bpf: Use bpf_is_subprog to check for subprogs

We would like to know whether a bpf_prog corresponds to the main prog or
one of the subprogs. The current JIT implementations simply check this
using the func_idx in bpf_prog->aux->func_idx. When the index is 0, it
belongs to the main program, otherwise it corresponds to some
subprogram.

This will also be necessary to halt exception propagation while walking
the stack when an exception is thrown, so we add a simple helper
function to check this, named bpf_is_subprog, and convert existing JIT
implementations to also make use of it.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20230912233214.1518551-2-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

arch/arm64/net/bpf_jit_comp.c

@@ -288,7 +288,7 @@ static bool is_lsi_offset(int offset, int scale)
 static int build_prologue(struct jit_ctx *ctx, bool ebpf_from_cbpf)
 {
 	const struct bpf_prog *prog = ctx->prog;
-	const bool is_main_prog = prog->aux->func_idx == 0;
+	const bool is_main_prog = !bpf_is_subprog(prog);
 	const u8 r6 = bpf2a64[BPF_REG_6];
 	const u8 r7 = bpf2a64[BPF_REG_7];
 	const u8 r8 = bpf2a64[BPF_REG_8];

arch/s390/net/bpf_jit_comp.c

@@ -556,7 +556,7 @@ static void bpf_jit_prologue(struct bpf_jit *jit, struct bpf_prog *fp,
 	EMIT6_PCREL_RILC(0xc0040000, 0, jit->prologue_plt);
 	jit->prologue_plt_ret = jit->prg;
 
-	if (fp->aux->func_idx == 0) {
+	if (!bpf_is_subprog(fp)) {
 		/* Initialize the tail call counter in the main program. */
 		/* xc STK_OFF_TCCNT(4,%r15),STK_OFF_TCCNT(%r15) */
 		_EMIT6(0xd703f000 | STK_OFF_TCCNT, 0xf000 | STK_OFF_TCCNT);

arch/x86/net/bpf_jit_comp.c

@@ -1049,7 +1049,7 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image, u8 *rw_image
 
 	emit_prologue(&prog, bpf_prog->aux->stack_depth,
 		      bpf_prog_was_classic(bpf_prog), tail_call_reachable,
-		      bpf_prog->aux->func_idx != 0);
+		      bpf_is_subprog(bpf_prog));
 	push_callee_regs(&prog, callee_regs_used);
 
 	ilen = prog - temp;

include/linux/bpf.h

@@ -3194,4 +3194,9 @@ static inline gfp_t bpf_memcg_flags(gfp_t flags)
 	return flags;
 }
 
+static inline bool bpf_is_subprog(const struct bpf_prog *prog)
+{
+	return prog->aux->func_idx != 0;
+}
+
 #endif /* _LINUX_BPF_H */