powerpc/mm: Cleanup check for stack expansion

When hitting below a VM_GROWSDOWN vma (typically growing the stack),
we check whether it's a valid stack-growing instruction and we
check the distance to GPR1. This is largely open coded with lots
of comments, so move it out to a helper.

While at it, make store_update_sp a boolean.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/mm/fault.c

@@ -71,15 +71,15 @@ static inline bool notify_page_fault(struct pt_regs *regs)
  * Check whether the instruction at regs->nip is a store using
  * an update addressing form which will update r1.
  */
-static int store_updates_sp(struct pt_regs *regs)
+static bool store_updates_sp(struct pt_regs *regs)
 {
 	unsigned int inst;
 
 	if (get_user(inst, (unsigned int __user *)regs->nip))
-		return 0;
+		return false;
 	/* check for 1 in the rA field */
 	if (((inst >> 16) & 0x1f) != 1)
-		return 0;
+		return false;
 	/* check major opcode */
 	switch (inst >> 26) {
 	case 37:	/* stwu */
@@ -87,7 +87,7 @@ static int store_updates_sp(struct pt_regs *regs)
 	case 45:	/* sthu */
 	case 53:	/* stfsu */
 	case 55:	/* stfdu */
-		return 1;
+		return true;
 	case 62:	/* std or stdu */
 		return (inst & 3) == 1;
 	case 31:
@@ -99,10 +99,10 @@ static int store_updates_sp(struct pt_regs *regs)
 		case 439:	/* sthux */
 		case 695:	/* stfsux */
 		case 759:	/* stfdux */
-			return 1;
+			return true;
 		}
 	}
-	return 0;
+	return false;
 }
 /*
  * do_page_fault error handling helpers
@@ -222,6 +222,43 @@ static bool bad_kernel_fault(bool is_exec, unsigned long error_code,
 	return is_exec || (address >= TASK_SIZE);
 }
 
+static bool bad_stack_expansion(struct pt_regs *regs, unsigned long address,
+				struct vm_area_struct *vma,
+				bool store_update_sp)
+{
+	/*
+	 * N.B. The POWER/Open ABI allows programs to access up to
+	 * 288 bytes below the stack pointer.
+	 * The kernel signal delivery code writes up to about 1.5kB
+	 * below the stack pointer (r1) before decrementing it.
+	 * The exec code can write slightly over 640kB to the stack
+	 * before setting the user r1.  Thus we allow the stack to
+	 * expand to 1MB without further checks.
+	 */
+	if (address + 0x100000 < vma->vm_end) {
+		/* get user regs even if this fault is in kernel mode */
+		struct pt_regs *uregs = current->thread.regs;
+		if (uregs == NULL)
+			return true;
+
+		/*
+		 * A user-mode access to an address a long way below
+		 * the stack pointer is only valid if the instruction
+		 * is one which would update the stack pointer to the
+		 * address accessed if the instruction completed,
+		 * i.e. either stwu rs,n(r1) or stwux rs,r1,rb
+		 * (or the byte, halfword, float or double forms).
+		 *
+		 * If we don't check this then any write to the area
+		 * between the last mapped region and the stack will
+		 * expand the stack rather than segfaulting.
+		 */
+		if (address + 2048 < uregs->gpr[1] && !store_update_sp)
+			return true;
+	}
+	return false;
+}
+
 static bool access_error(bool is_write, bool is_exec,
 			 struct vm_area_struct *vma)
 {
@@ -350,7 +387,7 @@ static int __do_page_fault(struct pt_regs *regs, unsigned long address,
 	int is_user = user_mode(regs);
 	int is_write = page_fault_is_write(error_code);
 	int fault, major = 0;
-	int store_update_sp = 0;
+	bool store_update_sp = false;
 
 #ifdef CONFIG_PPC_ICSWX
 	/*
@@ -458,36 +495,11 @@ retry:
 	if (unlikely(!(vma->vm_flags & VM_GROWSDOWN)))
 		return bad_area(regs, address);
 
-	/*
-	 * N.B. The POWER/Open ABI allows programs to access up to
-	 * 288 bytes below the stack pointer.
-	 * The kernel signal delivery code writes up to about 1.5kB
-	 * below the stack pointer (r1) before decrementing it.
-	 * The exec code can write slightly over 640kB to the stack
-	 * before setting the user r1.  Thus we allow the stack to
-	 * expand to 1MB without further checks.
-	 */
-	if (address + 0x100000 < vma->vm_end) {
-		/* get user regs even if this fault is in kernel mode */
-		struct pt_regs *uregs = current->thread.regs;
-		if (uregs == NULL)
-			return bad_area(regs, address);
+	/* The stack is being expanded, check if it's valid */
+	if (unlikely(bad_stack_expansion(regs, address, vma, store_update_sp)))
+		return bad_area(regs, address);
 
-		/*
-		 * A user-mode access to an address a long way below
-		 * the stack pointer is only valid if the instruction
-		 * is one which would update the stack pointer to the
-		 * address accessed if the instruction completed,
-		 * i.e. either stwu rs,n(r1) or stwux rs,r1,rb
-		 * (or the byte, halfword, float or double forms).
-		 *
-		 * If we don't check this then any write to the area
-		 * between the last mapped region and the stack will
-		 * expand the stack rather than segfaulting.
-		 */
-		if (address + 2048 < uregs->gpr[1] && !store_update_sp)
-			return bad_area(regs, address);
-	}
+	/* Try to expand it */
 	if (unlikely(expand_stack(vma, address)))
 		return bad_area(regs, address);