libbpf: fix LDX/STX/ST CO-RE relocation size adjustment logic

Libbpf has a somewhat obscure feature of automatically adjusting the
"size" of LDX/STX/ST instruction (memory store and load instructions),
based on originally recorded access size (u8, u16, u32, or u64) and the
actual size of the field on target kernel. This is meant to facilitate
using BPF CO-RE on 32-bit architectures (pointers are always 64-bit in
BPF, but host kernel's BTF will have it as 32-bit type), as well as
generally supporting safe type changes (unsigned integer type changes
can be transparently "relocated").

One issue that surfaced only now, 5 years after this logic was
implemented, is how this all works when dealing with fields that are
arrays. This isn't all that easy and straightforward to hit (see
selftests that reproduce this condition), but one of sched_ext BPF
programs did hit it with innocent looking loop.

Long story short, libbpf used to calculate entire array size, instead of
making sure to only calculate array's element size. But it's the element
that is loaded by LDX/STX/ST instructions (1, 2, 4, or 8 bytes), so
that's what libbpf should check. This patch adjusts the logic for
arrays and fixed the issue.

Reported-by: Emil Tsalapatis <emil@etsalapatis.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20250207014809.1573841-1-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/lib/bpf/relo_core.c

@@ -683,7 +683,7 @@ static int bpf_core_calc_field_relo(const char *prog_name,
 {
 	const struct bpf_core_accessor *acc;
 	const struct btf_type *t;
-	__u32 byte_off, byte_sz, bit_off, bit_sz, field_type_id;
+	__u32 byte_off, byte_sz, bit_off, bit_sz, field_type_id, elem_id;
 	const struct btf_member *m;
 	const struct btf_type *mt;
 	bool bitfield;
@@ -706,8 +706,14 @@ static int bpf_core_calc_field_relo(const char *prog_name,
 	if (!acc->name) {
 		if (relo->kind == BPF_CORE_FIELD_BYTE_OFFSET) {
 			*val = spec->bit_offset / 8;
-			/* remember field size for load/store mem size */
-			sz = btf__resolve_size(spec->btf, acc->type_id);
+			/* remember field size for load/store mem size;
+			 * note, for arrays we care about individual element
+			 * sizes, not the overall array size
+			 */
+			t = skip_mods_and_typedefs(spec->btf, acc->type_id, &elem_id);
+			while (btf_is_array(t))
+				t = skip_mods_and_typedefs(spec->btf, btf_array(t)->type, &elem_id);
+			sz = btf__resolve_size(spec->btf, elem_id);
 			if (sz < 0)
 				return -EINVAL;
 			*field_sz = sz;
@@ -767,7 +773,17 @@ static int bpf_core_calc_field_relo(const char *prog_name,
 	case BPF_CORE_FIELD_BYTE_OFFSET:
 		*val = byte_off;
 		if (!bitfield) {
-			*field_sz = byte_sz;
+			/* remember field size for load/store mem size;
+			 * note, for arrays we care about individual element
+			 * sizes, not the overall array size
+			 */
+			t = skip_mods_and_typedefs(spec->btf, field_type_id, &elem_id);
+			while (btf_is_array(t))
+				t = skip_mods_and_typedefs(spec->btf, btf_array(t)->type, &elem_id);
+			sz = btf__resolve_size(spec->btf, elem_id);
+			if (sz < 0)
+				return -EINVAL;
+			*field_sz = sz;
 			*type_id = field_type_id;
 		}
 		break;