[PATCH] sanitize unshare_files/reset_files_struct

* let unshare_files() give caller the displaced files_struct
* don't bother with grabbing reference only to drop it in the
  caller if it hadn't been shared in the first place
* in that form unshare_files() is trivially implemented via
  unshare_fd(), so we eliminate the duplicate logics in fork.c
* reset_files_struct() is not just only called for current;
  it will break the system if somebody ever calls it for anything
  else (we can't modify ->files of somebody else).  Lose the
  task_struct * argument.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

5 files changed, 34 insertions, 47 deletions

diff --git a/fs/exec.c b/fs/exec.c
index 475543002f13..b152029f18f6 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1269,19 +1269,13 @@ int do_execve(char * filename,
         struct linux_binprm *bprm;
         struct file *file;
         unsigned long env_p;
-        struct files_struct *files;
+        struct files_struct *displaced;
         int retval;
 
-        files = current->files;
-        retval = unshare_files();
+        retval = unshare_files(&displaced);
         if (retval)
                 goto out_ret;
 
-        if (files == current->files) {
-                put_files_struct(files);
-                files = NULL;
-        }
-
         retval = -ENOMEM;
         bprm = kzalloc(sizeof(*bprm), GFP_KERNEL);
         if (!bprm)
@@ -1340,8 +1334,8 @@ int do_execve(char * filename,
                 security_bprm_free(bprm);
                 acct_update_integrals(current);
                 kfree(bprm);
-                if (files)
-                        put_files_struct(files);
+                if (displaced)
+                        put_files_struct(displaced);
                 return retval;
         }
 
@@ -1363,8 +1357,8 @@ out_kfree:
         kfree(bprm);
 
 out_files:
-        if (files)
-                reset_files_struct(current, files);
+        if (displaced)
+                reset_files_struct(displaced);
 out_ret:
         return retval;
 }
diff --git a/include/linux/file.h b/include/linux/file.h
index 653477021e4c..69baf5a4f0a5 100644
--- a/include/linux/file.h
+++ b/include/linux/file.h
@@ -117,7 +117,8 @@ struct task_struct;
 
 struct files_struct *get_files_struct(struct task_struct *);
 void put_files_struct(struct files_struct *fs);
-void reset_files_struct(struct task_struct *, struct files_struct *);
+void reset_files_struct(struct files_struct *);
+int unshare_files(struct files_struct **);
 
 extern struct kmem_cache *files_cachep;
 
diff --git a/include/linux/fs.h b/include/linux/fs.h
index ad41d0bbcb4d..e057438a05ad 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2033,9 +2033,6 @@ static inline ino_t parent_ino(struct dentry *dentry)
         return res;
 }
 
-/* kernel/fork.c */
-extern int unshare_files(void);
-
 /* Transaction based IO helpers */
 
 /*
diff --git a/kernel/exit.c b/kernel/exit.c
index 3d320003cc03..97f609f574b1 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -507,8 +507,9 @@ void put_files_struct(struct files_struct *files)
         }
 }
 
-void reset_files_struct(struct task_struct *tsk, struct files_struct *files)
+void reset_files_struct(struct files_struct *files)
 {
+        struct task_struct *tsk = current;
         struct files_struct *old;
 
         old = tsk->files;
diff --git a/kernel/fork.c b/kernel/fork.c
index 2fc11f2e2b21..efb618fc8ffe 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -840,36 +840,6 @@ static int copy_io(unsigned long clone_flags, struct task_struct *tsk)
         return 0;
 }
 
-/*
- *      Helper to unshare the files of the current task.
- *      We don't want to expose copy_files internals to
- *      the exec layer of the kernel.
- */
-
-int unshare_files(void)
-{
-        struct files_struct *files  = current->files;
-        struct files_struct *newf;
-        int error = 0;
-
-        BUG_ON(!files);
-
-        /* This can race but the race causes us to copy when we don't
-           need to and drop the copy */
-        if(atomic_read(&files->count) == 1)
-        {
-                atomic_inc(&files->count);
-                return 0;
-        }
-        newf = dup_fd(files, &error);
-        if (newf) {
-                task_lock(current);
-                current->files = newf;
-                task_unlock(current);
-        }
-        return error;
-}
-
 static int copy_sighand(unsigned long clone_flags, struct task_struct *tsk)
 {
         struct sighand_struct *sig;
@@ -1807,3 +1777,27 @@ bad_unshare_cleanup_thread:
 bad_unshare_out:
         return err;
 }
+
+/*
+ *      Helper to unshare the files of the current task.
+ *      We don't want to expose copy_files internals to
+ *      the exec layer of the kernel.
+ */
+
+int unshare_files(struct files_struct **displaced)
+{
+        struct task_struct *task = current;
+        struct files_struct *copy;
+        int error;
+
+        error = unshare_fd(CLONE_FILES, &copy);
+        if (error || !copy) {
+                *displaced = NULL;
+                return error;
+        }
+        *displaced = task->files;
+        task_lock(task);
+        task->files = copy;
+        task_unlock(task);
+        return 0;
+}