Re: Linux 4.15.0-rc3 perf probe/uprobe issue with address randomization

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Dec 15, 2017 at 03:14:37PM +0100, Thomas-Mich Richter wrote:
> During debugging of perf probe tool I discovered an issue with
> uprobes and address randomization.
> 
> To set a uprobe on a function named inet_pton in libc library, you
> obtain the address of the symbol inet_pton using command nm and
> then use the following command to set the uprobe:
> 
> # echo "p:probe_libc/inet_pton /usr/lib64/libc-2.26.so:0x142060"
> 	> /sys/kernel/debug/tracing/uprobe_events
> 
> 0x142060 is the address of inet_pton on my system.
> This works nicely and the uprobe is usable.
> 
> The issue is with the output:
> # cat /sys/kernel/debug/tracing/uprobe_events
> p:probe_libc/inet_pton /usr/lib64/libc-2.26.so:0x000000002d0f8952
> #
> 
> The displayed address 0x000000002d0f8952 is wrong, probably
> randomized and post processing of this output with the perf
> probe tool fails due to this random address:
> 
> # linux/tools/perf/perf probe -l
> Failed to find debug information for address 2d0f8952
>   probe_libc:inet_pton (on 0x2d0f8952 in /usr/lib64/libc-2.26.so)
> # 
> 
> So how to fix this (if at all)?
> Is replacing %p by %llx in line 612 of file kernel/trace/trace_uprobe.c
>    seq_printf(m, "0x%p", (void *)tu->offset)
> an option?
> Or is this broken by design and intention?

So recently %p got changed to hash pointers in order to avoid leaking
kernel addresses.

  ad67b74d2469 ("printk: hash addresses printed with %p")

I'm not sure what privilidges are required for reading that kprobe
state, but I suspect its root only, so changing this to %px might be
what is needed.