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>

diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
index e924484..4e1f5e3 100644 (file)
--- a/arch/powerpc/mm/fault.c
+++ b/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);