(-)
>
> --
> 2.30.2
>

Fixed \n\ alignment and made a few small tweaks. Applied to bpf-next, thanks.