It takes some work to figure out how to load symbol tables at the correct offsets for relocatable object modules in GDB. This is done automatically for normal (non-remote) targets, but for the remote target, you have to do it yourself. You need to find out the actual address at which the shared module is mapped in the current context on the simulated machine, and then calculate the offset to use for GDB's add-symbol-file command.
To find the addresses of the shared libraries mapped into a process'
memory space under Solaris, use the /usr/proc/bin/pmap
pid command. The start address of the text segment can be
obtained from the Addr field in the .text line of
the output from dump -h file.
Under Linux, the list of memory mappings can be found in
the file /proc/pid/maps (plain text format). The
VMA column of the .text line of the output from
objdump -h file contains the start address of the
text segment.
Using these two values, map address and text address, you should use map address + text address as the offset to add-symbol-file (it has to be done this way to compensate for how GDB handles symbol loading).
To show you how it works, we will work through a simple example. The
example uses a simple program with a simple shared
library. The program can be found
in [qsp-x86]/targets/qsp-x86/images/hello
and the shared library is the libgreeter.so file in the
same directory. Here and in the rest of this
section [qsp-x86] refers to the location where the
QSP-x86 package is installed.
Start by booting the firststeps machine. Then mount the host file system and copy the program and shared library onto the machine. This should be done on the target:
~ # mount /host ~ # cp /host/[qsp-x86]/targets/qsp-x86/images/hello . ~ # cp /host/[qsp-x86]/targets/qsp-x86/images/libgreeter.so .
Then run the program in the background. The program will enter the infinite loop in the shared library.
Now we need the map address and the text address of the shared library. To get the map address, look in the process file system to see where it has mapped the shared library:
~ # ./hello & [1] 13104 ~ # cat /proc/13104/maps 00400000-00401000 r-xp 00000000 08:02 9442 /home/root/hello 00600000-00601000 r--p 00000000 08:02 9442 /home/root/hello 00601000-00602000 rw-p 00001000 08:02 9442 /home/root/hello 7f686c5ed000-7f686c786000 r-xp 00000000 08:02 10049 /lib/libc-2.21.so 7f686c786000-7f686c986000 ---p 00199000 08:02 10049 /lib/libc-2.21.so 7f686c986000-7f686c98a000 r--p 00199000 08:02 10049 /lib/libc-2.21.so 7f686c98a000-7f686c98c000 rw-p 0019d000 08:02 10049 /lib/libc-2.21.so 7f686c98c000-7f686c990000 rw-p 00000000 00:00 0 7f686c990000-7f686c991000 r-xp 00000000 08:02 10247 /home/root/libgreeter.so 7f686c991000-7f686cb90000 ---p 00001000 08:02 10247 /home/root/libgreeter.so 7f686cb90000-7f686cb91000 r--p 00000000 08:02 10247 /home/root/libgreeter.so 7f686cb91000-7f686cb92000 rw-p 00001000 08:02 10247 /home/root/libgreeter.so 7f686cb92000-7f686cbb4000 r-xp 00000000 08:02 9919 /lib/ld-2.21.so 7f686cdad000-7f686cdb0000 rw-p 00000000 00:00 0 7f686cdb2000-7f686cdb3000 rw-p 00000000 00:00 0 7f686cdb3000-7f686cdb4000 r--p 00021000 08:02 9919 /lib/ld-2.21.so 7f686cdb4000-7f686cdb5000 rw-p 00022000 08:02 9919 /lib/ld-2.21.so 7f686cdb5000-7f686cdb6000 rw-p 00000000 00:00 0 7fffde6a7000-7fffde6c8000 rw-p 00000000 00:00 0 [stack] 7fffde7ff000-7fffde800000 r-xp 00000000 00:00 0 [vdso] ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0 [vsyscall]
From this output you can see that the program is running with PID 13104 and that the map address is 0x7f686c990000. The exact PID may differ, adapt the commands accordingly.
To get the text address we use objdump. This should be run on a host computer with objdump installed:
> objdump -h [qsp-x86]/targets/qsp-x86/images/libgreeter.so
libgreeter.so: file format elf64-x86-64
Sections:
Idx Name Size VMA LMA File off Algn
:
10 .text 00000134 00000000000005e0 00000000000005e0 000005e0 2**4
CONTENTS, ALLOC, LOAD, READONLY, CODE
:
The .text symbols starts at address
0x5e0 and this is what we call the text address, so if
we connect GDB to Simics we have to add the symbols with an offset of
0x7f686c990000 + 0x5e0 = 0x7f686c9905e0.
Now we can set up GDB, connect it to Simics, and debug the program:
(gdb) dir [qsp-x86]/targets/qsp-x86/ Source directories searched: [qsp-x86]/targets/qsp-x86:$cdir:$cwd (gdb) add-symbol-file [qsp-x86]/targets/qsp-x86/images/libgreeter.so 0x7f686c9905e0 add symbol table from file "[qsp-x86]/targets/qsp-x86/images/libgreeter.so" at .text_addr = 0x6c9905e0 (y or n) y Reading symbols from [qsp-x86]/targets/qsp-x86/images/libgreeter.so...done. (gdb) set endian little The target is assumed to be little endian (gdb) target remote localhost:9123 Remote debugging using localhost:9123 greet (name=0x4008eb "World") at ../greeter.c:12 14 while (loop); /* Loop until the loop variable is reset by gdb */ (gdb)
This is just a toy program written to make it possible to debug it without any OS awareness. Normally you would use the OS awareness functionality described in Simics Analyzer User's Guide to debug user space programs.