[PATCH bpf-next v6 07/12] bpftool: Generate skeleton for global percpu data

[Leon Hwang <leon.hwang@linux.dev>]

Enhance bpftool to generate skeletons that properly handle global percpu
variables. The generated skeleton now includes a dedicated structure for
percpu data, allowing users to initialize and access percpu variables more
efficiently.

For global percpu variables, the skeleton now includes a nested
structure, e.g.:

struct test_global_percpu_data {
	struct bpf_object_skeleton *skeleton;
	struct bpf_object *obj;
	struct {
		struct bpf_map *percpu;
	} maps;
	// ...
	struct test_global_percpu_data__percpu {
		int data;
		char run;
		struct {
			char set;
			int i;
			int nums[7];
		} struct_data;
		int nums[7];
	} *percpu;

	// ...
};

  * The "struct test_global_percpu_data__percpu *percpu" points to
    initialized data, which is actually "maps.percpu->mmaped".
  * Before loading the skeleton, updating the
    "struct test_global_percpu_data__percpu *percpu" modifies the initial
    value of the corresponding global percpu variables.
  * After loading the skeleton, "maps.percpu->mmaped" has been marked as
    read-only in libbpf. If users want to update the global percpu
    variables, they have to update the "maps.percpu" map instead.
  * For lightweight skeleton, "lskel->percpu" will be protected by
    "mprotect(p, sz, PROT_READ)".
  * For subskeleton, those variables of global percpu data will be
    skipped.

Assisted-by: Codex:gpt-5.5-xhigh
Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
---
 tools/bpf/bpftool/gen.c       | 43 +++++++++++++++++++++++++++--------
 tools/lib/bpf/skel_internal.h | 24 +++++++++++++++++--
 2 files changed, 56 insertions(+), 11 deletions(-)

diff --git a/tools/bpf/bpftool/gen.c b/tools/bpf/bpftool/gen.c
index 6ae7262ebe0c..2200ab004d2b 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", ".percpu", ".kconfig" };
 	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", ".percpu", ".kconfig" };
 	int i, n;
 
 	/* recognize hard coded LLVM section name */
@@ -254,6 +254,11 @@ static const struct btf_type *find_type_for_map(struct btf *btf, const char *map
 	return NULL;
 }
 
+static bool bpf_map_is_percpu_data(const struct bpf_map *map)
+{
+	return bpf_map__is_internal(map) && bpf_map__type(map) == BPF_MAP_TYPE_PERCPU_ARRAY;
+}
+
 static bool is_mmapable_map(const struct bpf_map *map, char *buf, size_t sz)
 {
 	size_t tmp_sz;
@@ -263,13 +268,12 @@ static bool is_mmapable_map(const struct bpf_map *map, char *buf, size_t sz)
 		return true;
 	}
 
-	if (!bpf_map__is_internal(map) || !(bpf_map__map_flags(map) & BPF_F_MMAPABLE))
-		return false;
-
-	if (!get_map_ident(map, buf, sz))
-		return false;
+	if (bpf_map__is_internal(map) &&
+	    ((bpf_map__map_flags(map) & BPF_F_MMAPABLE) || bpf_map_is_percpu_data(map)) &&
+	    get_map_ident(map, buf, sz))
+		return true;
 
-	return true;
+	return false;
 }
 
 static int codegen_datasecs(struct bpf_object *obj, const char *obj_name)
@@ -343,6 +347,9 @@ static int codegen_subskel_datasecs(struct bpf_object *obj, const char *obj_name
 		if (!is_mmapable_map(map, map_ident, sizeof(map_ident)))
 			continue;
 
+		if (bpf_map_is_percpu_data(map))
+			continue;
+
 		sec = find_type_for_map(btf, map_ident);
 		if (!sec)
 			continue;
@@ -669,7 +676,7 @@ static void codegen_destroy(struct bpf_object *obj, const char *obj_name)
 		if (!get_map_ident(map, ident, sizeof(ident)))
 			continue;
 		if (bpf_map__is_internal(map) &&
-		    (bpf_map__map_flags(map) & BPF_F_MMAPABLE))
+		    ((bpf_map__map_flags(map) & BPF_F_MMAPABLE) || bpf_map_is_percpu_data(map)))
 			printf("\tskel_free_map_data(skel->%1$s, skel->maps.%1$s.initial_value, %2$zu);\n",
 			       ident, bpf_map_mmap_sz(map));
 		codegen("\
@@ -850,6 +857,20 @@ static int gen_trace(struct bpf_object *obj, const char *obj_name, const char *h
 		if (!is_mmapable_map(map, ident, sizeof(ident)))
 			continue;
 
+		if (bpf_map_is_percpu_data(map)) {
+			codegen("\
+		\n\
+			err = skel_protect_map_data(skel->%1$s, &skel->maps.%1$s.initial_value, %2$zd);\n\
+			if (err)					    \n\
+				return err;				    \n\
+		#ifdef __KERNEL__					    \n\
+			skel->%1$s = NULL;				    \n\
+		#endif							    \n\
+			",
+			ident, bpf_map_mmap_sz(map));
+			continue;
+		}
+
 		if (bpf_map__map_flags(map) & BPF_F_RDONLY_PROG)
 			mmap_flags = "PROT_READ";
 		else
@@ -1740,6 +1761,8 @@ static int do_subskeleton(int argc, char **argv)
 
 		if (!is_mmapable_map(map, ident, sizeof(ident)))
 			continue;
+		if (bpf_map_is_percpu_data(map))
+			continue;
 
 		map_type_id = bpf_map__btf_value_type_id(map);
 		if (map_type_id <= 0) {
@@ -1863,6 +1886,8 @@ static int do_subskeleton(int argc, char **argv)
 	bpf_object__for_each_map(map, obj) {
 		if (!is_mmapable_map(map, ident, sizeof(ident)))
 			continue;
+		if (bpf_map_is_percpu_data(map))
+			continue;
 
 		map_type_id = bpf_map__btf_value_type_id(map);
 		if (map_type_id <= 0)
diff --git a/tools/lib/bpf/skel_internal.h b/tools/lib/bpf/skel_internal.h
index 74503d358bc8..485b0cd17017 100644
--- a/tools/lib/bpf/skel_internal.h
+++ b/tools/lib/bpf/skel_internal.h
@@ -135,8 +135,10 @@ static inline void skel_free_map_data(void *p, __u64 addr, size_t sz)
 {
 	if (addr != ~0ULL)
 		kvfree(p);
-	/* When addr == ~0ULL the 'p' points to
-	 * ((struct bpf_array *)map)->value. See skel_finalize_map_data.
+	/*
+	 * When addr == ~0ULL the init buffer has already been released.
+	 * For skel_finalize_map_data(), 'p' points to
+	 * ((struct bpf_array *)map)->value.
 	 */
 }
 
@@ -174,6 +176,15 @@ static inline void *skel_finalize_map_data(__u64 *init_val, size_t mmap_sz, int
 	return addr;
 }
 
+static inline int skel_protect_map_data(void *p, __u64 *init_val, size_t sz)
+{
+	(void)sz;
+
+	kvfree(p);
+	*init_val = ~0ULL;
+	return 0;
+}
+
 #else
 
 static inline void *skel_alloc(size_t size)
@@ -212,6 +223,15 @@ static inline void *skel_finalize_map_data(__u64 *init_val, size_t mmap_sz, int
 		return NULL;
 	return addr;
 }
+
+static inline int skel_protect_map_data(void *p, __u64 *init_val, size_t sz)
+{
+	(void)init_val;
+
+	if (mprotect(p, sz, PROT_READ))
+		return -errno;
+	return 0;
+}
 #endif
 
 static inline int skel_closenz(int fd)
-- 
2.54.0