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

From: Johannes Meixner <[email-masked]>
Date: Tue, 16 Jun 2015 14:28:42 +0200 (CEST)

>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 */
+               }
+
                if (lseek (fd, cur, SEEK_SET) < 0)
                  suppressible_error (filename, errno);
                if (hole_start < st->st_size)