Subject: bug#20826: SEEK_HOLE not supported for ext4 for kernel < 3.1

From: Eric Blake <[email-masked]>
Date: Tue, 16 Jun 2015 06:47:40 -0600

[adding bug-gnulib]

On 06/16/2015 06:28 AM, Johannes Meixner wrote:
> 
>> From one of our (SUSE) kernel developers
> I even got a proposal for a workaround in grep:
> 
> --- a/src/grep.c
> +++ b/src/grep.c
> @@ -575,6 +575,17 @@ file_textbin (char *buf, size_t size, in
>            off_t hole_start = lseek (fd, cur, SEEK_HOLE);
>            if (0 <= hole_start)
>              {
> +             /* if hole_start is identical with cur, it's more likely a
> buggy
> +              * lseek(SEEK_HOLE) implementation in kernel filesystem;
> +              * check whether it really reads a null byte.
> +              */
> +             if (hole_start == cur)
> +               {
> +                 char c;
> +                 if (read (fd, &c, 1) != 1 || c != 0)
> +                   hole_start = st->st_size; /* no hole */
> +               }
> +

Sounds like we need to update the gnulib lseek module to work around
bugs like this in the wild.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org