[PATCH] mm: kill check_user_page_readable

check_user_page_readable is a problematic variant of follow_page.  It's used
only by oprofile's i386 and arm backtrace code, at interrupt time, to
establish whether a userspace stackframe is currently readable.

This is problematic, because we want to push the page_table_lock down inside
follow_page, and later split it; whereas oprofile is doing a spin_trylock on
it (in the i386 case, forgotten in the arm case), and needs that to pin
perhaps two pages spanned by the stackframe (which might be covered by
different locks when we split).

I think oprofile is going about this in the wrong way: it doesn't need to know
the area is readable (neither i386 nor arm uses read protection of user
pages), it doesn't need to pin the memory, it should simply
__copy_from_user_inatomic, and see if that succeeds or not.  Sorry, but I've
not got around to devising the sparse __user annotations for this.

Then we can eliminate check_user_page_readable, and return to a single
follow_page without the __follow_page variants.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 13 insertions, 25 deletions

diff --git a/arch/i386/oprofile/backtrace.c b/arch/i386/oprofile/backtrace.c
index 65dfd2edb671..21654be3f73f 100644
--- a/arch/i386/oprofile/backtrace.c
+++ b/arch/i386/oprofile/backtrace.c
@@ -12,6 +12,7 @@
 #include <linux/sched.h>
 #include <linux/mm.h>
 #include <asm/ptrace.h>
+#include <asm/uaccess.h>
 
 struct frame_head {
         struct frame_head * ebp;
@@ -21,26 +22,22 @@ struct frame_head {
 static struct frame_head *
 dump_backtrace(struct frame_head * head)
 {
-        oprofile_add_trace(head->ret);
+        struct frame_head bufhead[2];
 
-        /* frame pointers should strictly progress back up the stack
-         * (towards higher addresses) */
-        if (head >= head->ebp)
+        /* Also check accessibility of one struct frame_head beyond */
+        if (!access_ok(VERIFY_READ, head, sizeof(bufhead)))
+                return NULL;
+        if (__copy_from_user_inatomic(bufhead, head, sizeof(bufhead)))
                 return NULL;
 
-        return head->ebp;
-}
-
-/* check that the page(s) containing the frame head are present */
-static int pages_present(struct frame_head * head)
-{
-        struct mm_struct * mm = current->mm;
+        oprofile_add_trace(bufhead[0].ret);
 
-        /* FIXME: only necessary once per page */
-        if (!check_user_page_readable(mm, (unsigned long)head))
-                return 0;
+        /* frame pointers should strictly progress back up the stack
+         * (towards higher addresses) */
+        if (head >= bufhead[0].ebp)
+                return NULL;
 
-        return check_user_page_readable(mm, (unsigned long)(head + 1));
+        return bufhead[0].ebp;
 }
 
 /*
@@ -97,15 +94,6 @@ x86_backtrace(struct pt_regs * const regs, unsigned int depth)
                 return;
         }
 
-#ifdef CONFIG_SMP
-        if (!spin_trylock(&current->mm->page_table_lock))
-                return;
-#endif
-
-        while (depth-- && head && pages_present(head))
+        while (depth-- && head)
                 head = dump_backtrace(head);
-
-#ifdef CONFIG_SMP
-        spin_unlock(&current->mm->page_table_lock);
-#endif
 }