On Mon, 18 Dec 2017 16:32:25 +0100
Thomas Richter <[email protected]> wrote:
> Commit ad67b74d2469 ("printk: hash addresses printed with %p")
> changed %p to hash pointers in order to avoid leaking
> kernel addresses.
>
> This breaks the tool perf probe.
>
> To set a uprobe on a function named inet_pton in libc library,
> 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
>
> However the output shows a randomized address:
> # cat /sys/kernel/debug/tracing/uprobe_events
> p:probe_libc/inet_pton /usr/lib64/libc-2.26.so:0x000000002d0f8952
>
> The displayed address 0x000000002d0f8952 is incorrect and breaks
> tools post processing it:
Hmm, since the scrambling of %p is to prevent kernel addresses from
leaking, I wonder if it would be OK to make it only scramble the address
if the address is a kernel address. It should be fine to print user
space addresses unscrambled. Shouldn't it?
-- Steve
>
> # 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)
>
> Using the %px printk format string fixes this issue for root
> and shows the correct address allowing the perf probe tool
> to resolve the address to the symbol:
>
> # echo "p:probe_libc/inet_pton /usr/lib64/libc-2.26.so:0x142060"
> > /sys/kernel/debug/tracing/uprobe_events
> # cat /sys/kernel/debug/tracing/uprobe_events
> p:probe_libc/inet_pton /usr/lib64/libc-2.26.so:0x0000000000142060
> # linux/tools/perf/perf probe -l
> probe_libc:inet_pton (on __inet_pton@resolv/inet_pton.c
> in /usr/lib64/libc-2.26.so)
>
> Signed-off-by: Thomas Richter <[email protected]>
> Suggested-by: Peter Zijlstra <[email protected]>
> ---
> kernel/trace/trace_uprobe.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/trace/trace_uprobe.c b/kernel/trace/trace_uprobe.c
> index 40592e7b3568..268029ae1be6 100644
> --- a/kernel/trace/trace_uprobe.c
> +++ b/kernel/trace/trace_uprobe.c
> @@ -608,7 +608,7 @@ static int probes_seq_show(struct seq_file *m, void *v)
>
> /* Don't print "0x (null)" when offset is 0 */
> if (tu->offset) {
> - seq_printf(m, "0x%p", (void *)tu->offset);
> + seq_printf(m, "0x%px", (void *)tu->offset);
> } else {
> switch (sizeof(void *)) {
> case 4:
> --
> 2.13.4
