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

Jens Axboe
2024-01-23T22:07:56Z

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