Avoid excess lseek etc.

* src/buffer.c, src/delete.c: Do not include system-ioctl.h.
* src/buffer.c (guess_seekable_archive): Remove.  This is now done
by get_archive_status, in a different way.
(get_archive_status): New function that gets archive_stat
unless remote, and sets seekable_archive etc.
(_open_archive): Prefer bool for boolean.
(_open_archive, new_volume): Get archive status consistently
by calling get_archive_status in both places.
* src/buffer.c (backspace_output):
* src/compare.c (verify_volume):
* src/delete.c (move_archive):
Let mtioseek worry about mtio.
* src/common.h (archive_stat): New global, replacing ar_dev and
ar_ino.  All uses changed.
* src/delete.c (move_archive): Check for integer overflow.
Also report overflow if the archive position would go negative.
* src/system.c: Include system-ioctl.h, for MTIOCTOP etc.
(mtioseek): New function, which also checks for integer overflow.
(sys_save_archive_dev_ino): Remove.
(archive_stat): Now
(sys_get_archive_stat): Also initialize mtioseekable_archive.
(sys_file_is_archive): Don’t return true if the archive is /dev/null
since it’s not a problem in that case.
(sys_detect_dev_null_output): Cache dev_null_stat.

doc: omit MS-DOS mentions in doc
It’s really FAT32 we’re worried about now, not MS-DOS.
And doschk is no longer a GNU program.

5 files changed, 119 insertions, 153 deletions

diff --git a/src/buffer.c b/src/buffer.c
index a4467aa2..36b9c9c0 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -20,7 +20,6 @@
    Written by John Gilmore, on 1985-08-25.  */
 
 #include <system.h>
-#include <system-ioctl.h>
 
 #include <signal.h>
 
@@ -421,37 +420,6 @@ check_compressed_archive (bool *pshort)
   return ct_none;
 }
 
-/* Guess if the archive is seekable. */
-static void
-guess_seekable_archive (void)
-{
-  struct stat st;
-
-  if (subcommand_option == DELETE_SUBCOMMAND)
-    {
-      /* The current code in delete.c is based on the assumption that
-	 skip_member() reads all data from the archive. So, we should
-	 make sure it won't use seeks. On the other hand, the same code
-	 depends on the ability to backspace a record in the archive,
-	 so setting seekable_archive to false is technically incorrect.
-         However, it is tested only in skip_member(), so it's not a
-	 problem. */
-      seekable_archive = false;
-    }
-
-  if (seek_option != -1)
-    {
-      seekable_archive = !!seek_option;
-      return;
-    }
-
-  if (!multi_volume_option && !use_compress_program_option
-      && fstat (archive, &st) == 0)
-    seekable_archive = S_ISREG (st.st_mode);
-  else
-    seekable_archive = false;
-}
-
 /* Open an archive named archive_name_array[0]. Detect if it is
    a compressed archive of known type and use corresponding decompression
    program if so */
@@ -703,12 +671,41 @@ check_tty (enum access_mode mode)
     }
 }
 
+/* Fetch the status of the archive, accessed via WANTED_STATUS.  */
+
+static void
+get_archive_status (enum access_mode wanted_access, bool backed_up_flag)
+{
+  if (!sys_get_archive_stat ())
+    {
+      int saved_errno = errno;
+
+      if (backed_up_flag)
+        undo_last_backup ();
+      errno = saved_errno;
+      open_fatal (archive_name_array[0]);
+    }
+
+  seekable_archive
+    = (! (multi_volume_option || use_compress_program_option)
+       && (seek_option < 0
+	   ? (_isrmt (archive)
+	      || S_ISREG (archive_stat.st_mode)
+	      || S_ISBLK (archive_stat.st_mode))
+	   : seek_option));
+
+  if (wanted_access != ACCESS_READ)
+    sys_detect_dev_null_output ();
+
+  SET_BINARY_MODE (archive);
+}
+
 /* Open an archive file.  The argument specifies whether we are
    reading or writing, or both.  */
 static void
 _open_archive (enum access_mode wanted_access)
 {
-  int backed_up_flag = 0;
+  bool backed_up_flag = false;
 
   if (record_size == 0)
     FATAL_ERROR ((0, 0, _("Invalid value for record_size")));
@@ -797,15 +794,13 @@ _open_archive (enum access_mode wanted_access)
       {
       case ACCESS_READ:
         archive = open_compressed_archive ();
-	if (archive >= 0)
-	  guess_seekable_archive ();
         break;
 
       case ACCESS_WRITE:
         if (backup_option)
           {
             maybe_backup_file (archive_name_array[0], 1);
-            backed_up_flag = 1;
+            backed_up_flag = true;
           }
 	if (verify_option)
 	  archive = rmtopen (archive_name_array[0], O_RDWR | O_CREAT | O_BINARY,
@@ -833,20 +828,7 @@ _open_archive (enum access_mode wanted_access)
         break;
       }
 
-  if (archive < 0
-      || (! _isrmt (archive) && !sys_get_archive_stat ()))
-    {
-      int saved_errno = errno;
-
-      if (backed_up_flag)
-        undo_last_backup ();
-      errno = saved_errno;
-      open_fatal (archive_name_array[0]);
-    }
-
-  sys_detect_dev_null_output ();
-  sys_save_archive_dev_ino ();
-  SET_BINARY_MODE (archive);
+  get_archive_status (wanted_access, backed_up_flag);
 
   switch (wanted_access)
     {
@@ -1049,18 +1031,8 @@ flush_archive (void)
 static void
 backspace_output (void)
 {
-#ifdef MTIOCTOP
-  {
-    struct mtop operation;
-
-    operation.mt_op = MTBSR;
-    operation.mt_count = 1;
-    if (rmtioctl (archive, MTIOCTOP, (char *) &operation) >= 0)
-      return;
-    if (errno == EIO && rmtioctl (archive, MTIOCTOP, (char *) &operation) >= 0)
-      return;
-  }
-#endif
+  if (mtioseek (false, -1))
+    return;
 
   {
     off_t position = rmtlseek (archive, (off_t) 0, SEEK_CUR);
@@ -1373,7 +1345,6 @@ new_volume (enum access_mode mode)
       case ACCESS_READ:
         archive = rmtopen (*archive_name_cursor, O_RDONLY, MODE_RW,
                            rsh_command_option);
-	guess_seekable_archive ();
         break;
 
       case ACCESS_WRITE:
@@ -1398,7 +1369,7 @@ new_volume (enum access_mode mode)
       goto tryagain;
     }
 
-  SET_BINARY_MODE (archive);
+  get_archive_status (mode, false);
 
   return true;
 }
diff --git a/src/common.h b/src/common.h
index 62149113..58c1b6c1 100644
--- a/src/common.h
+++ b/src/common.h
@@ -392,9 +392,8 @@ struct name
     char *caname;               /* canonical name */
   };
 
-/* Obnoxious test to see if dimwit is trying to dump the archive.  */
-GLOBAL dev_t ar_dev;
-GLOBAL ino_t ar_ino;
+/* Status of archive file, or all zeros if remote.  */
+GLOBAL struct stat archive_stat;
 
 /* Flags for reading, searching, and fstatatting files.  */
 GLOBAL int open_read_flags;
@@ -402,6 +401,9 @@ GLOBAL int open_searchdir_flags;
 GLOBAL int fstatat_flags;
 
 GLOBAL int seek_option;
+
+/* true if archive if lseek should be used on the archive, 0 if it
+   should not be used.  */
 GLOBAL bool seekable_archive;
 
 GLOBAL dev_t root_device;
@@ -896,7 +898,6 @@ void xattr_map_free (struct xattr_map *xattr_map);
 /* Module system.c */
 
 void sys_detect_dev_null_output (void);
-void sys_save_archive_dev_ino (void);
 void sys_wait_for_child (pid_t, bool);
 void sys_spawn_shell (void);
 bool sys_compare_uid (struct stat *a, struct stat *b);
@@ -914,6 +915,7 @@ int sys_exec_info_script (const char **archive_name, int volume_number);
 void sys_exec_checkpoint_script (const char *script_name,
 				 const char *archive_name,
 				 int checkpoint_number);
+bool mtioseek (bool count_files, off_t count);
 
 /* Module compare.c */
 void report_difference (struct tar_stat_info *st, const char *message, ...)
diff --git a/src/compare.c b/src/compare.c
index 752eb15a..90043765 100644
--- a/src/compare.c
+++ b/src/compare.c
@@ -565,31 +565,12 @@ verify_volume (void)
   ioctl (archive, FDFLUSH);
 #endif
 
-#ifdef MTIOCTOP
-  {
-    struct mtop operation;
-    int status;
-
-    operation.mt_op = MTBSF;
-    operation.mt_count = 1;
-    if (status = rmtioctl (archive, MTIOCTOP, (char *) &operation), status < 0)
-      {
-	if (errno != EIO
-	    || (status = rmtioctl (archive, MTIOCTOP, (char *) &operation),
-		status < 0))
-	  {
-#endif
-	    if (rmtlseek (archive, (off_t) 0, SEEK_SET) != 0)
-	      {
-		/* Lseek failed.  Try a different method.  */
-		seek_warn (archive_name_array[0]);
-		return;
-	      }
-#ifdef MTIOCTOP
-	  }
-      }
-  }
-#endif
+  if (!mtioseek (true, -1) && rmtlseek (archive, 0, SEEK_SET) != 0)
+    {
+      /* Lseek failed.  Try a different method.  */
+      seek_warn (archive_name_array[0]);
+      return;
+    }
 
   access_mode = ACCESS_READ;
   now_verifying = 1;
diff --git a/src/delete.c b/src/delete.c
index fdf18af9..1d4d2681 100644
--- a/src/delete.c
+++ b/src/delete.c
@@ -18,7 +18,6 @@
    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 
 #include <system.h>
-#include <system-ioctl.h>
 
 #include "common.h"
 #include <rmt.h>
@@ -50,41 +49,25 @@ move_archive (off_t count)
   if (count == 0)
     return;
 
-#ifdef MTIOCTOP
-  {
-    struct mtop operation;
-
-    if (count < 0
-	? (operation.mt_op = MTBSR,
-	   operation.mt_count = -count,
-	   operation.mt_count == -count)
-	: (operation.mt_op = MTFSR,
-	   operation.mt_count = count,
-	   operation.mt_count == count))
-      {
-	if (0 <= rmtioctl (archive, MTIOCTOP, (char *) &operation))
-	  return;
+  if (mtioseek (false, count))
+    return;
 
-	if (errno == EIO
-	    && 0 <= rmtioctl (archive, MTIOCTOP, (char *) &operation))
+  off_t position0 = rmtlseek (archive, 0, SEEK_CUR), position = 0;
+  if (0 <= position0)
+    {
+      off_t increment;
+      if (INT_MULTIPLY_WRAPV (record_size, count, &increment)
+	  || INT_ADD_WRAPV (position0, increment, &position)
+	  || position < 0)
+	{
+	  ERROR ((0, EOVERFLOW, "lseek: %s", archive_name_array[0]));
 	  return;
-      }
-  }
-#endif /* MTIOCTOP */
-
-  {
-    off_t position0 = rmtlseek (archive, (off_t) 0, SEEK_CUR);
-    off_t increment = record_size * (off_t) count;
-    off_t position = position0 + increment;
-
-    if (increment / count != record_size
-	|| (position < position0) != (increment < 0)
-	|| (position = position < 0 ? 0 : position,
-	    rmtlseek (archive, position, SEEK_SET) != position))
-      seek_error_details (archive_name_array[0], position);
-
-    return;
-  }
+	}
+      else if (rmtlseek (archive, position, SEEK_SET) == position)
+	return;
+    }
+  if (!_isrmt (archive))
+    seek_error_details (archive_name_array[0], position);
 }
 
 /* Write out the record which has been filled.  If MOVE_BACK_FLAG,
diff --git a/src/system.c b/src/system.c
index 1f560783..9f8bbe4c 100644
--- a/src/system.c
+++ b/src/system.c
@@ -16,6 +16,7 @@
    with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 
 #include <system.h>
+#include <system-ioctl.h>
 
 #include "common.h"
 #include <priv-set.h>
@@ -37,6 +38,35 @@ xexec (const char *cmd)
   exec_fatal (cmd);
 }
 
+/* True if the archive is seekable via ioctl and MTIOCTOP,
+   or if it is not known whether it is seekable.
+   False if it is known to be not seekable.  */
+static bool mtioseekable_archive;
+
+bool
+mtioseek (bool count_files, off_t count)
+{
+  if (mtioseekable_archive)
+    {
+#ifdef MTIOCTOP
+      struct mtop operation;
+      operation.mt_op = (count_files
+			 ? (count < 0 ? MTBSF : MTFSF)
+			 : (count < 0 ? MTBSR : MTFSR));
+      if (! (count < 0
+	     ? INT_SUBTRACT_WRAPV (0, count, &operation.mt_count)
+	     : INT_ADD_WRAPV (count, 0, &operation.mt_count))
+	  && (0 <= rmtioctl (archive, MTIOCTOP, &operation)
+	      || (errno == EIO
+		  && 0 <= rmtioctl (archive, MTIOCTOP, &operation))))
+	return true;
+#endif
+
+      mtioseekable_archive = false;
+    }
+  return false;
+}
+
 #if MSDOS
 
 bool
@@ -52,11 +82,6 @@ sys_file_is_archive (struct tar_stat_info *p)
 }
 
 void
-sys_save_archive_dev_ino (void)
-{
-}
-
-void
 sys_detect_dev_null_output (void)
 {
   static char const dev_null[] = "nul";
@@ -128,31 +153,34 @@ sys_child_open_for_uncompress (void)
 
 extern union block *record_start; /* FIXME */
 
-static struct stat archive_stat; /* stat block for archive file */
-
 bool
 sys_get_archive_stat (void)
 {
-  return fstat (archive, &archive_stat) == 0;
+  bool remote = _isrmt (archive);
+  mtioseekable_archive = true;
+  if (!remote && 0 <= archive && fstat (archive, &archive_stat) == 0)
+    {
+      if (!S_ISCHR (archive_stat.st_mode))
+	mtioseekable_archive = false;
+      return true;
+    }
+  else
+    {
+      /* FIXME: This memset should not be needed.  It is present only
+	 because other parts of tar may incorrectly access
+	 archive_stat even if it's not the archive status.  */
+      memset (&archive_stat, 0, sizeof archive_stat);
+
+      return remote;
+    }
 }
 
 bool
 sys_file_is_archive (struct tar_stat_info *p)
 {
-  return (ar_dev && p->stat.st_dev == ar_dev && p->stat.st_ino == ar_ino);
-}
-
-/* Save archive file inode and device numbers */
-void
-sys_save_archive_dev_ino (void)
-{
-  if (!_isrmt (archive) && S_ISREG (archive_stat.st_mode))
-    {
-      ar_dev = archive_stat.st_dev;
-      ar_ino = archive_stat.st_ino;
-    }
-  else
-    ar_dev = 0;
+  return (!dev_null_output && !_isrmt (archive)
+	  && p->stat.st_dev == archive_stat.st_dev
+	  && p->stat.st_ino == archive_stat.st_ino);
 }
 
 /* Detect if outputting to "/dev/null".  */
@@ -160,14 +188,15 @@ void
 sys_detect_dev_null_output (void)
 {
   static char const dev_null[] = "/dev/null";
-  struct stat dev_null_stat;
+  static struct stat dev_null_stat;
 
   dev_null_output = (strcmp (archive_name_array[0], dev_null) == 0
 		     || (! _isrmt (archive)
 			 && S_ISCHR (archive_stat.st_mode)
-			 && stat (dev_null, &dev_null_stat) == 0
-			 && archive_stat.st_dev == dev_null_stat.st_dev
-			 && archive_stat.st_ino == dev_null_stat.st_ino));
+			 && (dev_null_stat.st_ino != 0
+			     || stat (dev_null, &dev_null_stat) == 0)
+			 && archive_stat.st_ino == dev_null_stat.st_ino
+			 && archive_stat.st_dev == dev_null_stat.st_dev));
 }
 
 void