From 1209d61ac0c940bdd2dd11448cc9e1b3e9b59a29 Mon Sep 17 00:00:00 2001
From: Jim Meyering <address@hidden>
Date: Sat, 28 Nov 2015 18:02:05 -0800
Subject: [PATCH] diff --brief no longer mistakenly reports diff. with 0-sized
 /proc/ files
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* src/analyze.c (diff_2_files): Don't declare difference when any
st_size is 0.
* src/diff.c (compare_files): Likewise.
* tests/brief-vs-proc-stat-zero: New test.
* tests/Makefile.am: Add it.
* NEWS (Bug fixes): Describe it.
Reported by Stephan Müller in http://debbugs.gnu.org/21942
---
 NEWS                                 |  8 ++++++++
 src/analyze.c                        |  2 ++
 src/diff.c                           |  4 +++-
 tests/Makefile.am                    |  1 +
 tests/brief-vs-stat-zero-kernel-lies | 30 ++++++++++++++++++++++++++++++
 5 files changed, 44 insertions(+), 1 deletion(-)
 create mode 100755 tests/brief-vs-stat-zero-kernel-lies

diff --git a/NEWS b/NEWS
index 088f13b..1724c74 100644
--- a/NEWS
+++ b/NEWS
@@ -23,6 +23,14 @@ GNU diffutils NEWS                                    -*- outline -*-
   diff -B no longer generates incorrect output if the two inputs
   each end with a one-byte incomplete line.

+  diff --brief no longer reports a difference for unusual identical files.
+  For example, when comparing a file like /proc/cmdline (for which the linux
+  kernel reports st_size of 0 even though it is not an empty file) to a
+  copy of that file's contents residing on a "normal" file system:
+    $ f=/proc/cmdline; cp $f k; diff --brief $f k
+    Files /proc/cmdline and k differ
+
+
 ** Performance changes

   diff's default algorithm has been adjusted to output higher-quality
diff --git a/src/analyze.c b/src/analyze.c
index 893d07c..2622810 100644
--- a/src/analyze.c
+++ b/src/analyze.c
@@ -477,6 +477,8 @@ diff_2_files (struct comparison *cmp)
     {
       /* Files with different lengths must be different.  */
       if (cmp->file[0].stat.st_size != cmp->file[1].stat.st_size
+	  && 0 < cmp->file[0].stat.st_size
+	  && 0 < cmp->file[1].stat.st_size
 	  && (cmp->file[0].desc < 0 || S_ISREG (cmp->file[0].stat.st_mode))
 	  && (cmp->file[1].desc < 0 || S_ISREG (cmp->file[1].stat.st_mode)))
 	changes = 1;
diff --git a/src/diff.c b/src/diff.c
index 46ac99d..cff23f0 100644
--- a/src/diff.c
+++ b/src/diff.c
@@ -1377,7 +1377,9 @@ compare_files (struct comparison const *parent,
   else if (files_can_be_treated_as_binary
 	   && S_ISREG (cmp.file[0].stat.st_mode)
 	   && S_ISREG (cmp.file[1].stat.st_mode)
-	   && cmp.file[0].stat.st_size != cmp.file[1].stat.st_size)
+	   && cmp.file[0].stat.st_size != cmp.file[1].stat.st_size
+	   && 0 < cmp.file[0].stat.st_size
+	   && 0 < cmp.file[1].stat.st_size)
     {
       message ("Files %s and %s differ\n",
 	       file_label[0] ? file_label[0] : cmp.file[0].name,
diff --git a/tests/Makefile.am b/tests/Makefile.am
index 805ccc2..b070a76 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -4,6 +4,7 @@ TESTS = \
   basic \
   bignum \
   binary \
+  brief-vs-stat-zero-kernel-lies \
   colliding-file-names \
   excess-slash \
   help-version	\
diff --git a/tests/brief-vs-stat-zero-kernel-lies b/tests/brief-vs-stat-zero-kernel-lies
new file mode 100755
index 0000000..9b272c6
--- /dev/null
+++ b/tests/brief-vs-stat-zero-kernel-lies
@@ -0,0 +1,30 @@
+#!/bin/sh
+# Before diff-3.4, diff --brief could mistakenly declare a difference.
+# For example, when comparing a file like /proc/cmdline (for which the linux
+# kernel reports a st_size of 0 even though it is not an empty file) to a
+# copy of that file's contents residing on a "normal" file system.
+
+. "${srcdir=.}/init.sh"; path_prepend_ ../src
+
+fail=0
+
+# Skip the test unless we have an appropriate file.
+boot=/proc/cmdline
+test -f $boot || skip_ no $boot file
+sz=$(stat --format %s $boot) || skip_ stat --format %s does not work
+test $sz = 0 || skip_ $boot has nonzero size
+
+# There are two code paths to test: one for non-binary and one for binary files.
+# $boot is non-binary.
+cat $boot > ref || framework_failure_
+diff --brief $boot ref > out 2>&1 || fail=1
+compare /dev/null out || fail=1
+
+# /proc/self/cmdline is a NUL-terminated list of argv values,
+# so construct the expected output here:
+printf 'diff\0--brief\0/proc/self/cmdline\0bin\0' > bin || framework_failure_
+# And run the command that is embedded in that output:
+diff --brief /proc/self/cmdline bin > out 2>&1 || fail=1
+compare /dev/null out || fail=1
+
+Exit $fail
-- 
2.6.2