[PATCH bpf-next v8 17/22] libbpf: Carry the exception cleanup table through the light skeleton

[Yonghong Song <yonghong.song@linux.dev>]

A light skeleton does not call bpf_prog_load(). bpf_gen__prog_load() builds
its own union bpf_attr field by field, and the loader program it emits is
what issues BPF_PROG_LOAD when the skeleton runs -- so a program loaded
this way reached the kernel without the table the previous patch collected
for it, and the verifier refused it with "unreachable insn", which names
neither the skeleton nor the table.

Carry it the way func_info and line_info are carried: the records go into
the loader's blob of bytes, the count and record size into the attr, and a
relocation stores the blob's address into attr.cleanup_info once that
address is known. For a program with records the attr grows to its new
last field, cleanup_info_cnt. One without keeps the old size: the longer
attr also takes in log_true_size, which the kernel writes back, into the
loader's read-only data.

Records are 4-byte fields like the other info blobs, so a cross-endian
build has to swap them too.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
---
 tools/lib/bpf/gen_loader.c      | 34 ++++++++++++++++++++++++++++++---
 tools/lib/bpf/libbpf_internal.h |  7 +++++++
 2 files changed, 38 insertions(+), 3 deletions(-)

diff --git a/tools/lib/bpf/gen_loader.c b/tools/lib/bpf/gen_loader.c
index 251392aa8b41..4017fb366384 100644
--- a/tools/lib/bpf/gen_loader.c
+++ b/tools/lib/bpf/gen_loader.c
@@ -994,13 +994,15 @@ static void cleanup_relos(struct bpf_gen *gen, int insns)
 	cleanup_core_relo(gen);
 }
 
-/* Convert func, line, and core relo info blobs to target endianness */
+/* Convert func, line, core relo and cleanup info blobs to target endianness */
 static void info_blob_bswap(struct bpf_gen *gen, int func_info, int line_info,
-			    int core_relos, struct bpf_prog_load_opts *load_attr)
+			    int core_relos, int cleanup_info,
+			    struct bpf_prog_load_opts *load_attr)
 {
 	struct bpf_func_info *fi = gen->data_start + func_info;
 	struct bpf_line_info *li = gen->data_start + line_info;
 	struct bpf_core_relo *cr = gen->data_start + core_relos;
+	struct bpf_cleanup_info *ci = gen->data_start + cleanup_info;
 	int i;
 
 	for (i = 0; i < load_attr->func_info_cnt; i++)
@@ -1011,6 +1013,9 @@ static void info_blob_bswap(struct bpf_gen *gen, int func_info, int line_info,
 
 	for (i = 0; i < gen->core_relo_cnt; i++)
 		bpf_core_relo_bswap(cr++);
+
+	for (i = 0; i < load_attr->cleanup_info_cnt; i++)
+		bpf_cleanup_info_bswap(ci++);
 }
 
 void bpf_gen__prog_load(struct bpf_gen *gen,
@@ -1024,10 +1029,20 @@ void bpf_gen__prog_load(struct bpf_gen *gen,
 			       load_attr->line_info_rec_size;
 	int core_relo_tot_sz = gen->core_relo_cnt *
 			       sizeof(struct bpf_core_relo);
+	int cleanup_info_tot_sz = load_attr->cleanup_info_cnt *
+				  load_attr->cleanup_info_rec_size;
 	int prog_load_attr, license_off, insns_off, func_info, line_info, core_relos;
 	int attr_size = offsetofend(union bpf_attr, core_relo_rec_size);
+	int cleanup_info;
 	union bpf_attr attr;
 
+	/*
+	 * Reach the cleanup fields only when there are records: the attr then
+	 * also covers log_true_size, which the kernel writes back, and the
+	 * attr lives in the loader's read-only data.
+	 */
+	if (load_attr->cleanup_info_cnt)
+		attr_size = offsetofend(union bpf_attr, cleanup_info_cnt);
 	memset(&attr, 0, attr_size);
 	/* add license string to blob of bytes */
 	license_off = add_data(gen, license, strlen(license) + 1);
@@ -1074,9 +1089,17 @@ void bpf_gen__prog_load(struct bpf_gen *gen,
 		 core_relos, gen->core_relo_cnt,
 		 sizeof(struct bpf_core_relo));
 
+	attr.cleanup_info_rec_size = tgt_endian(load_attr->cleanup_info_rec_size);
+	attr.cleanup_info_cnt = tgt_endian(load_attr->cleanup_info_cnt);
+	cleanup_info = add_data(gen, load_attr->cleanup_info, cleanup_info_tot_sz);
+	pr_debug("gen: prog_load: cleanup_info: off %d cnt %u rec size %u\n",
+		 cleanup_info, load_attr->cleanup_info_cnt,
+		 load_attr->cleanup_info_rec_size);
+
 	/* convert all info blobs to target endianness */
 	if (gen->swapped_endian && !gen->error)
-		info_blob_bswap(gen, func_info, line_info, core_relos, load_attr);
+		info_blob_bswap(gen, func_info, line_info, core_relos, cleanup_info,
+				load_attr);
 
 	libbpf_strlcpy(attr.prog_name, prog_name, sizeof(attr.prog_name));
 	prog_load_attr = add_data(gen, &attr, attr_size);
@@ -1098,6 +1121,11 @@ void bpf_gen__prog_load(struct bpf_gen *gen,
 	/* populate union bpf_attr with a pointer to core_relos */
 	emit_rel_store(gen, attr_field(prog_load_attr, core_relos), core_relos);
 
+	/* with no records there is no blob of them to point the attr at */
+	if (load_attr->cleanup_info_cnt)
+		emit_rel_store(gen, attr_field(prog_load_attr, cleanup_info),
+			       cleanup_info);
+
 	/* populate union bpf_attr fd_array with a pointer to data where map_fds are saved */
 	emit_rel_store(gen, attr_field(prog_load_attr, fd_array), gen->fd_array);
 
diff --git a/tools/lib/bpf/libbpf_internal.h b/tools/lib/bpf/libbpf_internal.h
index f1630f03d5f5..9d341839ca74 100644
--- a/tools/lib/bpf/libbpf_internal.h
+++ b/tools/lib/bpf/libbpf_internal.h
@@ -572,6 +572,13 @@ static inline void bpf_core_relo_bswap(struct bpf_core_relo *i)
 	i->kind = bswap_32(i->kind);
 }
 
+static inline void bpf_cleanup_info_bswap(struct bpf_cleanup_info *i)
+{
+	i->begin_off = bswap_32(i->begin_off);
+	i->end_off = bswap_32(i->end_off);
+	i->landing_pad_off = bswap_32(i->landing_pad_off);
+}
+
 enum btf_field_iter_kind {
 	BTF_FIELD_ITER_IDS,
 	BTF_FIELD_ITER_STRS,
-- 
2.53.0-Meta