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bug#2844: infinite loop in boyer_moore()

From: Alexandre Oliva
Subject: bug#2844: infinite loop in boyer_moore()
Date: Fri, 27 Mar 2009 00:05:34 -0300


Gnus has been entering infinite loops for me while splitting mail.  Today I got
a chance to look into it.  The problem is in boyer_moore(), in search.c:

    /* Use signed comparison if appropriate
       to make cursor+infinity sure to be > p_limit.
       Assuming that the buffer lies in a range of addresses
       that are all "positive" (as ints) or all "negative",
       either kind of comparison will work as long
       as we don't step by infinity.  So pick the kind
       that works when we do step by infinity.  */
    if ((EMACS_INT) (p_limit + infinity) > (EMACS_INT) p_limit)
      while ((EMACS_INT) cursor <= (EMACS_INT) p_limit)
        cursor += BM_tab[*cursor];
      while ((EMACS_UINT) cursor <= (EMACS_UINT) p_limit)
        cursor += BM_tab[*cursor];

it takes the signed (EMACS_INT) loop, but that fails because cursor is
(unsigned char *) 0x7fffc440, whereas p_limit is (unsigned char *) 0x80001260.

infinity, computed earlier in that function, is 0x37dac21, but I don't see how
a positive value would have helped.  It seems to me that we have to check that
we won't be crossing this boundary starting at cursor rather than p_limit, or
maybe both.  I haven't thought much about it.

I suppose checking that

  (EMACS_INT)(cursor + 20000) > (EMACS_INT)(cursor)

would also be necessary before choosing the EMACS_INT variant of the loop.

In GNU Emacs 22.3.1 (i386-redhat-linux-gnu, GTK+ Version 2.14.7)
 of 2009-02-09 on x86-5.fedora.phx.redhat.com
Windowing system distributor `The X.Org Foundation', version 11.0.10503000
configured using `configure  '--build=i386-redhat-linux-gnu' 
'--host=i386-redhat-linux-gnu' '--target=i386-redhat-linux-gnu' 
'--program-prefix=' '--prefix=/usr' '--exec-prefix=/usr' '--bindir=/usr/bin' 
'--sbindir=/usr/sbin' '--sysconfdir=/etc' '--datadir=/usr/share' 
'--includedir=/usr/include' '--libdir=/usr/lib' '--libexecdir=/usr/libexec' 
'--localstatedir=/var' '--sharedstatedir=/usr/com' '--mandir=/usr/share/man' 
'--infodir=/usr/share/info' '--with-x-toolkit=gtk' 
'build_alias=i386-redhat-linux-gnu' 'host_alias=i386-redhat-linux-gnu' 
'target_alias=i386-redhat-linux-gnu' 'CFLAGS=-DMAIL_USE_LOCKF -O2 -g -pipe 
-Wall -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector 
--param=ssp-buffer-size=4 -m32 -march=i386 -mtune=generic 

Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8
  default-enable-multibyte-characters: t

Alexandre Oliva           http://www.lsd.ic.unicamp.br/~oliva/
You must be the change you wish to see in the world. -- Gandhi
Be Free! -- http://FSFLA.org/   FSF Latin America board member
Free Software Evangelist      Red Hat Brazil Compiler Engineer

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