How to point a shared object to debug information in GDB without altering the files? - linux

Question

I have two files - a shared object file and debug information file.
How can I tell GDB to use the debug information file for that shared object without altering the files, file names or creating links?
Is it even possible?
I just want to tell GDB about it, not to change anything.
EDIT: Here is what I am trying to do (on Ubuntu 16.04, x86_64)
I am taking the libc and libc debug information files from my system, and copy them to a new directory. Then, I preload the moved libc to a process and attach to it with GDB.
sudo apt install libc6-dbg
cp /usr/lib/debug/lib/x86_64-linux-gnu/libc-2.23.so debug_file
cp /lib/x86_64-linux-gnu/libc.so.6 .
cat << EOF > traceme.c
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
int main(void) {
printf("trace me:\nsudo gdb -p %d\n", getpid());
sleep(20);
return 0;
}
EOF
gcc -o traceme traceme.c
LD_PRELOAD=./libc.so.6 ./traceme &
sudo gdb -p 28163
Now, my GDB sessions is this:
(gdb) info sharedlibrary
From To Syms Read Shared Object Library
0x00007ff8e77c68b0 0x00007ff8e7919ac4 Yes (*) ./libc.so.6
0x00007ff8e7b71ac0 0x00007ff8e7b8f810 Yes /lib64/ld-linux-x86-64.so.2
(*): Shared library is missing debugging information.
(gdb) add-symbol-file debug_file 0x00007ff8e77c68b0
add symbol table from file "debug_file" at
.text_addr = 0x7ff8e77c68b0
(y or n) y
Reading symbols from debug_file...done.
(gdb) p &main_arena
$1 = (struct malloc_state *) 0x3c4b20 <main_arena>
(gdb) p main_arena
Cannot access memory at address 0x3c4b20
(gdb) info proc mappings
process 28163
Mapped address spaces:
Start Addr End Addr Size Offset objfile
0x400000 0x401000 0x1000 0x0 /home/ubuntu/tmp/z/traceme
0x600000 0x601000 0x1000 0x0 /home/ubuntu/tmp/z/traceme
0x601000 0x602000 0x1000 0x1000 /home/ubuntu/tmp/z/traceme
0xff8000 0x1019000 0x21000 0x0 [heap]
0x7ff8e77a7000 0x7ff8e7967000 0x1c0000 0x0 /home/ubuntu/tmp/z/libc.so.6
0x7ff8e7967000 0x7ff8e7b67000 0x200000 0x1c0000 /home/ubuntu/tmp/z/libc.so.6
0x7ff8e7b67000 0x7ff8e7b6b000 0x4000 0x1c0000 /home/ubuntu/tmp/z/libc.so.6
0x7ff8e7b6b000 0x7ff8e7b6d000 0x2000 0x1c4000 /home/ubuntu/tmp/z/libc.so.6
0x7ff8e7b6d000 0x7ff8e7b71000 0x4000 0x0
0x7ff8e7b71000 0x7ff8e7b97000 0x26000 0x0 /lib/x86_64-linux-gnu/ld-2.23.so
0x7ff8e7d91000 0x7ff8e7d96000 0x5000 0x0
0x7ff8e7d96000 0x7ff8e7d97000 0x1000 0x25000 /lib/x86_64-linux-gnu/ld-2.23.so
0x7ff8e7d97000 0x7ff8e7d98000 0x1000 0x26000 /lib/x86_64-linux-gnu/ld-2.23.so
0x7ff8e7d98000 0x7ff8e7d99000 0x1000 0x0
0x7ffe53a5a000 0x7ffe53a7b000 0x21000 0x0 [stack]
0x7ffe53b3a000 0x7ffe53b3c000 0x2000 0x0 [vvar]
0x7ffe53b3c000 0x7ffe53b3e000 0x2000 0x0 [vdso]
0xffffffffff600000 0xffffffffff601000 0x1000 0x0 [vsyscall]
For some reason, the main_arena symbol is not within the mapping of the libc.

Answer 1

How can I tell GDB to use the symbols file for that shared object without altering the files, file names or creating links?
(gdb) info shared
Will tell you at what address your foo.so is loaded. Say it's $addr.
(gdb) add-symbol-file /path/to/foo.so.debug $addr
will tell GDB to add debug symbols for foo.so from foo.so.debug
Update:
(gdb) p main_arena
Cannot access memory at address 0x3c4b20
I am pretty sure this is a bug in GDB. You are correct: it's not relocating the .data section when it should.
Fortunately, there is a workaround:
(gdb) add-symbol-file debug_file 0x00007ff8e77c68b0 -s .data 0x7ff8e77a7000
(The first address is from info shared. The second address is from info proc map for the (first) address where libc.so.6 is loaded.)