uprobes: Move BUG_ON(UPROBE_SWBP_INSN_SIZE) from write_opcode() to install_breakpoint()

write_opcode() ensures that UPROBE_SWBP_INSN doesn't cross the
page boundary. This looks a bit confusing, the check does not
depend on vaddr and it is enough to do it only once right after
install_breakpoint()->arch_uprobe_analyze_insn().

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Acked-by: Ananth N Mavinakayanahalli <ananth@in.ibm.com>
Cc: Anton Arapov <anton@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20120615154350.GA9611@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/events/uprobes.c

@@ -210,7 +210,6 @@ static int write_opcode(struct arch_uprobe *auprobe, struct mm_struct *mm,
 	void *vaddr_old, *vaddr_new;
 	struct vm_area_struct *vma;
 	struct uprobe *uprobe;
-	unsigned long pgoff;
 	int ret;
 retry:
 	/* Read the page with vaddr into memory */
@@ -251,11 +250,7 @@ retry:
 	vaddr_new = kmap_atomic(new_page);
 
 	memcpy(vaddr_new, vaddr_old, PAGE_SIZE);
-
-	/* poke the new insn in, ASSUMES we don't cross page boundary */
-	pgoff = (vaddr & ~PAGE_MASK);
-	BUG_ON(pgoff + UPROBE_SWBP_INSN_SIZE > PAGE_SIZE);
-	memcpy(vaddr_new + pgoff, &opcode, UPROBE_SWBP_INSN_SIZE);
+	memcpy(vaddr_new + (vaddr & ~PAGE_MASK), &opcode, UPROBE_SWBP_INSN_SIZE);
 
 	kunmap_atomic(vaddr_new);
 	kunmap_atomic(vaddr_old);
@@ -699,6 +694,10 @@ install_breakpoint(struct uprobe *uprobe, struct mm_struct *mm,
 		if (ret)
 			return ret;
 
+		/* write_opcode() assumes we don't cross page boundary */
+		BUG_ON((uprobe->offset & ~PAGE_MASK) +
+				UPROBE_SWBP_INSN_SIZE > PAGE_SIZE);
+
 		uprobe->flags |= UPROBE_COPY_INSN;
 	}