Electric Fence
helps you detect two common programming bugs:
software that overruns the boundaries of a malloc() memory
allocation, and software that touches a memory allocation that has been
released by free(). Unlike other malloc() debuggers, Electric Fence will
detect
read
accesses as well as writes, and it will pinpoint the exact instruction that
causes an error. It has been in use at Pixar since 1987, and at many other
sites for years.
Electric Fence uses the virtual memory hardware of your computer to place an
inaccessible memory page immediately after (or before, at the user's option)
each memory allocation. When software reads or writes this inaccessible page,
the
hardware issues a segmentation fault, stopping the program at the offending
instruction. It is then trivial to find the erroneous statement using your
favorite debugger. In a similar manner, memory that has been released by
free() is made inaccessible, and any code that touches it will get a
segmentation fault.
Simply linking your application with libefence.a will allow you to detect
most, but not all, malloc buffer overruns and accesses of free memory.
If you want to be reasonably sure that you've found
all
bugs of this type, you'll have to read and understand the rest of this
man page.
USAGE
Link your program with the library
libefence.a .
Make sure you are
not
linking with
-lmalloc,-lmallocdebug,
or with other malloc-debugger or malloc-enhancer libraries.
You can only use one at a time.
If your system administrator
has installed Electric Fence for public use, you'll be able to use the
-lefence
argument to the linker, otherwise you'll have to put the path-name for
libefence.a
in the linker's command line.
You can also use dynamic linking. If you're using a Bourne shell, the
statement
export LD_PRELOAD=libefence.so.0.0
will cause Electric Fence to be loaded to run all dynamic executables.
The command
efcommand
runs a single command under Electric Fence.
Some systems will require special arguments to the linker to assure that
you are using the Electric Fence malloc() and not the one from your C library.
Run your program
using a debugger.
It's easier to work this way than to create a
core
file and post-mortem debug it. Electric Fence can create
huge
core files, and some operating systems will thus take minutes simply to dump
core! Some operating systems will not create usable core files from programs
that are linked with Electric Fence.
If your program has one of the errors detected by Electric Fence, it will
get a segmentation fault (SIGSEGV) at the offending instruction. Use the
debugger to locate the erroneous statement, and repair it.
GLOBAL AND ENVIRONMENT VARIABLES
Electric Fence has four configuration switches that can be enabled via
the shell environment, or by setting the value of global integer variables
using a debugger. These switches change what bugs Electric Fence will detect,
so it's important that you know how to use them.
EF_ALIGNMENT
This is an integer that specifies the alignment for any memory allocations
that will be returned by malloc(), calloc(), and realloc().
The value is specified in
bytes, thus a value of 4 will cause memory to be aligned to 32-bit boundaries
unless your system doesn't have a 8-bit characters. EF_ALIGNMENT is set to
sizeof(int) by default, since that is generally the word-size of your CPU.
If your program requires that allocations be aligned to 64-bit
boundaries and you have a 32-bit
int
you'll have to set this value to 8. This is the case when compiling with the
-mips2
flag on MIPS-based systems such as those from SGI.
The memory allocation that is returned by Electric Fence malloc() is aligned
using the value in EF_ALIGNMENT, and
its size the multiple ofthat value
that is greater than or equal to the requested size.
For this reason, you will sometimes want to set EF_ALIGNMENT to 0 (no
alignment), so that
you can detect overruns of less than your CPU's word size. Be sure to read
the section
WORD-ALIGNMENT AND OVERRUN DETECTION
in this manual page before you try this.
To change this value, set EF_ALIGNMENT in the shell environment to an
integer value, or assign
to the global integer variable EF_ALIGNMENT using a debugger.
EF_PROTECT_BELOW
Electric Fence usually places an inaccessible page immediately after each
memory allocation, so that software that runs past the end of the allocation
will be detected. Setting EF_PROTECT_BELOW to 1 causes Electric Fence
to place the inaccessible page
before
the allocation in the address space, so that under-runs will be detected
instead of over-runs.
When EF_PROTECT_BELOW is set, the EF_ALIGNMENT parameter is ignored.
All allocations will be aligned to virtual-memory-page boundaries, and
their size will be the exact size that was requested.
To change this value, set EF_PROTECT_BELOW in the shell environment to an
integer value, or assign to the global integer variable EF_PROTECT_BELOW using
a debugger.
EF_PROTECT_FREE
Electric Fence usually returns free memory to a pool from which it may be
re-allocated. If you suspect that a program may be touching free memory,
set EF_PROTECT_FREE to 1. This will cause Electric Fence to never re-allocate
memory once it has been freed, so that any access to free memory will be
detected. Some programs will use tremendous amounts of memory when this
parameter is set.
To change this value, set EF_PROTECT_FREE in the shell environment to an
integer value, or assign to the global integer variable EF_PROTECT_FREE using
a debugger.
EF_ALLOW_MALLOC_0
By default, Electric Fence traps calls to malloc() with a size of zero, because
they are often the result of a software bug. If EF_ALLOW_MALLOC_0 is non-zero,
the software will not trap calls to malloc() with a size of zero.
To change this value, set EF_ALLOC_MALLOC_0 in the shell environment to an
integer value, or assign to the global integer variable EF_ALLOC_MALLOC_0 using
a debugger.
EF_FILL
When set to a value between 0 and 255, every byte of allocated memory is
initialized to that value. This can help detect reads of uninitialized memory.
When set to -1, some memory is filled with zeroes
(the operating system default on most systems) and some memory will retain
the values written to it during its last use.
WORD-ALIGNMENT AND OVERRUN DETECTION
There is a conflict between the alignment restrictions that malloc() operates
under and the debugging strategy used by Electric Fence. When detecting
overruns, Electric Fence malloc() allocates two or more virtual memory
pages for each allocation. The last page is made inaccessible in such a way
that any read, write, or execute access will cause a segmentation fault.
Then, Electric Fence malloc() will return an address such that the first
byte after
the end of the allocation is on the inaccessible page.
Thus, any overrun
of the allocation will cause a segmentation fault.
It follows that the
address returned by malloc() is the address of the inaccessible page minus
the size of the memory allocation.
Unfortunately, malloc() is required to return
word-aligned
allocations, since many CPUs can only access a word when its address is aligned.
The conflict happens when software makes a memory allocation using a size that
is not a multiple of the word size, and expects to do word accesses to that
allocation. The location of the inaccessible page is fixed by hardware at
a word-aligned address. If Electric Fence malloc() is to return an aligned
address, it must increase the size of the allocation to a multiple of the
word size.
In addition, the functions memalign() and valloc() must honor explicit
specifications on the alignment of the memory allocation, and this, as well
can only be implemented by increasing the size of the allocation.
Thus, there will be situations in which the end of a memory allocation
contains some padding space, and accesses of that padding space will not
be detected, even if they are overruns.
Electric Fence provides the variable EF_ALIGNMENT so that the user can
control the default alignment used by malloc(), calloc(), and realloc().
To debug overruns as small as a single byte, you can set EF_ALIGNMENT to
zero. This will result in Electric Fence malloc() returning unaligned
addresses for allocations with sizes that are not a multiple of the word
size. This is not a problem in most cases, because compilers must pad the
size of objects so that alignment restrictions are honored when storing
those objects in arrays. The problem surfaces when software allocates
odd-sized buffers for objects that must be word-aligned. One case of this
is software that allocates a buffer to contain a structure and a
string, and the string has an odd size (this example was in a popular TIFF
library). If word references are made to un-aligned buffers, you will see
a bus error (SIGBUS) instead of a segmentation fault. The only way to fix
this is to re-write the offending code to make byte references or not make
odd-sized allocations, or to set EF_ALIGNMENT to the word size.
Another example of software incompatible with
EF_ALIGNMENT < word-size
is the strcmp() function and other string functions on SunOS (and probably
Solaris), which make word-sized accesses to character strings, and may
attempt to access up to three bytes beyond the end of a string. These
result in a segmentation fault (SIGSEGV). The only way around this is to
use versions of the string functions that perform byte references instead
of word references.
INSTRUCTIONS FOR DEBUGGING YOUR PROGRAM
1.
Link with libefence.a as explained above.
2.
Run your program in a debugger and fix any overruns or accesses to free memory.
3.
Quit the debugger.
4.
Set EF_PROTECT_BELOW = 1 in the shell environment.
5.
Repeat step 2, this time repairing underruns if they occur.
6.
Quit the debugger.
7.
Read the restrictions in the section on
WORD-ALIGNMENT AND OVERRUN DETECTION.
See if you can
set EF_ALIGNMENT to 0 and repeat step 2. Sometimes this will be too much work,
or there will be problems with library routines for which you don't have the
source, that will prevent you from doing this.
MEMORY USAGE AND EXECUTION SPEED
Since Electric Fence uses at least two virtual memory pages for each of its
allocations, it's a terrible memory hog. I've sometimes found it necessary to
add a swap file using swapon(8) so that the system would have enough virtual
memory to debug my program. Also, the way we manipulate memory results in
various cache and translation buffer entries being flushed with each call
to malloc or free. The end result is that your program will be much slower
and use more resources while you are debugging it with Electric Fence.
Don't leave libefence.a linked into production software! Use it only
for debugging.
I have tried to do as good a job as I can on this software, but I doubt
that it is even theoretically possible to make it bug-free.
This software has no warranty. It will not detect some bugs that you might
expect it to detect, and will indicate that some non-bugs are bugs.
LICENSE
Copyright 1987-1999 Bruce Perens. All rights reserved.
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License, Version 2,
as published by the Free Software Foundation. A copy of this license is
distributed with this software in the file "COPYING".
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. Read the
file "COPYING" for more details.
CONTACTING THE AUTHOR
Bruce Perens
1563 Solano Ave. #349
Berkeley, CA 94707
Telephone: 510-526-1165
Internet: [email protected]
Segmentation Fault: Examine the offending statement for violation of the
boundaries of a memory allocation.
Bus Error: See the section on
WORD-ALIGNMENT AND OVERRUN DETECTION.
in this manual page.
BUGS
My explanation of the alignment issue could be improved.
Some Sun systems running SunOS 4.1 were reported to signal an access to a
protected page with
SIGBUS
rather than
SIGSEGV,
I suspect this is an undocumented feature of a particular Sun hardware
version, not just the operating system.
On these systems, eftest will fail with a bus error until you modify the
Makefile to define
PAGE_PROTECTION_VIOLATED_SIGNAL
as
SIGBUS.
There are, without doubt, other bugs and porting issues. Please contact me via
e-mail if you have any bug reports, ideas, etc.
WHAT'S BETTER
Purify
does a much more thorough job than Electric Fence, and does not have
the huge memory overhead.
Checkergcc,
a modified version of the GNU C Compiler that instruments all memory
references,
is available on Linux systems and where GCC is used. It performs some of the
same tasks as Purify, but only on code that it has compiled.
