arm64: compat: Work around uninitialized variable warning

Dan reports that smatch complains about a potential uninitialized
variable being used in the compat alignment fixup code.

The logic is not wrong per se, but we do end up using an uninitialized
variable if reading the instruction that triggered the alignment fault
from user space faults, even if the fault ensures that the uninitialized
value doesn't propagate any further.

Given that we just give up and return 1 if any fault occurs when reading
the instruction, let's get rid of the 'success handling' pattern that
captures the fault in a variable and aborts later, and instead, just
return 1 immediately if any of the get_user() calls result in an
exception.

Fixes: 3fc24ef3 ("arm64: compat: Implement misalignment fixups for multiword loads")
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <error27@gmail.com>
Link: https://lore.kernel.org/r/202304021214.gekJ8yRc-lkp@intel.com/Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20230404103625.2386382-1-ardb@kernel.orgSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/compat_alignment.c

@@ -314,36 +314,32 @@ int do_compat_alignment_fixup(unsigned long addr, struct pt_regs *regs)
 	int (*handler)(unsigned long addr, u32 instr, struct pt_regs *regs);
 	unsigned int type;
 	u32 instr = 0;
-	u16 tinstr = 0;
 	int isize = 4;
 	int thumb2_32b = 0;
-	int fault;
 
 	instrptr = instruction_pointer(regs);
 
 	if (compat_thumb_mode(regs)) {
 		__le16 __user *ptr = (__le16 __user *)(instrptr & ~1);
+		u16 tinstr, tinst2;
 
-		fault = alignment_get_thumb(regs, ptr, &tinstr);
-		if (!fault) {
-			if (IS_T32(tinstr)) {
-				/* Thumb-2 32-bit */
-				u16 tinst2;
-				fault = alignment_get_thumb(regs, ptr + 1, &tinst2);
-				instr = ((u32)tinstr << 16) | tinst2;
-				thumb2_32b = 1;
-			} else {
-				isize = 2;
-				instr = thumb2arm(tinstr);
-			}
+		if (alignment_get_thumb(regs, ptr, &tinstr))
+			return 1;
+
+		if (IS_T32(tinstr)) { /* Thumb-2 32-bit */
+			if (alignment_get_thumb(regs, ptr + 1, &tinst2))
+				return 1;
+			instr = ((u32)tinstr << 16) | tinst2;
+			thumb2_32b = 1;
+		} else {
+			isize = 2;
+			instr = thumb2arm(tinstr);
 		}
 	} else {
-		fault = alignment_get_arm(regs, (__le32 __user *)instrptr, &instr);
+		if (alignment_get_arm(regs, (__le32 __user *)instrptr, &instr))
+			return 1;
 	}
 
-	if (fault)
-		return 1;
-
 	switch (CODING_BITS(instr)) {
 	case 0x00000000:	/* 3.13.4 load/store instruction extensions */
 		if (LDSTHD_I_BIT(instr))