[PATCH] Add lockless helpers for remove_suid()

Right now users have to grab i_mutex before calling remove_suid(), in the
unlikely event that a call to ->setattr() may be needed. Split up the
function in two parts:

- One to check if we need to remove suid
- One to actually remove it

The first we can call lockless.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

2 files changed, 24 insertions, 8 deletions

diff --git a/include/linux/fs.h b/include/linux/fs.h
index d695ba2346a3..2fe6e3f900ba 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1712,6 +1712,8 @@ extern void __iget(struct inode * inode);
 extern void clear_inode(struct inode *);
 extern void destroy_inode(struct inode *);
 extern struct inode *new_inode(struct super_block *);
+extern int __remove_suid(struct dentry *, int);
+extern int should_remove_suid(struct dentry *);
 extern int remove_suid(struct dentry *);
 extern void remove_dquot_ref(struct super_block *, int, struct list_head *);
 
diff --git a/mm/filemap.c b/mm/filemap.c
index 3464b681f844..7c7addb9333c 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1884,11 +1884,10 @@ repeat:
  *      if suid or (sgid and xgrp)
  *              remove privs
  */
-int remove_suid(struct dentry *dentry)
+int should_remove_suid(struct dentry *dentry)
 {
         mode_t mode = dentry->d_inode->i_mode;
         int kill = 0;
-        int result = 0;
 
         /* suid always must be killed */
         if (unlikely(mode & S_ISUID))
@@ -1901,13 +1900,28 @@ int remove_suid(struct dentry *dentry)
         if (unlikely((mode & S_ISGID) && (mode & S_IXGRP)))
                 kill |= ATTR_KILL_SGID;
 
-        if (unlikely(kill && !capable(CAP_FSETID))) {
-                struct iattr newattrs;
+        if (unlikely(kill && !capable(CAP_FSETID)))
+                return kill;
 
-                newattrs.ia_valid = ATTR_FORCE | kill;
-                result = notify_change(dentry, &newattrs);
-        }
-        return result;
+        return 0;
+}
+
+int __remove_suid(struct dentry *dentry, int kill)
+{
+        struct iattr newattrs;
+
+        newattrs.ia_valid = ATTR_FORCE | kill;
+        return notify_change(dentry, &newattrs);
+}
+
+int remove_suid(struct dentry *dentry)
+{
+        int kill = should_remove_suid(dentry);
+
+        if (unlikely(kill))
+                return __remove_suid(dentry, kill);
+
+        return 0;
 }
 EXPORT_SYMBOL(remove_suid);