linux-kernel - Re: kprobes string reading broken on s390

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Date:   Fri, 5 Jun 2020 19:44:33 +0200
From:   Sven Schnelle <svens@...ux.ibm.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Christoph Hellwig <hch@....de>, linux-kernel@...r.kernel.org
Subject: Re: kprobes string reading broken on s390

Masami,

On Sat, Jun 06, 2020 at 01:58:06AM +0900, Masami Hiramatsu wrote:
> Hi Sven,
> 
> On Fri, 5 Jun 2020 15:25:41 +0200
> Christoph Hellwig <hch@....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.

Which SHA1 should the Fixes tag carry? The one from linux-next?

Regards
Sven