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SubjectRe: kprobes string reading broken on s390
Hi Sven,

On Fri, 5 Jun 2020 15:25:41 +0200
Christoph Hellwig <hch@lst.de> wrote:

> Yes, this looks correct. You probably want to write a small changelog
> and add a Fixes tag, though.
>
> On Fri, Jun 05, 2020 at 01:05:34PM +0200, Sven Schnelle wrote:
> > Hi Christoph,
> >
> > with the latest linux-next i noticed that some tests in the
> > ftrace test suites are failing on s390, namely:
> >
> > [FAIL] Kprobe event symbol argument
> > [FAIL] Kprobe event with comm arguments
> >
> > The following doesn't work anymore:
> >
> > cd /sys/kernel/tracing
> > echo 'p:testprobe _do_fork comm=$comm ' >kprobe_events
> > echo 1 >/sys/kernel/tracing/events/kprobes/testprobe/enable
> > cat /sys/kernel/tracing/trace
> >
> > it will just show
> >
> > test.sh-519 [012] .... 18.580625: testprobe: (_do_fork+0x0/0x3c8) comm=(fault)
> >
> > Looking at d411a9c4e95a ("tracing/kprobes: handle mixed kernel/userspace probes
> > better") i see that there are two helpers for reading strings:
> >
> > fetch_store_string_user() -> read string from user space
> > fetch_store_string() -> read string from kernel space(?)
> >
> > but in the end both are using strncpy_from_user_nofault(), but i would
> > think that fetch_store_string() should use strncpy_from_kernel_nofault().
> > However, i'm not sure about the exact semantics of fetch_store_string(),
> > as there where a lot of wrong assumptions in the past, especially since
> > on x86 you usually don't fail if you use the same function for accessing kernel
> > and userspace although it's technically wrong.

Thanks for fixing!
This report can be a good changelog.
Please resend it with Fixed tag as Christoph said.

Christoph, it seems your series in the akpm tree, so this also should
be sent to Andrew?

Thank you,

> >
> > Regards,
> > Sven
> >
> > commit 81408eab8fcc79dc0871a95462b13176d3446f5e
> > Author: Sven Schnelle <svens@linux.ibm.com>
> > Date: Fri Jun 5 13:01:24 2020 +0200
> >
> > kprobes: use strncpy_from_kernel_nofault() in fetch_store_string()
> >
> > Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
> >
> > diff --git a/kernel/trace/trace_kprobe.c b/kernel/trace/trace_kprobe.c
> > index b1f21d558e45..ea8d0b094f1b 100644
> > --- a/kernel/trace/trace_kprobe.c
> > +++ b/kernel/trace/trace_kprobe.c
> > @@ -1278,7 +1278,7 @@ fetch_store_string(unsigned long addr, void *dest, void *base)
> > * Try to get string again, since the string can be changed while
> > * probing.
> > */
> > - ret = strncpy_from_user_nofault(__dest, (void *)addr, maxlen);
> > + ret = strncpy_from_kernel_nofault(__dest, (void *)addr, maxlen);
> > if (ret >= 0)
> > *(u32 *)dest = make_data_loc(ret, __dest - base);
> >
> ---end quoted text---


--
Masami Hiramatsu <mhiramat@kernel.org>

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