bpf: Allow a variable in DATASEC that is smaller than its type

LLVM splits a static of a Rust program into pieces. Every piece is a VAR
with the type of the whole static:

  [223] STRUCT 'BpfCell<core::option::Option<...>>' size=32 vlen=1
  [236] VAR '..scx_cosmos9TASK_CTXS.0' type_id=223, linkage=static
  [248] DATASEC '.bss' size=0 vlen=9
          type_id=236 offset=24648 size=1 (VAR '..TASK_CTXS.0')

and the kernel rejects such BTF with "Invalid size".

Allow it. The size of a variable in DATASEC is used in two places:

- btf_find_datasec_var() looks for timers, spin locks, kptrs and other
  special fields. btf_find_field_one() skips a variable when its size is
  not the size of its type, so there are no special fields in a piece.

- btf_datasec_show() prints variables by their types. It would read
  past the piece and past the end of the map value when the piece is the
  last one. Don't print the pieces.

"global data test #8" and "#10" in prog_tests/btf.c expect "Invalid
size". BTF of both is loaded now. The map of #10 is not created, since
its value is larger than the section.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20261002124714.180012-10-alexei.starovoitov@gmail.com
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

1 files changed, 3 insertions, 6 deletions

diff --git a/tools/testing/selftests/bpf/prog_tests/btf.c b/tools/testing/selftests/bpf/prog_tests/btf.c
index 642800b80c74..24ab62b2834a 100644
--- a/tools/testing/selftests/bpf/prog_tests/btf.c
+++ b/tools/testing/selftests/bpf/prog_tests/btf.c
@@ -424,7 +424,7 @@ static struct btf_raw_test raw_tests[] = {
 	.err_str = "Invalid type",
 },
 {
-	.descr = "global data test #8, invalid var size",
+	.descr = "global data test #8, var is smaller than its type",
 	.raw_types = {
 		/* int */
 		BTF_TYPE_INT_ENC(0, BTF_INT_SIGNED, 0, 32, 4),	/* [1] */
@@ -457,8 +457,6 @@ static struct btf_raw_test raw_tests[] = {
 	.key_type_id = 0,
 	.value_type_id = 7,
 	.max_entries = 1,
-	.btf_load_err = true,
-	.err_str = "Invalid size",
 },
 {
 	.descr = "global data test #9, invalid var size",
@@ -498,7 +496,7 @@ static struct btf_raw_test raw_tests[] = {
 	.err_str = "Invalid size",
 },
 {
-	.descr = "global data test #10, invalid var size",
+	.descr = "global data test #10, section is smaller than map value",
 	.raw_types = {
 		/* int */
 		BTF_TYPE_INT_ENC(0, BTF_INT_SIGNED, 0, 32, 4),	/* [1] */
@@ -531,8 +529,7 @@ static struct btf_raw_test raw_tests[] = {
 	.key_type_id = 0,
 	.value_type_id = 7,
 	.max_entries = 1,
-	.btf_load_err = true,
-	.err_str = "Invalid size",
+	.map_create_err = true,
 },
 {
 	.descr = "global data test #11, multiple section members",