Re: [PATCH v3 1/2] Add ftruncate_file that truncates a struct file*

[Jens Axboe <axboe@kernel.dk>]

On 1/23/24 2:19 PM, Tony Solomonik wrote:
> do_sys_ftruncate receives a file descriptor, fgets the struct file*, and
> finally actually truncates the file.

Just do struct file and get rid of '*', kernel style would otherwise
dictate it should be struct file * but there's no point in mentioning
this is a pointer. It's the only case that makes sense.

> ftruncate_file allows for truncating a file without fgets.

I'd rephrase that last sentence, as it reads as you could do this
without holding a file reference. That is obviously not true. You
could make it:

ftruncate_file allows for passing in a file directly, with the
caller already holding a reference to it.

> 
> Signed-off-by: Tony Solomonik <tony.solomonik@gmail.com>
> ---
>  fs/internal.h |  1 +
>  fs/open.c     | 51 ++++++++++++++++++++++++++++++---------------------
>  2 files changed, 31 insertions(+), 21 deletions(-)
> 
> diff --git a/fs/internal.h b/fs/internal.h
> index 58e43341aebf..78a641ebd16e 100644
> --- a/fs/internal.h
> +++ b/fs/internal.h
> @@ -182,6 +182,7 @@ extern struct open_how build_open_how(int flags, umode_t mode);
>  extern int build_open_flags(const struct open_how *how, struct open_flags *op);
>  extern struct file *__close_fd_get_file(unsigned int fd);
>  
> +long ftruncate_file(struct file *file, loff_t length, int small);
>  long do_sys_ftruncate(unsigned int fd, loff_t length, int small);
>  int chmod_common(const struct path *path, umode_t mode);
>  int do_fchownat(int dfd, const char __user *filename, uid_t user, gid_t group,
> diff --git a/fs/open.c b/fs/open.c
> index 02dc608d40d8..0c505402e93d 100644
> --- a/fs/open.c
> +++ b/fs/open.c
> @@ -154,47 +154,56 @@ 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 ftruncate_file(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))
> +		return error;
>  
>  	error = -EINVAL;
>  	/* Cannot ftruncate over 2^31 bytes without large file support */
>  	if (small && length > MAX_NON_LFS)
> -		goto out_putf;
> +		return error;
>  
>  	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)))
> +		return error;
>  	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:
> +
> +  return error;

White space issue here with 'error'. And see below comments for error
assignment in general.

> +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 = ftruncate_file(f.file, length, small);
> +
>  	fdput(f);
>  out:
>  	return error;

No reason for the goto's here anymore, just do:

long do_sys_ftruncate(unsigned int fd, loff_t length, int small)
{
	struct fd f;
	int error;

	if (length < 0)
		return -EINVAL;
	error = -EBADF;
	f = fdget(fd);
	if (f.file)
		error = ftruncate_file(f.file, length, small);
	fdput(f);
	return error;
}

Same for the above helper, save error for when you actually need it
rather than do:

	error = -EFOO;
	if (some_error)
		return error;

That only really makes sense when you assign error through eg calling a
function, not when you know what error you are returning. Makes it
easier to read the code as well.

-- 
Jens Axboe