Here is an example:
> gdb example64
(gdb) start
Breakpoint 1 at 0x10000734: file example.c, line 10.
Starting program: /home/pgilliam/example64
[Thread debugging using libthread_db enabled]
[New Thread 4398046665456 (LWP 9443)]
Warning:
Cannot insert breakpoint -2.
Error accessing memory address 0x9ce0: Input/output
error.
Cannot insert breakpoint -3.
Error accessing memory address 0x9cf0: Input/output
error.
(gdb)
Here is the problem:
1) In linux-thread-db.c: enable_thread_event(), The routine
"td_ta_event_addr" in the library
"thread_db" gets called and
returns a function descriptor for the address at which to
set
the breakpoints for the "create" and
"delete" thread events
in the "pthread" library.
2) These address point at PLT entries in the '.opd'
section.
3) 'dereferencing' the function descriptor should give the
actual address at which to set a breakpoint, but gives
instead
the offset within the "pthread" library where
the breakpoint
should be placed.
The attached patch 'fixes' the problem by looking up the
load address of the
"pthread" library and adding that to the address
from the PLT. This seems to
do the trick, but THIS HAS ONLY BEEN TESTED WITH A 64-BIT
GDB AND A 64-BIT
TARGET. And it's a real HACK!!!
But it does illustrate the problem.
So, should I try to change GDB so that enable_thread_event()
gets called after
the dynamic loader has has a chance to relocate the .opd?
or
Find a better place way to do the relocation for just these
two things?
-=# Paul Gilliam #=-
--- /home/pgilliam/linux-thread-db.c 2006-08-17
02:27:05.000000000 -0700
+++ hacked.linux-thread-db.c 2006-08-17 02:29:16.000000000
-0700
 -497,6
+497,7
static td_err_e
enable_thread_event (td_thragent_t *thread_agent, int
event, CORE_ADDR *bp)
{
+ static CORE_ADDR thread_lib_reloc = 0;
td_notify_t notify;
td_err_e err;
-514,7
+515,24
? (CORE_ADDR) (intptr_t) notify.u.bptaddr
: (CORE_ADDR) (uintptr_t) notify.u.bptaddr),
¤t_target));
- create_thread_event_breakpoint ((*bp));
+ if (! thread_lib_reloc) {
+ char tbuf[1024];
+ FILE *pmf;
+
+ sprintf (tbuf, "/proc/%d/maps",
proc_handle.pid);
+ pmf = fopen (tbuf, "r");
+ if (pmf) {
+ while (fgets( tbuf, sizeof(tbuf), pmf)) {
+ char *cp = rindex (tbuf, '/');
+ if (cp && strncmp (cp+1, "libpthread",
10) == 0) {
+ thread_lib_reloc = (CORE_ADDR) strtol (tbuf, 0, 16);
+ break;
+ }
+ }
+ fclose (pmf);
+ }
+ }
+ create_thread_event_breakpoint ((*bp) +
thread_lib_reloc);
return TD_OK;
}
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