[PATCH 4/9] powerpc/bpf: Handle large branch ranges with BPF_EXIT

[Naveen N. Rao]

In some scenarios, it is possible that the program epilogue is outside
the branch range for a BPF_EXIT instruction. Instead of rejecting such
programs, emit an indirect branch. We track the size of the bpf program
emitted after the initial run and do a second pass since BPF_EXIT can
end up emitting different number of instructions depending on the
program size.

Suggested-by: Jordan Niethe <jniethe5 at gmail.com>
Signed-off-by: Naveen N. Rao <naveen.n.rao at linux.vnet.ibm.com>
---
 arch/powerpc/net/bpf_jit.h        |  3 +++
 arch/powerpc/net/bpf_jit_comp.c   | 22 +++++++++++++++++++++-
 arch/powerpc/net/bpf_jit_comp32.c |  2 +-
 arch/powerpc/net/bpf_jit_comp64.c |  2 +-
 4 files changed, 26 insertions(+), 3 deletions(-)

diff --git a/arch/powerpc/net/bpf_jit.h b/arch/powerpc/net/bpf_jit.h
index 89bd744c2bffd4..4023de1698b9f5 100644
--- a/arch/powerpc/net/bpf_jit.h
+++ b/arch/powerpc/net/bpf_jit.h
@@ -126,6 +126,7 @@
 
 #define SEEN_FUNC	0x20000000 /* might call external helpers */
 #define SEEN_TAILCALL	0x40000000 /* uses tail calls */
+#define SEEN_BIG_PROG	0x80000000 /* large prog, >32MB */
 
 #define SEEN_VREG_MASK	0x1ff80000 /* Volatile registers r3-r12 */
 #define SEEN_NVREG_MASK	0x0003ffff /* Non volatile registers r14-r31 */
@@ -179,6 +180,8 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, struct codegen_context *
 void bpf_jit_build_prologue(u32 *image, struct codegen_context *ctx);
 void bpf_jit_build_epilogue(u32 *image, struct codegen_context *ctx);
 void bpf_jit_realloc_regs(struct codegen_context *ctx);
+int bpf_jit_emit_exit_insn(u32 *image, struct codegen_context *ctx,
+					int tmp_reg, unsigned long exit_addr);
 
 #endif
 
diff --git a/arch/powerpc/net/bpf_jit_comp.c b/arch/powerpc/net/bpf_jit_comp.c
index fcbf7a917c566e..3204872fbf2738 100644
--- a/arch/powerpc/net/bpf_jit_comp.c
+++ b/arch/powerpc/net/bpf_jit_comp.c
@@ -72,6 +72,21 @@ static int bpf_jit_fixup_subprog_calls(struct bpf_prog *fp, u32 *image,
 	return 0;
 }
 
+int bpf_jit_emit_exit_insn(u32 *image, struct codegen_context *ctx,
+					int tmp_reg, unsigned long exit_addr)
+{
+	if (!(ctx->seen & SEEN_BIG_PROG) && is_offset_in_branch_range(exit_addr)) {
+		PPC_JMP(exit_addr);
+	} else {
+		ctx->seen |= SEEN_BIG_PROG;
+		PPC_FUNC_ADDR(tmp_reg, (unsigned long)image + exit_addr);
+		EMIT(PPC_RAW_MTCTR(tmp_reg));
+		EMIT(PPC_RAW_BCTR());
+	}
+
+	return 0;
+}
+
 struct powerpc64_jit_data {
 	struct bpf_binary_header *header;
 	u32 *addrs;
@@ -155,12 +170,17 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *fp)
 		goto out_addrs;
 	}
 
+	if (!is_offset_in_branch_range((long)cgctx.idx * 4))
+		cgctx.seen |= SEEN_BIG_PROG;
+
 	/*
 	 * If we have seen a tail call, we need a second pass.
 	 * This is because bpf_jit_emit_common_epilogue() is called
 	 * from bpf_jit_emit_tail_call() with a not yet stable ctx->seen.
+	 * We also need a second pass if we ended up with too large
+	 * a program so as to fix branches.
 	 */
-	if (cgctx.seen & SEEN_TAILCALL) {
+	if (cgctx.seen & (SEEN_TAILCALL | SEEN_BIG_PROG)) {
 		cgctx.idx = 0;
 		if (bpf_jit_build_body(fp, 0, &cgctx, addrs, false)) {
 			fp = org_fp;
diff --git a/arch/powerpc/net/bpf_jit_comp32.c b/arch/powerpc/net/bpf_jit_comp32.c
index a74d52204f8da2..d2a67574a23066 100644
--- a/arch/powerpc/net/bpf_jit_comp32.c
+++ b/arch/powerpc/net/bpf_jit_comp32.c
@@ -852,7 +852,7 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, struct codegen_context *
 			 * we'll just fall through to the epilogue.
 			 */
 			if (i != flen - 1)
-				PPC_JMP(exit_addr);
+				bpf_jit_emit_exit_insn(image, ctx, tmp_reg, exit_addr);
 			/* else fall through to the epilogue */
 			break;
 
diff --git a/arch/powerpc/net/bpf_jit_comp64.c b/arch/powerpc/net/bpf_jit_comp64.c
index f06c62089b1457..3351a866ef6207 100644
--- a/arch/powerpc/net/bpf_jit_comp64.c
+++ b/arch/powerpc/net/bpf_jit_comp64.c
@@ -761,7 +761,7 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, struct codegen_context *
 			 * we'll just fall through to the epilogue.
 			 */
 			if (i != flen - 1)
-				PPC_JMP(exit_addr);
+				bpf_jit_emit_exit_insn(image, ctx, b2p[TMP_REG_1], exit_addr);
 			/* else fall through to the epilogue */
 			break;
 
-- 
2.33.0