GDB: breakpoint in linux built-in module fails

Question

I am debugging linux kernel using two virtual machines connected via serial port.

Target machine awaits connection from remote gdb, by inserting kgdbwait() and kgdboc=ttyS0,115200 in the correct entry in /boot/grub/grub.cfg.

In Host machine

sudo gdb ./vmlinux

Symbols are read and I am supposed to be able to put breakpoints on function names.

(gdb) break oom_kill_process
Breakpoint 1 at 0xc1172ef0: file mm/oom_kill.c, line 843.

Works fine!

However, if I set a breakpoint at htb_dequeue_tree which is found here, I get the following error:

(gdb) break htb_dequeue_tree

No symbol "htb_dequeue_tree" in current context.

this function may be inlined. Try to set a break on a concrete address (b *0x12345678) — Alex Hoppus
@AlexHoppus I tried break net/sched/sch_htb.c:htb_dequeue_tree and it didn't work. When I did this for mm/oom_kill.c:oom_kill_process it succeeded so for some reason sch_htb.c is not recognized? anyway, thanks for your comment, but how do I find that address of the function? — Tony Tannous
use objdump vmlinux . I didn't get how what you are saying refutes my assumption about inlined function. — Alex Hoppus
@AlexHoppus didn't mean to refute your suggestion, I wanted to add that sch_htb.c is not found. I am getting No source file named net/sched/sch_htb.c — Tony Tannous
if you can rebuild the kernel use attribute__((optimize("O0"))) and __attribute ((noinline)) — Alex Hoppus

employee of the month employee of the month · Accepted Answer · 2018-05-31T07:53:20

The most typical cause is the function getting inlined - then there is no symbol to begin with. Quite often you can put a break on a file+line pair (e.g. foo.c:42).

In your particular case the function is static and has only one user: htp_dequeue. If you disassemble it you will probably see the code from htp_dequeue_tree slurped in. If not, there is some other breakage going on(does breaking on other functions work?)

GDB: breakpoint in linux built-in module fails

2 Answers