NAME

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;

extern int EF_ALLOW_MALLOC_0;

extern int EF_FILL;

DESCRIPTION

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

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

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.

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

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

Don't leave libefence.a linked into production software! Use it only for debugging.

MAILING LIST

AUTHOR

WARNINGS

LICENSE

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: bruce@perens.com

FILES

SEE ALSO

DIAGNOSTICS

BUGS

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.

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