bug#6936: Bug#594666: /usr/bin/tac: tac aborts

[Jim Meyering]

Salvo Tomaselli wrote:
> Package: coreutils
> Version: 8.5-1
> Severity: normal
> File: /usr/bin/tac
>
> Tac aborts when using it on a particular file.
>
> *** glibc detected *** tac: double free or corruption (top): 
> 0x00000000025c5030 ***
...

Thank you for the report!
That is indeed a bug in the very latest.

For a stand-alone, minimal demonstrator, run this:

    valgrind tac <(printf %0$((2**14 + 1))d 0) > /dev/null

It prints this:

     Invalid free() / delete / delete[]
        at 0x4A04D72: free (vg_replace_malloc.c:325)
        by 0x402294: main (tac.c:669)
      Address 0x4c30040 is 0 bytes inside a block of size 16,388 free'd
        at 0x4A05255: realloc (vg_replace_malloc.c:476)
        by 0x4117B8: xrealloc (xmalloc.c:57)
        by 0x401A68: tac_seekable (tac.c:319)
        by 0x402379: main (tac.c:515)

Here is a fix:

>From b3959fc691e606857a3c6e9b316ec34819972245 Mon Sep 17 00:00:00 2001
From: Jim Meyering <address@hidden>
Date: Sat, 28 Aug 2010 17:45:29 +0200
Subject: [PATCH] tac: avoid double free

* src/tac.c (main): Reading a line longer than 16KiB would cause
tac to realloc its primary buffer.  Then, just before exit, tac
would mistakenly free the original (now free'd) buffer.
This bug was introduced by commit be6c13e7, "maint: always free a
buffer, to avoid even semblance of a leak".
* NEWS (Bug fixes): Mention it.
* tests/misc/tac (double-free): New test, to exercise this.
Reported by Salvo Tomaselli in <http://bugs.debian.org/594666>.
---
 NEWS           |    3 +++
 src/tac.c      |    6 ++++--
 tests/misc/tac |    6 ++++++
 3 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/NEWS b/NEWS
index 85f55a2..f29d311 100644
--- a/NEWS
+++ b/NEWS
@@ -11,6 +11,9 @@ GNU coreutils NEWS                                    -*- 
outline -*-
   du -H and -L now consistently count pointed-to files instead of
   symbolic links, and correctly diagnose dangling symlinks.

+  tac would perform a double-free when given an input line longer than 16KiB.
+  [bug introduced in coreutils-8.3]
+
 ** New features

   cp now accepts the --attributes-only option to not copy file data,
diff --git a/src/tac.c b/src/tac.c
index cec9736..859e006 100644
--- a/src/tac.c
+++ b/src/tac.c
@@ -633,7 +633,6 @@ main (int argc, char **argv)
   if (! (read_size < half_buffer_size && half_buffer_size < G_buffer_size))
     xalloc_die ();
   G_buffer = xmalloc (G_buffer_size);
-  void *buf = G_buffer;
   if (sentinel_length)
     {
       strcpy (G_buffer, separator);
@@ -666,6 +665,9 @@ main (int argc, char **argv)
       error (0, errno, "-");
       ok = false;
     }
-  free (buf);
+
+  size_t offset = sentinel_length ? sentinel_length : 1;
+  free (G_buffer - offset);
+
   exit (ok ? EXIT_SUCCESS : EXIT_FAILURE);
 }
diff --git a/tests/misc/tac b/tests/misc/tac
index 7631049..4130c00 100755
--- a/tests/misc/tac
+++ b/tests/misc/tac
@@ -24,6 +24,9 @@ my $prog = 'tac';

 my $bad_dir = 'no/such/dir';

+# This must be longer than 16KiB to trigger the double free in coreutils-8.5.
+my $long_line = 'o' x (16 * 1024 + 1);
+
 my @Tests =
 (
   ['segfault', '-r', {IN=>"a\n"}, {IN=>"b\n"}, {OUT=>"a\nb\n"}],
@@ -67,6 +70,9 @@ my @Tests =
    {ERR_SUBST => "s,`$bad_dir': .*,...,"},
    {ERR => "$prog: cannot create temporary file in ...\n"},
    {EXIT => 1}],
+
+  # coreutils-8.5's tac would double-free its primary buffer.
+  ['double-free', {IN=>$long_line}, {OUT=>$long_line}],
 );

 @Tests = triple_test address@hidden;
--
1.7.2.2.510.g7180a