I can debug some program (say /bin/ls) like this:
[ks@localhost ~]$ gdb -q --args /bin/ls Reading symbols from /bin/ls...Reading symbols from /bin/ls...(no debugging symbols found)...done. (no debugging symbols found)...done. Missing separate debuginfos, use: debuginfo-install coreutils-8.22-19.fc21.x86_64 (gdb) start Temporary breakpoint 1 at 0x402990 Starting program: /usr/bin/ls [Thread debugging using libthread_db enabled] Using host libthread_db library "/lib64/libthread_db.so.1". Temporary breakpoint 1, 0x0000000000402990 in main () (gdb)
Here I can set temporary breakpoint at main and stop at it.
But I have to run program with custom elf interpreter like this:
[ks@localhost ~]$ gdb -q --args /lib64/ld-linux-x86-64.so.2 /bin/ls Reading symbols from /lib64/ld-linux-x86-64.so.2...Reading symbols from /usr/lib/debug/usr/lib64/ld-2.20.so.debug...done. done. (gdb) start Function "main" not defined. Make breakpoint pending on future shared library load? (y or [n]) y Temporary breakpoint 1 (main) pending. Starting program: /usr/lib64/ld-linux-x86-64.so.2 /bin/ls [Thread debugging using libthread_db enabled] Using host libthread_db library "/lib64/libthread_db.so.1". 1234 glibc-2.20 python tmp [Inferior 1 (process 2610) exited normally] Missing separate debuginfos, use: debuginfo-install libacl-2.2.52-7.fc21.x86_64 libattr-2.4.47-9.fc21.x86_64 libcap-2.24-7.fc21.x86_64 pcre-8.35-8.fc21.x86_64 (gdb)
Here gdb did not stop at main because symbols for /bin/ls were not loaded. How can I force gdb to load symbols and stop at main in this case?
Here is how you can do it:
cat t.c #include <stdio.h> #include <stdlib.h> int main() { printf("Hello\n"); return 0; } gcc -g t.c gdb -q --args /usr/lib64/ld-linux-x86-64.so.2 ./a.out (gdb) start Function "main" not defined. Starting program: /usr/lib64/ld-linux-x86-64.so.2 ./a.out Hello [Inferior 1 (process 7134) exited normally]
So far everything is matching what you observed. Now for the solution:
(gdb) set stop-on-solib-events 1 (gdb) r Starting program: /usr/lib64/ld-linux-x86-64.so.2 ./a.out Stopped due to shared library event (no libraries added or removed) (gdb) c Continuing. Stopped due to shared library event: Inferior loaded /usr/lib64/ld-linux-x86-64.so.2 (gdb) c Continuing. Stopped due to shared library event: Inferior loaded /usr/lib64/libc.so.6
At this point, ./a.out has also been loaded, and you can confirm that with:
(gdb) info proc map process 7140 Mapped address spaces: Start Addr End Addr Size Offset objfile 0x400000 0x401000 0x1000 0x0 /tmp/a.out 0x600000 0x601000 0x1000 0x0 /tmp/a.out 0x601000 0x602000 0x1000 0x1000 /tmp/a.out 0x555555554000 0x555555579000 0x25000 0x0 /usr/lib64/ld-2.19.so 0x555555779000 0x55555577a000 0x1000 0x25000 /usr/lib64/ld-2.19.so 0x55555577a000 0x55555577c000 0x2000 0x26000 /usr/lib64/ld-2.19.so 0x7ffff7c2a000 0x7ffff7c2d000 0x3000 0x0 0x7ffff7c2d000 0x7ffff7df0000 0x1c3000 0x0 /usr/lib64/libc-2.19.so 0x7ffff7df0000 0x7ffff7fef000 0x1ff000 0x1c3000 /usr/lib64/libc-2.19.so 0x7ffff7fef000 0x7ffff7ff3000 0x4000 0x1c2000 /usr/lib64/libc-2.19.so 0x7ffff7ff3000 0x7ffff7ff5000 0x2000 0x1c6000 /usr/lib64/libc-2.19.so 0x7ffff7ff5000 0x7ffff7ff9000 0x4000 0x0 0x7ffff7ff9000 0x7ffff7ffa000 0x1000 0x0 /etc/ld.so.cache 0x7ffff7ffa000 0x7ffff7ffd000 0x3000 0x0 0x7ffff7ffd000 0x7ffff7fff000 0x2000 0x0 [vdso] 0x7ffffffde000 0x7ffffffff000 0x21000 0x0 [stack] 0xffffffffff600000 0xffffffffff601000 0x1000 0x0 [vsyscall]
Unfortunately, GDB does not understand that it should also load symbols for ./a.out. You have to tell it:
(gdb) add-symbol-file ./a.out The address where ./a.out has been loaded is missing
One would think that the address that GDB needs would be from the above info proc map: 0x400000. One would be wrong. The actual address GDB needs is the start of .text section, which you can get from readelf:
readelf -WS ./a.out | grep text [13] .text PROGBITS 0000000000400440 000440 000182 00 AX 0 0 16
Back to GDB:
(gdb) add-symbol-file ./a.out 0x0000000000400440 add symbol table from file "./a.out" at .text_addr = 0x400440 Reading symbols from ./a.out...done.
And now we can break on main:
(gdb) b main Breakpoint 1 at 0x400531: file t.c, line 6. (gdb) c Continuing. Breakpoint 1, main () at t.c:6 6 printf("Hello\n"); (gdb) n Hello 7 return 0;
Voila!
P.S. Re-running the binary may give you some glitches:
(gdb) r Starting program: /usr/lib64/ld-linux-x86-64.so.2 ./a.out Error in re-setting breakpoint 1: Cannot access memory at address 0x40052d Error in re-setting breakpoint 1: Cannot access memory at address 0x40052d Stopped due to shared library event (no libraries added or removed)
This is happening because the ld-linux is yet to map the ./a.out. But you can continue:
(gdb) c Continuing. Stopped due to shared library event: Inferior loaded /usr/lib64/ld-linux-x86-64.so.2 (gdb) c Continuing. Stopped due to shared library event: Inferior loaded /usr/lib64/libc.so.6
And now, ./a.out has also been loaded, so you can re-enable the breakpoint(s):
(gdb) enable (gdb) continue Continuing. Breakpoint 1, main () at t.c:6 6 printf("Hello\n");
