DUMA - DUMA Malloc Debugger
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 DUMA_ALIGNMENT;
extern int DUMA_PROTECT_BELOW;
extern int DUMA_PROTECT_FREE;
extern int DUMA_ALLOW_MALLOC_0;
extern int DUMA_FILL;
DUMA 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, DUMA 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
DUMA 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 libduma.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.
Link your program with the library libduma.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 DUMA for public use, you’ll be able
to use the -lduma argument to the linker, otherwise you’ll have to put
the path-name for libduma.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=libduma.so.0.0 will cause DUMA to be loaded to run
all dynamic executables. The command duma command runs a single
command under DUMA.
Some systems will require special arguments to the linker to assure
that you are using the DUMA malloc() and not the one from your C
Run your program using a debugger. It’s easier to work this way than
to create a core file and post-mortem debug it. DUMA 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 DUMA. If your program has one of
the errors detected by DUMA, 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
DUMA 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 DUMA will detect, so it’s
important that you know how to use them.
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. DUMA_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 DUMA malloc() is aligned
using the value in DUMA_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 DUMA_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 DUMA_ALIGNMENT in the shell
environment to an integer value, or assign to the global integer
variable DUMA_ALIGNMENT using a debugger.
DUMA 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 DUMA_PROTECT_BELOW to 1
causes DUMA to place the inaccessible page before the allocation
in the address space, so that under-runs will be detected
instead of over-runs. When DUMA_PROTECT_BELOW is set, the
DUMA_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
DUMA_PROTECT_BELOW in the shell environment to an integer value,
or assign to the global integer variable DUMA_PROTECT_BELOW
using a debugger.
DUMA 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 DUMA_PROTECT_FREE to 1. This will cause DUMA 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 DUMA_PROTECT_FREE in the shell
environment to an integer value, or assign to the global integer
variable DUMA_PROTECT_FREE using a debugger.
By default, DUMA traps calls to malloc() with a size of zero,
because they are often the result of a software bug. If
DUMA_ALLOW_MALLOC_0 is non-zero, the software will not trap
calls to malloc() with a size of zero. To change this value,
set DUMA_ALLOC_MALLOC_0 in the shell environment to an integer
value, or assign to the global integer variable
DUMA_ALLOC_MALLOC_0 using a debugger.
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
WORD-ALIGNMENT AND OVERRUN DETECTION
There is a conflict between the alignment restrictions that malloc()
operates under and the debugging strategy used by DUMA. When detecting
overruns, DUMA 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, DUMA 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 DUMA 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
DUMA provides the variable DUMA_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 DUMA_ALIGNMENT to
zero. This will result in DUMA 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 DUMA_ALIGNMENT
to the word size.
Another example of software incompatible with DUMA_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 libduma.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 DUMA_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 DUMA_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 DUMA 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 DUMA.
Don’t leave libduma.a linked into production software! Use it only for
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.
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
1563 Solano Ave. #349
Berkeley, CA 94707
/dev/zero: Source of memory pages (via mmap(2)).
malloc(3), mmap(2), mprotect(2), swapon(8)
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.
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, dumatest 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.
Purify does a much more thorough job than DUMA, 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.