[PATCH bpf-next v6 17/21] libbpf: Let the static linker carry .bpf_cleanup relocations

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

An object that carries a compiler-emitted exception cleanup table cannot be
linked today. The table's fields are byte offsets into a code section,
materialised by a 32-bit relocation against that section's symbol with the
offset itself as the implicit addend, and the linker rejects both halves of
that: the relocation type is not in the list it accepts, and a relocation
against an STT_SECTION symbol from a non-executable section is an outright
error.

Both spellings of that relocation have to be taken. LLVM emits
R_BPF_64_NODYLD32 for a .long against a section symbol; GNU as emits
R_BPF_64_ABS32, which is what bpf_reloc_type_lookup() maps BFD_RELOC_32 to.
They describe the same value, and the selftests are built with both
compilers.

Keying on the relocation type rather than the section name means any
non-executable section can reach the new arm, where an unconditional
refusal used to stand. That refusal was covering two things the arm now has
to do itself: a target may be SHT_NOBITS, which extend_sec() leaves with no
raw_data, and r_offset is alignment-checked only where the section holds
instructions. The arm rejects both, and bounds the offset against the
section size before writing through it.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
---
 tools/lib/bpf/linker.c | 36 +++++++++++++++++++++++++++++++++++-
 1 file changed, 35 insertions(+), 1 deletion(-)

diff --git a/tools/lib/bpf/linker.c b/tools/lib/bpf/linker.c
index 53f64a1a1f25..26a5e53df6f0 100644
--- a/tools/lib/bpf/linker.c
+++ b/tools/lib/bpf/linker.c
@@ -1036,7 +1036,8 @@ static int linker_sanity_check_elf_relos(struct src_obj *obj, struct src_sec *se
 		size_t sym_type = ELF64_R_TYPE(relo->r_info);
 
 		if (sym_type != R_BPF_64_64 && sym_type != R_BPF_64_32 &&
-		    sym_type != R_BPF_64_ABS64 && sym_type != R_BPF_64_ABS32) {
+		    sym_type != R_BPF_64_ABS64 && sym_type != R_BPF_64_ABS32 &&
+		    sym_type != R_BPF_64_NODYLD32) {
 			pr_warn("ELF relo #%d in section #%zu has unexpected type %zu in %s\n",
 				i, sec->sec_idx, sym_type, obj->filename);
 			return -EINVAL;
@@ -2258,6 +2259,7 @@ static int linker_append_elf_relos(struct bpf_linker *linker, struct src_obj *ob
 			if (ELF64_ST_TYPE(src_sym->st_info) == STT_SECTION) {
 				struct src_sec *sec = &obj->secs[src_sym->st_shndx];
 				struct bpf_insn *insn;
+				__u32 *val;
 
 				if (src_linked_sec->shdr->sh_flags & SHF_EXECINSTR) {
 					/* calls to the very first static function inside
@@ -2292,6 +2294,38 @@ static int linker_append_elf_relos(struct bpf_linker *linker, struct src_obj *ob
 					if (linker->swapped_endian)
 						off = bswap_64(off);
 					memcpy(ptr, &off, sizeof(off));
+				} else if (sym_type == R_BPF_64_NODYLD32 ||
+					   sym_type == R_BPF_64_ABS32) {
+					/*
+					 * A byte offset into a code section,
+					 * stored in place. LLVM spells this
+					 * relocation NODYLD32 and GNU as
+					 * spells it ABS32; being bytes, the
+					 * section's new start goes in as it
+					 * is, not scaled the way a call's
+					 * instruction index is above.
+					 *
+					 * r_offset is checked only for an
+					 * executable section, and SHT_NOBITS
+					 * has no raw_data, so bound it here --
+					 * subtracting, so it cannot wrap.
+					 */
+					if (!dst_linked_sec->raw_data ||
+					    dst_linked_sec->sec_sz < (int)sizeof(*val) ||
+					    dst_rel->r_offset % sizeof(*val) ||
+					    dst_rel->r_offset >
+					    (size_t)dst_linked_sec->sec_sz - sizeof(*val)) {
+						pr_warn("ELF relo #%d in section #%zu points outside the data of section '%s' in %s\n",
+							j, src_sec->sec_idx,
+							dst_linked_sec->sec_name,
+							obj->filename);
+						return -EINVAL;
+					}
+					val = dst_linked_sec->raw_data + dst_rel->r_offset;
+					if (linker->swapped_endian)
+						*val = bswap_32(bswap_32(*val) + sec->dst_off);
+					else
+						*val += sec->dst_off;
 				} else {
 					pr_warn("relocation against STT_SECTION in non-exec section is not supported!\n");
 					return -EINVAL;
-- 
2.53.0-Meta