>From b7b4926b2f52cfa2ab7d33742355beaf9a08c695 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?P=C3=A1draig=20Brady?= <P@draigBrady.com>
Date: Sun, 31 Oct 2021 15:38:29 +0000
Subject: [PATCH] copy: avoid SEEK_DATA corruption on ZFS

Avoid corruption when using SEEK_DATA on ZFS, as discussed in:
https://github.com/openzfs/zfs/issues/11900

* src/copy.c (functional_seek_data): A new function that
returns true when we're sure we're not copying from ZFS.
Note systems like solaris which doesn't have statfs()->f_type
available, will be assumed to not have a usable SEEK_DATA.
Most systems in use should have this info available.
(infer_scantype): After a file is determined to perhaps be sparse,
call functional_seek_data to ensure it's not on ZFS.
* init.cfg (seek_data_capable_): Skip on ZFS.
* NEWS: Mention the bug avoidance.
Addresses https://bugs.gnu.org/51433
---
 NEWS       |  4 ++++
 init.cfg   | 12 ++++++++++++
 src/copy.c | 36 +++++++++++++++++++++++++++++++++++-
 3 files changed, 51 insertions(+), 1 deletion(-)

diff --git a/NEWS b/NEWS
index 086da03ae..78a73ab05 100644
--- a/NEWS
+++ b/NEWS
@@ -4,6 +4,10 @@ GNU coreutils NEWS                                    -*- outline -*-
 
 ** Bug fixes
 
+  cp will avoid corruption bugs in the ZFS file system by avoiding use
+  of lseek(..., SEEK_DATA), thus using potentially slower hole detection.
+  [bug triggered in coreutils-9.0]
+
   chmod -R no longer exits with error status when encountering symlinks.
   All files would be processed correctly, but the exit status was incorrect.
   [bug introduced in coreutils-9.0]
diff --git a/init.cfg b/init.cfg
index b92f717f5..0f97a6712 100644
--- a/init.cfg
+++ b/init.cfg
@@ -541,6 +541,18 @@ seek_data_capable_()
       return 1
   fi
 
+  # Skip on zfs due to various SEEK_DATA issues in its implementation
+  fstype=$(stat -f -c%t "$@")  # Ensure we have f_type, as that's what copy uses
+  if test -z "$fstype" || test "$fstype" = '?'; then
+    warn_ 'seek_data_capable_: stat(1) failed: assuming not SEEK_DATA capable'
+    return 1
+  fi
+  fsname=$(stat -f -c%T "$@")
+  if test "$fsname" = 'zfs'; then
+    warn_ 'seek_data_capable_: zfs detected: SEEK_DATA is disabled'
+    return 1
+  fi
+
   # Use timeout if available to skip cases where SEEK_DATA takes a long time.
   # We saw FreeBSD 9.1 take 35s to return from SEEK_DATA for a 1TiB empty file.
   # Note lseek() is uninterruptible on FreeBSD 9.1, but it does eventually
diff --git a/src/copy.c b/src/copy.c
index a6523ed97..6e19b8740 100644
--- a/src/copy.c
+++ b/src/copy.c
@@ -1080,6 +1080,33 @@ union scan_inference
   off_t ext_start;
 };
 
+/* SEEK_DATA on ZFS has many issues as described at:
+   https://github.com/openzfs/zfs/issues/11900
+   so return FALSE for ZFS (or inability to detect fs type).  */
+
+#ifdef SEEK_HOLE
+# include "fs.h"
+# if HAVE_SYS_STATFS_H
+#  include <sys/statfs.h>
+# elif HAVE_SYS_VFS_H
+#  include <sys/vfs.h>
+# endif
+static bool
+functional_seek_data (int fd)
+{
+# if HAVE_FSTATFS && HAVE_STRUCT_STATFS_F_TYPE
+  struct statfs buf;
+  if (fstatfs (fd, &buf) == 0)
+    {
+      if (buf.f_type != S_MAGIC_ZFS)
+        return true;
+    }
+# endif
+
+  return false;
+}
+#endif
+
 /* Return how to scan a file with descriptor FD and stat buffer SB.
    Store any information gathered into *SCAN.  */
 static enum scantype
@@ -1092,7 +1119,14 @@ infer_scantype (int fd, struct stat const *sb,
     return PLAIN_SCANTYPE;
 
 #ifdef SEEK_HOLE
-  scan_inference->ext_start = lseek (fd, 0, SEEK_DATA);
+  if (! functional_seek_data (fd))
+    {
+      errno = ENOTSUP;
+      scan_inference->ext_start = -1;
+    }
+  else
+    scan_inference->ext_start = lseek (fd, 0, SEEK_DATA);
+
   if (0 <= scan_inference->ext_start || errno == ENXIO)
     return LSEEK_SCANTYPE;
   else if (errno != EINVAL && !is_ENOTSUP (errno))
-- 
2.26.2