Re: Multiple breakpoint issue when debugging loadable kernel module

[Vimal <j.vimal@gmail.com>]

Some more info.  This looks interesting.

1. find_pc_sect_symtab has changed quite a bit between the faulty
commit noted above (sha 2cdbbe44) and its parent (2cdbbe44^)!

2. Since the module is loaded into the kernel address space,
find_pc_sect_symtab seems to think the vmlinux module's symbol table
has symbol information for the function I am breaking into.   Some
prints from find_pc_sect_symtab for a LKM function seems to return
vmlinux's symtab! :(

find_pc_sect_symtab(pc=0xffffffffa00d5bb8,sec=0x20fb988)
objfile=0x11cbc40, name=/usr/src/linux-cfs-bw/vmlinux
Returning 0x2690cc0 (objfile=0x11cbc40)  # Which is vmlinux

So, I checked the BLOCK_START and BLOCK_END for the vmlinux image.  I got this:
start=0xffffffff8102eb10
end=0xffffffffff60017f

and pc=0xffffffffa00d5bb8 was very well within start and end, so it
chose vmlinux. :(

So, I tired this (without loading vmlinux's symbols)

(gdb) target remote localhost:1234
(gdb) add-symbol-file (for module)
(gdb) info scope module_function
(got info)
(gdb) break module_function
(correct break)
(gdb) info scope module_function
(got correct info!  it works!)

All locals/args info is retained.

So, to combat this, I presently do this (load vmlinux _after_ loading
module symbols)

gdb (without vmlinux)
(gdb) target remote ...
(gdb) add-symbol-file (for module)
(gdb) file vmlinux
(gdb) break vport_receive
Breakpoint 1, vport_receive (vport=0xffff880115fa7200,
skb=0xffff880117580900) at
/home/nikhilh/openvswitch/datapath/linux/vport.c:643
643             if (vport->ops->flags & VPORT_F_GEN_STATS) {

With full information.

I am also able to break into Linux functions, like sys_write for e.g.

Jan: can you try this and tell us if it works?

Thanks,
-- 
Vimal