[RFC/PATCH] libbpf: Store zero fd to fd_array for loader kfunc relocation

[Jiri Olsa <jolsa@kernel.org>]

When moving some of the test kfuncs to bpf_testmod I hit an issue
when some of the object's kfuncs are in module and some in vmlinux.

The problem is that both vmlinux and module kfuncs get btf_fd_idx
index into fd_array, but we store to it the BTF fd value only for
module's kfunc.

Then after the program is loaded we check if fd_array[btf_fd_idx] != 0
and close the fd.

When the object has kfuncs from both vmlinux and module, the fd from
fd_array[btf_fd_idx] from previous load will be there for vmlinux kfunc
and we close unrelated fd (of the program we just loaded in my case).

Not sure if there's easier way to clear the fd_array between the
loads, but the change below seems to fix the issue for me.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
---
 tools/lib/bpf/gen_loader.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/tools/lib/bpf/gen_loader.c b/tools/lib/bpf/gen_loader.c
index 83e8e3bfd8ff..e247f62aed49 100644
--- a/tools/lib/bpf/gen_loader.c
+++ b/tools/lib/bpf/gen_loader.c
@@ -703,17 +703,17 @@ static void emit_relo_kfunc_btf(struct bpf_gen *gen, struct ksym_relo_desc *relo
 	/* obtain fd in BPF_REG_9 */
 	emit(gen, BPF_MOV64_REG(BPF_REG_9, BPF_REG_7));
 	emit(gen, BPF_ALU64_IMM(BPF_RSH, BPF_REG_9, 32));
-	/* jump to fd_array store if fd denotes module BTF */
-	emit(gen, BPF_JMP_IMM(BPF_JNE, BPF_REG_9, 0, 2));
-	/* set the default value for off */
-	emit(gen, BPF_ST_MEM(BPF_H, BPF_REG_8, offsetof(struct bpf_insn, off), 0));
-	/* skip BTF fd store for vmlinux BTF */
-	emit(gen, BPF_JMP_IMM(BPF_JA, 0, 0, 4));
 	/* load fd_array slot pointer */
 	emit2(gen, BPF_LD_IMM64_RAW_FULL(BPF_REG_0, BPF_PSEUDO_MAP_IDX_VALUE,
 					 0, 0, 0, blob_fd_array_off(gen, btf_fd_idx)));
 	/* store BTF fd in slot */
 	emit(gen, BPF_STX_MEM(BPF_W, BPF_REG_0, BPF_REG_9, 0));
+	/* jump to insn[insn_idx].off store if fd denotes module BTF */
+	emit(gen, BPF_JMP_IMM(BPF_JNE, BPF_REG_9, 0, 2));
+	/* set the default value for off */
+	emit(gen, BPF_ST_MEM(BPF_H, BPF_REG_8, offsetof(struct bpf_insn, off), 0));
+	/* skip BTF fd store for vmlinux BTF */
+	emit(gen, BPF_JMP_IMM(BPF_JA, 0, 0, 1));
 	/* store index into insn[insn_idx].off */
 	emit(gen, BPF_ST_MEM(BPF_H, BPF_REG_8, offsetof(struct bpf_insn, off), btf_fd_idx));
 log:
-- 
2.40.1