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

From: Johannes Meixner <[email-masked]>
Date: Tue, 16 Jun 2015 12:39:39 +0200 (CEST)

Hello,

newer grep versions do "lseek (fd, cur, SEEK_HOLE)"
but that does not always work as expected,
at least not for ext4 for kernel < 3.1.

This can cause weird results when using a newer grep
for files on ext4 with kernel < 3.1.

Our (SUSE) kernel developers found and analyzed that issue.

Here some excerpts from their mails:
----------------------------------------------------------------------
> homer:/usr/local/src/kernel/linux-3.x.git # grep master xx
> Binary file xx matches
> homer:/usr/local/src/kernel/linux-3.x.git # split -n 2 xx
> homer:/usr/local/src/kernel/linux-3.x.git # grep master xaa
> CONFIG_LOCALVERSION="-master"
> homer:/usr/local/src/kernel/linux-3.x.git # grep master xab
> homer:/usr/local/src/kernel/linux-3.x.git #
>
> it doesn't like the first 2000 lines of .config (xx),
> but does like both halves.
>
> Made sure by adding that line to xab.
>
> homer:/usr/local/src/kernel/linux-3.x.git # grep master xaa
> CONFIG_LOCALVERSION="-master"
> homer:/usr/local/src/kernel/linux-3.x.git # grep master xab
> CONFIG_LOCALVERSION="-master"

I could reproduce it.

Now grepping the grep code, I found that it checks the binary
also by looking at the hole in a file:

           /* Look for a hole after the current location.  */
           off_t hole_start = lseek (fd, cur, SEEK_HOLE);
           if (0 <= hole_start)
             {
               if (lseek (fd, cur, SEEK_SET) < 0)
                 suppressible_error (filename, errno);
               if (hole_start < st->st_size)
                 return TEXTBIN_BINARY;
             }

And hole_start returns 32768 instead of an error or the end of file.

The reason is that ext4 of SLE11 doesn't support SEEK_HOLE,
it just ignores the flag and returns the current position as is.
----------------------------------------------------------------------

"SLE11" is "SUSE Linux Enterprise 11" which has kernel 3.0.

Kind Regards
Johannes Meixner
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