shred: overwrite inode storage used by some file systems

* doc/coreutils.texi (shred invocation): Mention some reasons
why clearing slack space might be useful.
* src/shred.c (do_wipefd): Add initial writes for each pass
for small regular files in case the storage for those is
in the inode, and thus a larger write up to a block size would
bypass that.  Move the direct I/O control to...
(dopass): ... here so we can avoid enabling it for these small
initial writes.  It's better to retry direct I/O for each pass
anyway to handle the case where direct I/O is disabled for only
the last portion of a file when the size is not a multiple of
the block size.  Note we don't avoid the sync for the initial
write as it will be small but more importantly could be on a
different part of the disk and so worth doing independently
to ensure the write is not discarded.
* tests/misc/shred-exact.sh: Check some more direct I/O cases.
* NEWS: Mention the improvements.
The inode storage issue was mentioned by Paul Eggert.

1 files changed, 3 insertions, 1 deletions

diff --git a/NEWS b/NEWS
index c6451b289..06e78a7a9 100644
--- a/NEWS
+++ b/NEWS
@@ -58,7 +58,9 @@ GNU coreutils NEWS                                    -*- outline -*-
   in case the look-up within the chroot fails due to library conflicts etc.
 
   shred now supports multiple passes on GNU/Linux tape devices by rewinding
-  the tape before each pass.  Also redundant writes to empty files are avoided.
+  the tape before each pass, avoids redundant writes to empty files,
+  uses direct I/O for all passes where possible, and attempts to clear
+  inode storage used for small files on some file systems.
 
   split avoids unnecessary input buffering, immediately writing input to output
   which is significant with --filter or when writing to fifos or stdout etc.