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.
