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hacking shlib/dlopened breakpoints
- From: caolan at csn dot ul dot ie (Caolan McNamara)
- To: gdb at sources dot redhat dot com
- Date: Fri, 21 Nov 2003 12:55:02 +0000
- Subject: hacking shlib/dlopened breakpoints
I'm looking at breakpoints in dlopened libraries at the moment,
setting a breakpoint after my library is dlopened works of course
and, as in the example below, I see that gdb can move the address of
the breakpoint in the .so when it is unloaded and reloaded during
execution, but on re-execution of the little program I get
"
Warning:
Cannot insert breakpoint X.
Error accessing memory address 0xe8535a: Input/output error.
"
Naturally the library isn't loaded at the start of re-execution, but I
hoped that the the breakpoint state would change to bp_shlib_disabled
and get reenabled when the .so reappears.
So I dig a little and see that this will only happen if
(DISABLE_UNSETTABLE_BREAK (b->address)) is true. But that queries to
see if the address is a valid loaded address, which it isn't anymore.
I naively turned this to if (1) to see what would happen, and was
happy to see this worked, the breakpoint in the .so gets re-enabled
and set to its new address when the affected .so reappears, and gdb
stops in it correctly.
But sadly when I continue everything only works as far as the dlclose
of that .so where I get...
Program received signal SIGSEGV, Segmentation fault.
Cannot remove breakpoints because program is no longer writable.
It might be running in another process.
Further execution is probably impossible.
0x008c4ea4 in _dl_debug_state_internal () from /lib/ld-linux.so.2
Any ideas/hints as to how to make it work, or what exactly might the
cause of my exciting crash above ? The reason I'm fiddling with this
is that I'm really after a working deferred break/future-break in gdb
to make debugging apps like OpenOffice/Mozilla with mountains of
dynamically loaded components a lot easier.
C.
Sample program/lib and patch follow...
/*---main.c--- start*/
#include <stdio.h>
#include <dlfcn.h>
int main(int argc, char **argv)
{
void *handle;
double (*cosine)(double);
char *error;
int i;
for (i = 0; i < 3; ++i)
{
handle = dlopen ("./libfoo.so", RTLD_LAZY);
if (!handle)
{
fprintf (stderr, "%s\n", dlerror());
exit(1);
}
cosine = dlsym(handle, "fakecos");
if ((error = dlerror()) != NULL)
{
fprintf (stderr, "%s\n", error);
exit(1);
}
printf ("%f\n", (*cosine)(2.0));
dlclose(handle);
}
return 0;
}
/*---main.c--- end*/
/*---libfoo.c--- start*/
double fakecos(double f)
{
return 100.0;
}
/*---libfoo.c--- end*/
gcc -g -rdynamic main.c -ldl
gcc -g -shared -nostartfiles -o libfoo.so
diff -u -r gdb-6.0/gdb/breakpoint.c gdb-6.0.stage1/gdb/breakpoint.c
--- gdb-6.0/gdb/breakpoint.c 2003-07-02 17:24:30.000000000 +0100
+++ gdb-6.0.stage1/gdb/breakpoint.c 2003-11-19 16:30:44.000000000 +0000
@@ -854,7 +854,7 @@
{
/* Can't set the breakpoint. */
#if defined (DISABLE_UNSETTABLE_BREAK)
- if (DISABLE_UNSETTABLE_BREAK (b->address))
+ if (1)
{
/* See also: disable_breakpoints_in_shlibs. */
val = 0;
--
Caolan McNamara | caolan@skynet.ie
http://www.skynet.ie/~caolan | +353 86 8161184
Once my power is secure, I will destroy all those pesky time-travel devices