proc: check vma->vm_file before dereferencing

Commit 7b540d0 ("proc_map_files_readdir(): don't bother with grabbing files") switched proc_map_files_readdir() to use @f_mode directly instead of grabbing @file reference, but same time the test for @vm_file presence was lost leading to nil dereference. The patch brings the test back. The all proc_map_files feature is CONFIG_CHECKPOINT_RESTORE wrapped (which is set to 'n' by default) so the bug doesn't affect regular kernels. The regression is 3.7-rc1 only as far as I can tell. [gorcunov@openvz.org: provided changelog] Signed-off-by: Stanislav Kinsbursky <skinsbursky@parallels.com> Acked-by: Cyrill Gorcunov <gorcunov@openvz.org> Cc: Al Viro <viro@zeniv.linux.org.uk> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: Stanislav Kinsbursky

fs/proc/base.c

@@ -1877,8 +1877,9 @@ static struct dentry *proc_map_files_lookup(struct inode *dir,
 if (!vma)
 goto out_no_vma;
 
-result = proc_map_files_instantiate(dir, dentry, task,
-(void *)(unsigned long)vma->vm_file->f_mode);
+if (vma->vm_file)
+result = proc_map_files_instantiate(dir, dentry, task,
+(void *)(unsigned long)vma->vm_file->f_mode);
 
 out_no_vma:
 up_read(&mm->mmap_sem);