[PATCH 31/32] s390/bpf: Simplify exception table entry handling

[Heiko Carstens]

With exception table lookup always pointing to the faulting instruction,
only a single exception table entry per probe instruction is needed.

Remove the second exception table entry, the extra nop instruction
emitted between/after probe instructions, and simplify probe handling
in the BPF JIT.

Signed-off-by: Heiko Carstens <hca@xxxxxxxxxxxxx>
---
 arch/s390/net/bpf_jit_comp.c | 78 ++++++++++--------------------------
 1 file changed, 21 insertions(+), 57 deletions(-)

diff --git a/arch/s390/net/bpf_jit_comp.c b/arch/s390/net/bpf_jit_comp.c
index c4b47070bb59..9f1632e068e1 100644
--- a/arch/s390/net/bpf_jit_comp.c
+++ b/arch/s390/net/bpf_jit_comp.c
@@ -723,7 +723,6 @@ bool ex_handler_bpf(const struct exception_table_entry *x, struct pt_regs *regs)
  */
 struct bpf_jit_probe {
 	int prg;	/* JITed instruction offset */
-	int nop_prg;	/* JITed nop offset */
 	int reg;	/* Register to clear on exception */
 	int arena_reg;	/* Register to use for arena addressing */
 };
@@ -731,29 +730,10 @@ struct bpf_jit_probe {
 static void bpf_jit_probe_init(struct bpf_jit_probe *probe)
 {
 	probe->prg = -1;
-	probe->nop_prg = -1;
 	probe->reg = -1;
 	probe->arena_reg = REG_0;
 }
 
-/*
- * Handlers of certain exceptions leave psw.addr pointing to the instruction
- * directly after the failing one. Therefore, create two exception table
- * entries and also add a nop in case two probing instructions come directly
- * after each other.
- */
-static void bpf_jit_probe_emit_nop(struct bpf_jit *jit,
-				   struct bpf_jit_probe *probe)
-{
-	if (probe->prg == -1 || probe->nop_prg != -1)
-		/* The probe is not armed or nop is already emitted. */
-		return;
-
-	probe->nop_prg = jit->prg;
-	/* bcr 0,%0 */
-	_EMIT2(0x0700);
-}
-
 static void bpf_jit_probe_load_pre(struct bpf_jit *jit, struct bpf_insn *insn,
 				   struct bpf_jit_probe *probe)
 {
@@ -815,47 +795,36 @@ static int bpf_jit_probe_post(struct bpf_jit *jit, struct bpf_prog *fp,
 			      struct bpf_jit_probe *probe)
 {
 	struct exception_table_entry *ex;
-	int i, prg;
 	s64 delta;
-	u8 *insn;
 
 	if (probe->prg == -1)
 		/* The probe is not armed. */
 		return 0;
-	bpf_jit_probe_emit_nop(jit, probe);
 	if (!fp->aux->extable)
 		/* Do nothing during early JIT passes. */
 		return 0;
-	insn = jit->prg_buf + probe->prg;
-	if (WARN_ON_ONCE(probe->prg + insn_length(*insn) != probe->nop_prg))
-		/* JIT bug - gap between probe and nop instructions. */
+	if (WARN_ON_ONCE(jit->excnt >= fp->aux->num_exentries))
+		/* Verifier bug - not enough entries. */
 		return -1;
-	for (i = 0; i < 2; i++) {
-		if (WARN_ON_ONCE(jit->excnt >= fp->aux->num_exentries))
-			/* Verifier bug - not enough entries. */
-			return -1;
-		ex = &fp->aux->extable[jit->excnt];
-		/* Add extable entries for probe and nop instructions. */
-		prg = i == 0 ? probe->prg : probe->nop_prg;
-		delta = jit->prg_buf + prg - (u8 *)&ex->insn;
-		if (WARN_ON_ONCE(delta < INT_MIN || delta > INT_MAX))
-			/* JIT bug - code and extable must be close. */
-			return -1;
-		ex->insn = delta;
-		/*
-		 * Land on the current instruction. Note that the extable
-		 * infrastructure ignores the fixup field; it is handled by
-		 * ex_handler_bpf().
-		 */
-		delta = jit->prg_buf + jit->prg - (u8 *)&ex->fixup;
-		if (WARN_ON_ONCE(delta < INT_MIN || delta > INT_MAX))
-			/* JIT bug - landing pad and extable must be close. */
-			return -1;
-		ex->fixup = delta;
-		ex->type = EX_TYPE_BPF;
-		ex->data = probe->reg;
-		jit->excnt++;
-	}
+	ex = &fp->aux->extable[jit->excnt];
+	delta = jit->prg_buf + probe->prg - (u8 *)&ex->insn;
+	if (WARN_ON_ONCE(delta < INT_MIN || delta > INT_MAX))
+		/* JIT bug - code and extable must be close. */
+		return -1;
+	ex->insn = delta;
+	/*
+	 * Land on the current instruction. Note that the extable
+	 * infrastructure ignores the fixup field; it is handled by
+	 * ex_handler_bpf().
+	 */
+	delta = jit->prg_buf + jit->prg - (u8 *)&ex->fixup;
+	if (WARN_ON_ONCE(delta < INT_MIN || delta > INT_MAX))
+		/* JIT bug - landing pad and extable must be close. */
+		return -1;
+	ex->fixup = delta;
+	ex->type = EX_TYPE_BPF;
+	ex->data = probe->reg;
+	jit->excnt++;
 	return 0;
 }
 
@@ -1705,7 +1674,6 @@ static noinline int bpf_jit_insn(struct bpf_jit *jit, struct bpf_prog *fp,
 			EMIT6_DISP_LH(0xe3000000,
 				      is32 ? 0x0058 : 0x0004, REG_W0, REG_0,
 				      load_probe.arena_reg, off);
-			bpf_jit_probe_emit_nop(jit, &load_probe);
 			/* Reuse {ly|lg}'s arena_reg for {csy|csg}. */
 			if (load_probe.prg != -1) {
 				probe.prg = jit->prg;
@@ -1716,7 +1684,6 @@ static noinline int bpf_jit_insn(struct bpf_jit *jit, struct bpf_prog *fp,
 			/* 0: {csy|csg} %w0,%src,off(%arena) */
 			EMIT6_DISP_LH(0xeb000000, is32 ? 0x0014 : 0x0030,
 				      REG_W0, src_reg, probe.arena_reg, off);
-			bpf_jit_probe_emit_nop(jit, &probe);
 			/* brc 4,0b */
 			EMIT4_PCREL_RIC(0xa7040000, 4, loop_start);
 			/* {llgfr|lgr} %src,%w0 */
@@ -2364,9 +2331,6 @@ static struct bpf_binary_header *bpf_jit_alloc(struct bpf_jit *jit,
 			 */
 			fp->aux->num_exentries += 1;
 	}
-	/* We need two entries per insn. */
-	fp->aux->num_exentries *= 2;
-
 	code_size = roundup(jit->size,
 			    __alignof__(struct exception_table_entry));
 	extable_size = fp->aux->num_exentries *
-- 
2.53.0