od --skip (-j) works even on files in /proc, when the kernel lies

* src/od.c (skip): Don't let kernel misinformation (nonempty files
in /proc with stat.st_size == 0) make "od -j N" misbehave.
Patch by Paul Eggert.
* NEWS: Document this work-around.
* tests/misc/od-zero-len: New file, test for the above.

ChangeLog

@@ -1,5 +1,12 @@
 2007-08-15  Jim Meyering  <jim@meyering.net>
 
+	od --skip (-j) works even on files in /proc, when the kernel lies
+	* src/od.c (skip): Don't let kernel misinformation (nonempty files
+	in /proc with stat.st_size == 0) make "od -j N" misbehave.
+	Patch by Paul Eggert.
+	* NEWS: Document this work-around.
+	* tests/misc/od-zero-len: New file, test for the above.
+
 	* src/printf.c (usage): Adjust summary to also mention OPTIONs.
 	From Karl Berry.
 

NEWS

@@ -98,6 +98,10 @@ GNU coreutils NEWS                                    -*- outline -*-
   ln=target attribute) would mistakenly output the string "target"
   before the name of each symlink.  [introduced in coreutils-6.0]
 
+  od's --skip (-j) option now works even when the kernel says that a
+  nonempty regular file has stat.st_size = 0.  This happens at least
+  with files in /proc and linux-2.6.22.
+
   "od -j L FILE" had a bug: when the number of bytes to skip, L, is exactly
   the same as the length of FILE, od would skip *no* bytes.  When the number
   of bytes to skip is exactly the sum of the lengths of the first N files,

src/od.c

@@ -1035,9 +1035,11 @@ skip (uintmax_t n_skip)
 	  /* The st_size field is valid only for regular files
 	     (and for symbolic links, which cannot occur here).
 	     If the number of bytes left to skip is larger than
-	     the size of the current file, we can decrement
-	     n_skip and go on to the next file.  */
-	  if (S_ISREG (file_stats.st_mode) && 0 <= file_stats.st_size)
+	     the size of the current file, we can decrement n_skip
+	     and go on to the next file.  Skip this optimization also
+	     when st_size is 0, because some kernels report that
+	     nonempty files in /proc have st_size == 0.  */
+	  if (S_ISREG (file_stats.st_mode) && 0 < file_stats.st_size)
 	    {
 	      if ((uintmax_t) file_stats.st_size < n_skip)
 		n_skip -= file_stats.st_size;

tests/misc/od-zero-len

@@ -0,0 +1,53 @@
+#!/bin/sh
+# ensure that od -j doesn't improperly fseek across a nonempty file in /proc
+
+# Copyright (C) 2007 Free Software Foundation, Inc.
+
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+if test "$VERBOSE" = yes; then
+  set -x
+  od --version
+fi
+
+pwd=`pwd`
+t0=`echo "$0"|sed 's,.*/,,'`.tmp; tmp=$t0/$$
+trap 'status=$?; cd "$pwd" && chmod -R u+rwx $t0 && rm -rf $t0 && exit $status' 0
+trap '(exit $?); exit $?' 1 2 13 15
+
+framework_failure=0
+mkdir -p $tmp || framework_failure=1
+cd $tmp || framework_failure=1
+printf e > f2 || framework_failure=1
+
+if test $framework_failure = 1; then
+  echo "$0: failure in testing framework" 1>&2
+  (exit 1); exit 1
+fi
+
+# Use a file in /proc whose size is not likely to
+# change between the wc and od invocations.
+f=/proc/version
+test -r $f || f=empty
+
+fail=0
+
+n=`wc -c < $f` || fail=1
+od -An -c -j $n $f f2 > out || fail=1
+echo '   e' > exp || fail=1
+
+cmp out exp || fail=1
+test $fail = 1 && diff out exp 2> /dev/null
+
+(exit $fail); exit $fail