Re: Ftrace, KASLR and gdb

[alexandre.ferrieux@xxxxxxxxxx]

On 10/05/2024 20:12, Steven Rostedt wrote:
>

Hi Steven. Honoured to be talking to the creator of my preferred toy !

> > Ftrace is a jewel to dig into the kernel, be it for troubleshooting, perf tuning 
> > or just understanding.
> > But when one needs to disassemble the running kernel (eg to move kprobes around 
> > in a function, in order to understand a given code path), KASLR makes it 
> > impossible for gdb to get useful symbol addresses, even with a debug image.
>
> Really? Can't you just use a function name plus offset? For instance, I
> do this all the time:

Yes, on the ftrace side, obviously name+offset are sufficient. I use them all 
the time, and am delighted with fetchargs.

The problem is rather, on the gdb side, with a production kernel (not the one 
you've just compiled), on a random machine you have a passing access to. First, 
the kernel itself is compressed (vmlinuz), and in many cases getting the vmlinux 
is a pain. So, what ELF binary do you pass to gdb ?

In my case:

  sym2elf /proc/kallsyms > /tmp/kallsyms.elf
  gdb /tmp/kallsyms.elf /proc/kcore
  (gdb) disass vfs_write
  ...

As an added bonus, I see (a snapshot of) globals:

  (gdb) p *(int *)jiffies
  $1 = 92299928

And also I see ftrace's surgical changes if by accident I disassemble a function 
I currently am sniffing ;-)

   echo "p:kprobe_tpacket_rcv tpacket_rcv+26 skb=%di:x64" >> kprobe_events
   (gdb) disass tcpaket_rcv
   Dump of assembler code for function tpacket_rcv:
     0xffffffffb9d79e10 <+0>:     endbr64
     ...
     0xffffffffb9d79e2a <+26>:    jmp    0xffffffffc1389000 <kprobe_optinsn_page>

Of course the latter only serves my curiosity; but it shows I'm looking at the 
"live" code. It matters in other cases of dynamic code, like static branches.

So, is there currently another method to reach the same effect ?

-Alex
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