efence(3) efence(3)
27-April-1993
NAME
efence - Electric Fence Malloc Debugger
SYNOPSIS
#include <stdlib.h>
void * malloc (size_t size);
void free (void *ptr);
void * realloc (void *ptr, size_t size);
void * calloc (size_t nelem, size_t elsize);
void * memalign (size_t alignment, size_t size);
void * valloc (size_t size);
extern int EF_ALIGNMENT;
extern int EF_PROTECT_BELOW;
extern int EF_PROTECT_FREE;
DESCRIPTION
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. 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. On AIX systems, you may have to use
the flags
-bnso -bnodelcsect -bI:/lib/syscalls.exp
On Sun systems running SunOS 4.X, you'll probably have to use -
Bstatic. 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
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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 of
that 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.
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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.
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,
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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.
PORTING
Electric Fence is written for ANSI C. You should be able to port it
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with simple changes to the Makefile and to page.c, which contains the
memory management primitives . Many POSIX platforms will require only
a re-compile. The operating system facilities required to port
Electric Fence are:
A way to allocate memory pages
A way to make selected pages inaccessible.
A way to make the pages accessible again.
A way to detect when a program touches an inaccessible page.
A way to print messages. Please e-mail me a copy of any changes
you have to make, so that I can merge them into the distribution.
AUTHOR
Bruce Perens
WARNINGS
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. Bruce Perens and/or Pixar will not be liable to any claims
resulting from the use of this software or the ideas within it. The
entire responsibility for its use must be assumed by the user. If you
use it and it results in loss of life and/or property, tough. If it
leads you on a wild goose chase and you waste two weeks debugging
something, too bad. If you can't deal with the above, please don't
use the software! I've written this in an attempt to help other
people, not to get myself sued or prosecuted.
LICENSE
Copyright 1987-1995 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
c/o Pixar
1001 West Cutting Blvd., Suite 200
Richmond, CA 94804
Telephone: 510-215-3502
Fax: 510-236-0388
Internet: Bruce@Pixar.com
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FILES
/dev/zero: Source of memory pages (via mmap(2)).
SEE ALSO
malloc(3), mmap(2), mprotect(2), swapon(8)
DIAGNOSTICS
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 are 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, from Purify Systems, does a much better job than Electric
Fence, and does much more. It's available at this writing on SPARC and
HP. I'm not affiliated with Purify, I just think it's a wonderful
product and you should check it out.
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