Commit a506d6ce authored by Alexei Starovoitov's avatar Alexei Starovoitov Committed by Andrii Nakryiko
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libbpf: Fix ld_imm64 copy logic for ksym in light skeleton. 



Unlike normal libbpf the light skeleton 'loader' program is doing
btf_find_by_name_kind() call at run-time to find ksym in the kernel and
populate its {btf_id, btf_obj_fd} pair in ld_imm64 insn. To avoid doing the
search multiple times for the same ksym it remembers the first patched ld_imm64
insn and copies {btf_id, btf_obj_fd} from it into subsequent ld_imm64 insn.
Fix a bug in copying logic, since it may incorrectly clear BPF_PSEUDO_BTF_ID flag.

Also replace always true if (btf_obj_fd >= 0) check with unconditional JMP_JA
to clarify the code.

Fixes: d995816b ("libbpf: Avoid reload of imm for weak, unresolved, repeating ksym")
Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
Signed-off-by: default avatarAndrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20230319203014.55866-1-alexei.starovoitov@gmail.com
parent 08ff1c9f

tools/lib/bpf/gen_loader.c

+6 −4
Original line number Diff line number Diff line
@@ -804,11 +804,13 @@ static void emit_relo_ksym_btf(struct bpf_gen *gen, struct ksym_relo_desc *relo, 
		return;

	/* try to copy from existing ldimm64 insn */

	if (kdesc->ref > 1) {

		move_blob2blob(gen, insn + offsetof(struct bpf_insn, imm), 4,

			       kdesc->insn + offsetof(struct bpf_insn, imm));

		move_blob2blob(gen, insn + sizeof(struct bpf_insn) + offsetof(struct bpf_insn, imm), 4,

			       kdesc->insn + sizeof(struct bpf_insn) + offsetof(struct bpf_insn, imm));

		/* jump over src_reg adjustment if imm is not 0, reuse BPF_REG_0 from move_blob2blob */

		move_blob2blob(gen, insn + offsetof(struct bpf_insn, imm), 4,

			       kdesc->insn + offsetof(struct bpf_insn, imm));

		/* jump over src_reg adjustment if imm (btf_id) is not 0, reuse BPF_REG_0 from move_blob2blob

		 * If btf_id is zero, clear BPF_PSEUDO_BTF_ID flag in src_reg of ld_imm64 insn

		 */

		emit(gen, BPF_JMP_IMM(BPF_JNE, BPF_REG_0, 0, 3));

		goto clear_src_reg;

	}
@@ -831,7 +833,7 @@ static void emit_relo_ksym_btf(struct bpf_gen *gen, struct ksym_relo_desc *relo, 
	emit(gen, BPF_STX_MEM(BPF_W, BPF_REG_8, BPF_REG_7,

			      sizeof(struct bpf_insn) + offsetof(struct bpf_insn, imm)));

	/* skip src_reg adjustment */

	emit(gen, BPF_JMP_IMM(BPF_JSGE, BPF_REG_7, 0, 3));

	emit(gen, BPF_JMP_IMM(BPF_JA, 0, 0, 3));

clear_src_reg:

	/* clear bpf_object__relocate_data's src_reg assignment, otherwise we get a verifier failure */

	reg_mask = src_reg_mask();