Re: [PATCH] tmpfs: let lseek return ENXIO with a negative offset

[William Kucharski <william.kucharski@oracle.com>]

> On Oct 24, 2018, at 8:22 PM, Yufen Yu <yuyufen@huawei.com> wrote:
> 
> For now, the others filesystems, such as ext4, f2fs, ubifs,
> all of them return ENXIO when lseek with a negative offset.
> It is better to let tmpfs return ENXIO too. After that, tmpfs
> can also pass generic/448.
> 
> Signed-off-by: Yufen Yu <yuyufen@huawei.com>
> ---
> mm/shmem.c | 4 +---
> 1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 0376c124..f37bf06 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -2608,9 +2608,7 @@ static loff_t shmem_file_llseek(struct file *file, loff_t offset, int whence)
> 	inode_lock(inode);
> 	/* We're holding i_mutex so we can access i_size directly */
> 
> -	if (offset < 0)
> -		offset = -EINVAL;
> -	else if (offset >= inode->i_size)
> +	if (offset < 0 || offset >= inode->i_size)
> 		offset = -ENXIO;
> 	else {
> 		start = offset >> PAGE_SHIFT;
> --

It's not at all clear what the proper thing to do is if a negative offset is passed.

The man page for lseek(2) states:

       SEEK_DATA
              Adjust the file offset to the next location in the file
              greater than or equal to offset containing data.  If offset
              points to data, then the file offset is set to offset.
       
       SEEK_HOLE
              Adjust the file offset to the next hole in the file greater
              than or equal to offset.  If offset points into the middle of
              a hole, then the file offset is set to offset.  If there is no
              hole past offset, then the file offset is adjusted to the end
              of the file (i.e., there is an implicit hole at the end of any
              file).

This seems to indicate that if passed a negative offset, a whence of either SEEK_DATA
or SEEK_HOLE should operate the same as if passed an offset of 0.

ENXIO just seems to be the wrong error code to return for a passed negative offset in
these cases (also from lseek(2)):

       ENXIO  whence is SEEK_DATA or SEEK_HOLE, and the file offset is
              beyond the end of the file.

but EINVAL isn't technically appropriate either:

       EINVAL whence is not valid.  Or: the resulting file offset would be
              negative, or beyond the end of a seekable device.

At the very least it seems the man page should be updated to reflect that ENXIO may be
returned if whence is SEEK_DATA or SEEK_HOLE and the passed offset is negative.

    William Kucharski