Building Library Interposers for Fun and Profit
by Greg Nakhimovsky
Library interposition is a useful technique for tuning performance, collecting runtime statistics, or debugging applications. This article offers helpful tips and tools for working with the technique and gets you started on your own interposer.
Greg is an employee of Sun Microsystems and can be contacted at [email protected].
Most of today's applications use shared libraries and dynamic linking, especially for such system libraries as the standard C library (libc), or the X Window or OpenGL libraries. Operating system vendors encourage this method because it provides many advantages. With dynamic linking, you can intercept any function call that an application makes to any shared library. Once you intercept it, you can do whatever you want in that function, as well as call the real function that the application originally intended to call. Performance tuning is one use of this technology. Even if you have access to profilers and other development tools, or the application's source code itself, having your own library interposer puts you completely in control. You can see exactly what you're doing and make adjustments at any time.
Building and Running Your First Interposer
To use library interposition, you need to create a special shared library and
set the LD_PRELOAD environment variable. When LD_PRELOAD is set, the dynamic
linker will use the specified library before any other when it searches for
shared libraries.
Let's create a simple interposer for malloc(), which is normally
a part of /usr/lib/libc.so.1, the standard C library. A message, displaying
the argument passed to each malloc() call, will be printed out each time
the application calls malloc().
Listing One (malloc_interposer.c) is
the source for this interposer. In Listing One, func is a function pointer to
the real malloc() routine, which is in /usr/lib/libc.so.1. The RTLD_NEXT
argument passed to dlsym(3X) tells the dynamic linker to find the next
reference to the specified function, using the normal dynamic linker search
sequence.
Now let's build and run this interposer, using ls(1) as our sample
application. We'll use the C-shell syntax for this and other examples.
% cc -o malloc_interposer.so -G -Kpic malloc_interposer.c % setenv LD_PRELOAD $cwd/malloc_interposer.so % ls -l malloc_interposer.so malloc(64) is called malloc(52) is called malloc(1072) is called -rwxr-xr-x 1 gregn 5224 Aug 3 15:21 malloc_interposer.so* % unsetenv LD_PRELOAD
Without access to the source code of ls(1), and without rebuilding it
in any way, we've just discovered which arguments the application used to call
malloc() in the test run. Note that LD_PRELOAD must specify the full
path to the interposer library, and that library interposition is disabled for
setuid programs in order to prevent security problems.
Collecting Runtime Statistics
Here's a more practical example of library interposition on malloc(), as
well as on a few other routines. It collects statistics about the size of the
memory blocks requested with the calls to malloc(), calloc(), and
realloc(), and prints out a histogram detailing their use upon exiting
the application.
Listing Two (malloc_hist.c) is the source code. Note that we
round up all memory-request sizes to the next power of two. To obtain the name
of the current executable, we use the proc(4) interface. The version of
the proc(4) interface used here works with Solaris 2.6 and above. We can
run this interposer with the CDE editor dtpad as the test application.
% setenv LD_PRELOAD $cwd/malloc_hist.so
% dtpad malloc_hist.c
% unsetenv LD_PRELOAD
Here are the results.
% cat /tmp/malloc_histogram.dtpad.15203
prog_name=dtpad.15203
******** malloc **********
1 76
2 105
4 450
8 667
16 2047
32 620
64 158
128 39
256 33
512 22
1024 32
2048 10
4096 14
8192 46
32768 2
131072 3
******** calloc **********
1 0
2 0
4 1676
8 40
16 21
32 12
64 34
128 4
256 2
512 0
1024 3
8192 7
******** realloc **********
1 0
2 0
4 2
8 2
16 11
32 11
64 14
128 1
256 0
512 0
If the application invokes more than one executable, you'll get a histogram in
the /tmp directory for each. Such histograms can be quite useful in application
performance tuning. We now know that dtpad (as used in this session) calls malloc()
to request 16-byte memory blocks more often than it requests other sizes. This
tool has been used with many real applications. It revealed that most of one application's
malloc() requests were for blocks of four bytes or less. There are two
performance problems with this pattern.
First of all, most malloc() implementations,
including the default Solaris malloc(3C), will waste a lot of memory when
used this way. malloc(3C) uses eight bytes of overhead for each memory
block it returns to the caller. When the application calls malloc, requesting
only four bytes of memory, the malloc() overhead is twice as large as the
useful memory consumed. This memory waste can easily lead to increased paging
to disk, ruining the application's performance. Second, it's possible to create
your own memory allocator specially geared towards small blocks. It can be made
a lot faster than the system's default malloc(), which is designed to deal
with a wide variety of block sizes.
Fixing a Bug
This is a true story. A major mechanical CAD application stopped working with
Solaris 2.6, but continued to work with Solaris 2.5.1. Debugging showed that the
reason for failure was a call to XOpenDisplay(3X11) that returned NULL.
Interposing on that routine revealed that the application was calling XOpenDisplay()
with the argument unix:0.0 instead of with the usual NULL.
The reason it didn't
work was a bug in X. It could also be considered a bug in the application, because
using unix:0.0 for DISPLAY is an old Unix technique which no longer makes
sense. In any case, we needed a quick workaround until the bug was fixed. The
application in question was old and complex. It called XOpenDisplay() many
times from different modules, so even tracking the troublesome calls was a challenge.
The following library interposer was our solution. Not only did this interposer
print out the argument passed to XOpenDisplay(), it actually changed the
XOpenDisplay() argument to NULL, fixing the problem; see Listing Three
(XOpenDisplay_interpose.c).
More Ideas
Now that you know how to build and use library interposers, here are some other things you can have them do:
- Compute the amount of time spent in malloc(), realloc(),
and free() routines, and print out the results upon exiting. A good
timing routine in Solaris is gethrtime(3C). Once you have this information,
you'll know if memory management is a performance bottleneck.
- Collect usage statistics for memmove(), memcpy(), and memset()
routines, and print out a histogram similar to the one for malloc().
This will tell you how much data the application is moving around, and whether
or not this area of the program is worth optimizing.
- You can also determine how many backward moves or copies the application performs. A backward move occurs when the source address is larger than the destination address. The performance of backward memory moves is often much worse than that of forward moves.
- Determine which environment variables the application is using. Interpose
on getenv(3C), and print its argument and return value.
- Test a different version of malloc(), or any other dynamically-linked
routine, with your application. All you have to do is interpose the test version
(assuming it's built as a shared library). To go back to using the original
version, simply undefine LD_PRELOAD.
Using the library interposers described in this article, you can monitor your applications' patterns of system-resource consumption and provide useful feedback to application developers.
Acknowledgments
I'd like to thank two of my colleagues at Sun Microsystems: Bart Smaalders, who wrote the original version of the interposer to collect malloc statistics, and Morgan Herrington, who generously helped in many ways.
Resources
"Profiling and Tracing Dynamic Library Usage via Interposition," Timothy W.
Curry (USENIX Conference Proceedings, Summer 1994): http://www.usenix.org/publications/library/proceedings/bos94/curry.html.
Listing One
/* Example of a library interposer: interpose on malloc().
* Build and use this interposer as following:
* cc -o malloc_interposer.so -G -Kpic malloc_interposer.c
* setenv LD_PRELOAD $cwd/malloc_interposer.so
* run the app
* unsetenv LD_PRELOAD
*/
#include <stdio.h>
#include <dlfcn.h>
void *malloc(size_t size)
{
static void * (*func)();
if(!func)
func = (void *(*)()) dlsym(RTLD_NEXT, "malloc");
printf("malloc(%d) is called\n", size);
return(func(size));
}
Listing Two
/* Library interposer to collect malloc/calloc/realloc statistics
* and produce a histogram of their use.
* cc -o malloc_hist.so -G -Kpic malloc_hist.c
* setenv LD_PRELOAD $cwd/malloc_hist.so
* run the application
* unsetenv LD_PRELOAD
*
* The results will be in /tmp/malloc_histogram.<prog_name>.<pid>
* for each process invoked by current application.
*/
#include <dlfcn.h>
#include <memory.h>
#include <assert.h>
#include <thread.h>
#include <stdio.h>
#include <procfs.h>
#include <fcntl.h>
typedef struct data {
int histogram[32];
char * caller;
} data_t;
data_t mdata = {
0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
"malloc"};
data_t cdata = {
0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
"calloc"};
data_t rdata = {
0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
"realloc"};
static FILE* output;
static int pid;
static char prog_name[32];
static char path[64];
static void done()
{
fprintf(output, "prog_name=%s\n", prog_name);
print_data(&mdata);
print_data(&cdata);
print_data(&rdata);
}
static int print_data(data_t * ptr)
{
int i;
fprintf(output, "******** %s **********\n", ptr->caller);
for(i=0;i<32;i++)
if(i < 10 || ptr->histogram[i])
fprintf(output, "%10u\t%10d\n", 1<<i, ptr->histogram[i]);
}
exit(int status)
{
char procbuf[32];
psinfo_t psbuf;
int fd;
/* Get current executable's name using proc(4) interface */
pid = (int)getpid();
(void)sprintf(procbuf, "/proc/%ld/psinfo", (long)pid);
if ((fd = open(procbuf, O_RDONLY)) != -1)
{
if (read(fd, &psbuf, sizeof(psbuf)) == sizeof(psbuf))
sprintf(prog_name, "%s.%d", psbuf.pr_fname, pid);
else
sprintf(prog_name, "%s.%d", "unknown", pid);
}
else
sprintf(prog_name, "%s.%d", "unknown", pid);
sprintf(path, "%s%s", "/tmp/malloc_histogram.", prog_name);
/* Open the file here since
the shell closes all file descriptors before calling exit() */
output = fopen(path, "w");
if(output)
done();
(*((void (*)())dlsym(RTLD_NEXT, "exit")))(status);
}
static int bump_counter(data_t * ptr, int size)
{
static mutex_t lock;
int size_orig;
int i = 0;
size_orig = size;
while(size /= 2)
i++;
if(1<<i < size_orig)
i++;
/* protect histogram data if application is multithreaded */
mutex_lock(&lock);
ptr->histogram[i]++;
mutex_unlock(&lock);
}
void * malloc(size_t size)
{
static void * (*func)();
void * ret;
if(!func) {
func = (void *(*)()) dlsym(RTLD_NEXT, "malloc");
}
ret = func(size);
bump_counter(&mdata, size);
return(ret);
}
void * calloc(size_t nelem, size_t elsize)
{
static void * (*func)();
void * ret;
int i;
if(!func)
func = (void *(*)()) dlsym(RTLD_NEXT, "calloc");
ret = func(nelem, elsize);
for(i=0;i<nelem;i++)
bump_counter(&cdata, elsize);
return(ret);
}
void * realloc(void *ptr, size_t size)
{
static void * (*func)();
void * ret;
if(!func)
func = (void *(*)()) dlsym(RTLD_NEXT, "realloc");
ret = func(ptr, size);
bump_counter(&rdata, size);
return(ret);
}
Listing Three
/*
* Library interposer for XOpenDisplay() and XCloseDisplay.
* cc -g -o XOpenDisplay_interpose.so -G -Kpic XOpenDisplay_interpose.c
* setenv LD_PRELOAD $cwd/XOpenDisplay_interpose.so
* run the app
* unsetenv LD_PRELOAD
*/
#include <stdio.h>
#include <X11/Xlib.h>
#include <dlfcn.h>
Display *XOpenDisplay(char *display_name)
{
static Display * (*func)(char *);
Display *ret;
if(!func)
func = (Display *(*)(char *))dlsym(RTLD_NEXT, "XOpenDisplay");
if(display_name)
printf("XOpenDisplay() is called with display_name=%s\n", display_name);
else
printf("XOpenDisplay() is called with display_name=NULL\n");
/*
ret = func(display_name);
*/
printf(" calling XOpenDisplay(NULL)\n");
ret = func(0);
printf("XOpenDisplay() returned %p\n", ret);
return(ret);
}
int XCloseDisplay(Display *display_name)
{
static int (*func)(Display *);
int ret;
if(!func)
func = (int (*)(Display *))dlsym(RTLD_NEXT, "XCloseDisplay");
ret = func(display_name);
printf("called XCloseDisplay(%p)\n", display_name);
return(ret);
}
This article first appeared in Unix Insider by ITworld.
