[PATCH v3 bpf-next 3/4] bpftool: generate externs datasec in BPF skeleton

[Andrii Nakryiko <andriin@fb.com>]

Add support for generation of mmap()-ed read-only view of libbpf-provided
extern variables. As externs are not supposed to be provided by user code
(that's what .data, .bss, and .rodata is for), don't mmap() it initially. Only
after skeleton load is performed, map .extern contents as read-only memory.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
---
 tools/bpf/bpftool/gen.c |  4 ++++
 tools/lib/bpf/libbpf.c  | 10 +++++-----
 2 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/tools/bpf/bpftool/gen.c b/tools/bpf/bpftool/gen.c
index 5d8755ef6acf..a7d472e37ddc 100644
--- a/tools/bpf/bpftool/gen.c
+++ b/tools/bpf/bpftool/gen.c
@@ -82,6 +82,8 @@ static const char *get_map_ident(const struct bpf_map *map)
 		return "rodata";
 	else if (str_has_suffix(name, ".bss"))
 		return "bss";
+	else if (str_has_suffix(name, ".extern"))
+		return "externs"; /* extern is a C keyword */
 	else
 		return NULL;
 }
@@ -109,6 +111,8 @@ static int codegen_datasec_def(struct bpf_object *obj,
 		sec_ident = "bss";
 	else if (strcmp(sec_name, ".rodata") == 0)
 		sec_ident = "rodata";
+	else if (strcmp(sec_name, ".extern") == 0)
+		sec_ident = "externs"; /* extern is a C keyword */
 	else
 		return 0;
 
diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index be5508d0acbd..fe758d670b22 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -7472,7 +7472,8 @@ int bpf_object__open_skeleton(struct bpf_object_skeleton *s,
 			return -ESRCH;
 		}
 
-		if (mmaped)
+		/* externs shouldn't be pre-setup from user code */
+		if (mmaped && (*map)->libbpf_type != LIBBPF_MAP_EXTERN)
 			*mmaped = (*map)->mmaped;
 	}
 
@@ -7505,7 +7506,6 @@ int bpf_object__load_skeleton(struct bpf_object_skeleton *s)
 		size_t mmap_sz = bpf_map_mmap_sz(map);
 		int prot, map_fd = bpf_map__fd(map);
 		void **mmaped = s->maps[i].mmaped;
-		void *remapped;
 
 		if (!mmaped)
 			continue;
@@ -7530,9 +7530,9 @@ int bpf_object__load_skeleton(struct bpf_object_skeleton *s)
 		 * as per normal clean up procedure, so we don't need to worry
 		 * about it from skeleton's clean up perspective.
 		 */
-		remapped = mmap(*mmaped, mmap_sz, prot, MAP_SHARED | MAP_FIXED,
-				map_fd, 0);
-		if (remapped == MAP_FAILED) {
+		*mmaped = mmap(map->mmaped, mmap_sz, prot,
+				MAP_SHARED | MAP_FIXED, map_fd, 0);
+		if (*mmaped == MAP_FAILED) {
 			err = -errno;
 			*mmaped = NULL;
 			pr_warn("failed to re-mmap() map '%s': %d\n",
-- 
2.17.1