[PATCH v2 1/2] Add __do_ftruncate that truncates a struct file*

[Tony Solomonik <tony.solomonik@gmail.com>]

do_sys_ftruncate receives a file descriptor, fgets the struct file*, and
finally actually truncates the file.

__do_ftruncate allows for truncating a file without fgets.
---
 fs/open.c                | 52 ++++++++++++++++++++++++----------------
 include/linux/syscalls.h |  2 ++
 2 files changed, 33 insertions(+), 21 deletions(-)

diff --git a/fs/open.c b/fs/open.c
index 02dc608d40d8..b32ac430666c 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -154,47 +154,57 @@ COMPAT_SYSCALL_DEFINE2(truncate, const char __user *, path, compat_off_t, length
 }
 #endif
 
-long do_sys_ftruncate(unsigned int fd, loff_t length, int small)
+long __do_ftruncate(struct file *file, loff_t length, int small)
 {
 	struct inode *inode;
 	struct dentry *dentry;
-	struct fd f;
 	int error;
 
-	error = -EINVAL;
-	if (length < 0)
-		goto out;
-	error = -EBADF;
-	f = fdget(fd);
-	if (!f.file)
-		goto out;
-
 	/* explicitly opened as large or we are on 64-bit box */
-	if (f.file->f_flags & O_LARGEFILE)
+	if (file->f_flags & O_LARGEFILE)
 		small = 0;
 
-	dentry = f.file->f_path.dentry;
+	dentry = file->f_path.dentry;
 	inode = dentry->d_inode;
 	error = -EINVAL;
-	if (!S_ISREG(inode->i_mode) || !(f.file->f_mode & FMODE_WRITE))
-		goto out_putf;
+	if (!S_ISREG(inode->i_mode) || !(file->f_mode & FMODE_WRITE))
+		goto out;
 
 	error = -EINVAL;
 	/* Cannot ftruncate over 2^31 bytes without large file support */
 	if (small && length > MAX_NON_LFS)
-		goto out_putf;
+		goto out;
 
 	error = -EPERM;
 	/* Check IS_APPEND on real upper inode */
-	if (IS_APPEND(file_inode(f.file)))
-		goto out_putf;
+	if (IS_APPEND(file_inode(file)))
+		goto out;
 	sb_start_write(inode->i_sb);
-	error = security_file_truncate(f.file);
+	error = security_file_truncate(file);
 	if (!error)
-		error = do_truncate(file_mnt_idmap(f.file), dentry, length,
-				    ATTR_MTIME | ATTR_CTIME, f.file);
+		error = do_truncate(file_mnt_idmap(file), dentry, length,
+				    ATTR_MTIME | ATTR_CTIME, file);
 	sb_end_write(inode->i_sb);
-out_putf:
+
+out:
+  return error;
+}
+
+long do_sys_ftruncate(unsigned int fd, loff_t length, int small)
+{
+	struct fd f;
+	int error;
+
+	error = -EINVAL;
+	if (length < 0)
+		goto out;
+	error = -EBADF;
+	f = fdget(fd);
+	if (!f.file)
+		goto out;
+
+	error = __do_ftruncate(f.file, length, small);
+
 	fdput(f);
 out:
 	return error;
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index fd9d12de7e92..e8c56986e751 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -1229,6 +1229,8 @@ static inline long ksys_lchown(const char __user *filename, uid_t user,
 			     AT_SYMLINK_NOFOLLOW);
 }
 
+extern long __do_ftruncate(struct file *file, loff_t length, int small);
+
 extern long do_sys_ftruncate(unsigned int fd, loff_t length, int small);
 
 static inline long ksys_ftruncate(unsigned int fd, loff_t length)
-- 
2.34.1