[PATCH v2 2/3] powerpc: sstep: Fix store and update emulation

[Sandipan Das <sandipan@linux.ibm.com>]

The Power ISA says that the fixed-point store and update
instructions must not use R0 for the base address (RA).
In this case, the instruction is invalid. This applies
to the following instructions.
  * Store Byte with Update (stbu)
  * Store Byte with Update Indexed (stbux)
  * Store Halfword with Update (sthu)
  * Store Halfword with Update Indexed (sthux)
  * Store Word with Update (stwu)
  * Store Word with Update Indexed (stwux)
  * Store Doubleword with Update (stdu)
  * Store Doubleword with Update Indexed (stdux)

To remove any inconsistencies, this adds an additional check
for the aforementioned instructions to make sure that they
are treated as unknown by the emulation infrastructure when
RA = 0. The kernel will then fallback to executing the
instruction on hardware.

Fixes: 0016a4cf5582 ("powerpc: Emulate most Book I instructions in emulate_step()")
Reviewed-by: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>
---
Previous versions can be found at:
v1: https://lore.kernel.org/linuxppc-dev/20201119054139.244083-2-sandipan@linux.ibm.com/

Changes in v2:
- Jump to unknown_opcode instead of returning -1 for invalid
  instruction forms.

---
 arch/powerpc/lib/sstep.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/arch/powerpc/lib/sstep.c b/arch/powerpc/lib/sstep.c
index db824fec6165..230d1ae77ef5 100644
--- a/arch/powerpc/lib/sstep.c
+++ b/arch/powerpc/lib/sstep.c
@@ -2301,17 +2301,23 @@ int analyse_instr(struct instruction_op *op, const struct pt_regs *regs,
 
 		case 149:	/* stdx */
 		case 181:	/* stdux */
+			if (u && ra == 0)
+				goto unknown_opcode;
 			op->type = MKOP(STORE, u, 8);
 			break;
 #endif
 
 		case 151:	/* stwx */
 		case 183:	/* stwux */
+			if (u && ra == 0)
+				goto unknown_opcode;
 			op->type = MKOP(STORE, u, 4);
 			break;
 
 		case 215:	/* stbx */
 		case 247:	/* stbux */
+			if (u && ra == 0)
+				goto unknown_opcode;
 			op->type = MKOP(STORE, u, 1);
 			break;
 
@@ -2340,6 +2346,8 @@ int analyse_instr(struct instruction_op *op, const struct pt_regs *regs,
 
 		case 407:	/* sthx */
 		case 439:	/* sthux */
+			if (u && ra == 0)
+				goto unknown_opcode;
 			op->type = MKOP(STORE, u, 2);
 			break;
 
@@ -2684,12 +2692,16 @@ int analyse_instr(struct instruction_op *op, const struct pt_regs *regs,
 
 	case 36:	/* stw */
 	case 37:	/* stwu */
+		if (u && ra == 0)
+			goto unknown_opcode;
 		op->type = MKOP(STORE, u, 4);
 		op->ea = dform_ea(word, regs);
 		break;
 
 	case 38:	/* stb */
 	case 39:	/* stbu */
+		if (u && ra == 0)
+			goto unknown_opcode;
 		op->type = MKOP(STORE, u, 1);
 		op->ea = dform_ea(word, regs);
 		break;
@@ -2712,6 +2724,8 @@ int analyse_instr(struct instruction_op *op, const struct pt_regs *regs,
 
 	case 44:	/* sth */
 	case 45:	/* sthu */
+		if (u && ra == 0)
+			goto unknown_opcode;
 		op->type = MKOP(STORE, u, 2);
 		op->ea = dform_ea(word, regs);
 		break;
@@ -2890,6 +2904,8 @@ int analyse_instr(struct instruction_op *op, const struct pt_regs *regs,
 			op->type = MKOP(STORE, 0, 8);
 			break;
 		case 1:		/* stdu */
+			if (ra == 0)
+				goto unknown_opcode;
 			op->type = MKOP(STORE, UPDATE, 8);
 			break;
 		case 2:		/* stq */
-- 
2.25.1