arm64: insn: Add return statements after BUG_ON()

Following a recent series of enhancements to the insn code the ARMv8
allnoconfig build has been generating a large number of warnings in the
form of:

arch/arm64/kernel/insn.c:689:8: warning: 'insn' may be used uninitialized in this function [-Wmaybe-uninitialized]

This is because BUG() and related macros can be compiled out so we get
execution paths which normally result in a panic compiling out to noops
instead.

I wasn't able to immediately identify a sensible return value to use in
these cases so just return AARCH64_BREAK_FAULT - this is all "should
never happen" code so hopefully it never has a practical impact.

Signed-off-by: Mark Brown <broonie@kernel.org>
[catalin.marinas@arm.com: AARCH64_BREAK_FAULT definition contributed by Daniel Borkmann]
[catalin.marinas@arm.com: replace return 0 with AARCH64_BREAK_FAULT]
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/include/asm/debug-monitors.h

@@ -48,9 +48,11 @@
 /*
  * #imm16 values used for BRK instruction generation
  * Allowed values for kgbd are 0x400 - 0x7ff
+ * 0x100: for triggering a fault on purpose (reserved)
  * 0x400: for dynamic BRK instruction
  * 0x401: for compile time BRK instruction
  */
+#define FAULT_BRK_IMM			0x100
 #define KGDB_DYN_DGB_BRK_IMM		0x400
 #define KDBG_COMPILED_DBG_BRK_IMM	0x401
 
@@ -60,6 +62,12 @@
  */
 #define AARCH64_BREAK_MON	0xd4200000
 
+/*
+ * BRK instruction for provoking a fault on purpose
+ * Unlike kgdb, #imm16 value with unallocated handler is used for faulting.
+ */
+#define AARCH64_BREAK_FAULT	(AARCH64_BREAK_MON | (FAULT_BRK_IMM << 5))
+
 /*
  * Extract byte from BRK instruction
  */

arch/arm64/kernel/insn.c

@@ -22,7 +22,9 @@
 #include <linux/smp.h>
 #include <linux/stop_machine.h>
 #include <linux/uaccess.h>
+
 #include <asm/cacheflush.h>
+#include <asm/debug-monitors.h>
 #include <asm/insn.h>
 
 #define AARCH64_INSN_SF_BIT	BIT(31)
@@ -388,6 +390,7 @@ u32 __kprobes aarch64_insn_gen_branch_imm(unsigned long pc, unsigned long addr,
 		break;
 	default:
 		BUG_ON(1);
+		return AARCH64_BREAK_FAULT;
 	}
 
 	return aarch64_insn_encode_immediate(AARCH64_INSN_IMM_26, insn,
@@ -413,6 +416,7 @@ u32 aarch64_insn_gen_comp_branch_imm(unsigned long pc, unsigned long addr,
 		break;
 	default:
 		BUG_ON(1);
+		return AARCH64_BREAK_FAULT;
 	}
 
 	switch (variant) {
@@ -423,6 +427,7 @@ u32 aarch64_insn_gen_comp_branch_imm(unsigned long pc, unsigned long addr,
 		break;
 	default:
 		BUG_ON(1);
+		return AARCH64_BREAK_FAULT;
 	}
 
 	insn = aarch64_insn_encode_register(AARCH64_INSN_REGTYPE_RT, insn, reg);
@@ -475,6 +480,7 @@ u32 aarch64_insn_gen_branch_reg(enum aarch64_insn_register reg,
 		break;
 	default:
 		BUG_ON(1);
+		return AARCH64_BREAK_FAULT;
 	}
 
 	return aarch64_insn_encode_register(AARCH64_INSN_REGTYPE_RN, insn, reg);
@@ -497,6 +503,7 @@ u32 aarch64_insn_gen_load_store_reg(enum aarch64_insn_register reg,
 		break;
 	default:
 		BUG_ON(1);
+		return AARCH64_BREAK_FAULT;
 	}
 
 	insn = aarch64_insn_encode_ldst_size(size, insn);
@@ -535,6 +542,7 @@ u32 aarch64_insn_gen_load_store_pair(enum aarch64_insn_register reg1,
 		break;
 	default:
 		BUG_ON(1);
+		return AARCH64_BREAK_FAULT;
 	}
 
 	switch (variant) {
@@ -553,6 +561,7 @@ u32 aarch64_insn_gen_load_store_pair(enum aarch64_insn_register reg1,
 		break;
 	default:
 		BUG_ON(1);
+		return AARCH64_BREAK_FAULT;
 	}
 
 	insn = aarch64_insn_encode_register(AARCH64_INSN_REGTYPE_RT, insn,
@@ -590,6 +599,7 @@ u32 aarch64_insn_gen_add_sub_imm(enum aarch64_insn_register dst,
 		break;
 	default:
 		BUG_ON(1);
+		return AARCH64_BREAK_FAULT;
 	}
 
 	switch (variant) {
@@ -600,6 +610,7 @@ u32 aarch64_insn_gen_add_sub_imm(enum aarch64_insn_register dst,
 		break;
 	default:
 		BUG_ON(1);
+		return AARCH64_BREAK_FAULT;
 	}
 
 	BUG_ON(imm & ~(SZ_4K - 1));
@@ -632,6 +643,7 @@ u32 aarch64_insn_gen_bitfield(enum aarch64_insn_register dst,
 		break;
 	default:
 		BUG_ON(1);
+		return AARCH64_BREAK_FAULT;
 	}
 
 	switch (variant) {
@@ -644,6 +656,7 @@ u32 aarch64_insn_gen_bitfield(enum aarch64_insn_register dst,
 		break;
 	default:
 		BUG_ON(1);
+		return AARCH64_BREAK_FAULT;
 	}
 
 	BUG_ON(immr & ~mask);
@@ -677,6 +690,7 @@ u32 aarch64_insn_gen_movewide(enum aarch64_insn_register dst,
 		break;
 	default:
 		BUG_ON(1);
+		return AARCH64_BREAK_FAULT;
 	}
 
 	BUG_ON(imm & ~(SZ_64K - 1));
@@ -692,6 +706,7 @@ u32 aarch64_insn_gen_movewide(enum aarch64_insn_register dst,
 		break;
 	default:
 		BUG_ON(1);
+		return AARCH64_BREAK_FAULT;
 	}
 
 	insn |= (shift >> 4) << 21;
@@ -725,6 +740,7 @@ u32 aarch64_insn_gen_add_sub_shifted_reg(enum aarch64_insn_register dst,
 		break;
 	default:
 		BUG_ON(1);
+		return AARCH64_BREAK_FAULT;
 	}
 
 	switch (variant) {
@@ -737,6 +753,7 @@ u32 aarch64_insn_gen_add_sub_shifted_reg(enum aarch64_insn_register dst,
 		break;
 	default:
 		BUG_ON(1);
+		return AARCH64_BREAK_FAULT;
 	}
 
 
@@ -769,6 +786,7 @@ u32 aarch64_insn_gen_data1(enum aarch64_insn_register dst,
 		break;
 	default:
 		BUG_ON(1);
+		return AARCH64_BREAK_FAULT;
 	}
 
 	switch (variant) {
@@ -779,6 +797,7 @@ u32 aarch64_insn_gen_data1(enum aarch64_insn_register dst,
 		break;
 	default:
 		BUG_ON(1);
+		return AARCH64_BREAK_FAULT;
 	}
 
 	insn = aarch64_insn_encode_register(AARCH64_INSN_REGTYPE_RD, insn, dst);
@@ -815,6 +834,7 @@ u32 aarch64_insn_gen_data2(enum aarch64_insn_register dst,
 		break;
 	default:
 		BUG_ON(1);
+		return AARCH64_BREAK_FAULT;
 	}
 
 	switch (variant) {
@@ -825,6 +845,7 @@ u32 aarch64_insn_gen_data2(enum aarch64_insn_register dst,
 		break;
 	default:
 		BUG_ON(1);
+		return AARCH64_BREAK_FAULT;
 	}
 
 	insn = aarch64_insn_encode_register(AARCH64_INSN_REGTYPE_RD, insn, dst);
@@ -852,6 +873,7 @@ u32 aarch64_insn_gen_data3(enum aarch64_insn_register dst,
 		break;
 	default:
 		BUG_ON(1);
+		return AARCH64_BREAK_FAULT;
 	}
 
 	switch (variant) {
@@ -862,6 +884,7 @@ u32 aarch64_insn_gen_data3(enum aarch64_insn_register dst,
 		break;
 	default:
 		BUG_ON(1);
+		return AARCH64_BREAK_FAULT;
 	}
 
 	insn = aarch64_insn_encode_register(AARCH64_INSN_REGTYPE_RD, insn, dst);
@@ -911,6 +934,7 @@ u32 aarch64_insn_gen_logical_shifted_reg(enum aarch64_insn_register dst,
 		break;
 	default:
 		BUG_ON(1);
+		return AARCH64_BREAK_FAULT;
 	}
 
 	switch (variant) {
@@ -923,6 +947,7 @@ u32 aarch64_insn_gen_logical_shifted_reg(enum aarch64_insn_register dst,
 		break;
 	default:
 		BUG_ON(1);
+		return AARCH64_BREAK_FAULT;
 	}