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, 5 insertions, 1 deletions

diff --git a/tests/misc/shred-exact.sh b/tests/misc/shred-exact.sh
index 5f2848e1d..5434229c4 100755
--- a/tests/misc/shred-exact.sh
+++ b/tests/misc/shred-exact.sh
@@ -40,6 +40,10 @@ done
 # (i.e. we want to test failed writes not at the start).
 truncate -s1MiB file.slop || framework_failure_
 truncate -s+1 file.slop || framework_failure_
-shred --exact -n1 file.slop || fail=1
+shred --exact -n2 file.slop || fail=1
+
+# make sure direct I/O is handled appropriately at start of file
+truncate -s1 file.slop || framework_failure_
+shred --exact -n2 file.slop || fail=1
 
 Exit $fail