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Date:   Fri, 14 Feb 2020 10:04:30 -0300
From:   Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Thomas Richter <tmricht@...ux.ibm.com>,
        linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
        gor@...ux.ibm.com, sumanthk@...ux.ibm.com,
        heiko.carstens@...ibm.com
Subject: Re: [PATCH v3] perf test: Fix test trace+probe_vfs_getname.sh

Em Fri, Feb 14, 2020 at 09:45:50AM +0900, Masami Hiramatsu escreveu:
> On Thu, 13 Feb 2020 15:11:40 -0300
> Arnaldo Carvalho de Melo <acme@...nel.org> wrote:
> 
> > Em Fri, Feb 14, 2020 at 02:01:51AM +0900, Masami Hiramatsu escreveu:
> > > On Thu, 13 Feb 2020 11:30:48 -0300 Arnaldo Carvalho de Melo <acme@...nel.org> wrote:
> >  
> > > > Em Thu, Feb 13, 2020 at 01:20:09PM +0100, Thomas Richter escreveu:
> > > > > This test places a kprobe to function getname_flags() in the kernel
> > > > > which has the following prototype:
> >  
> > > > >   struct filename *
> > > > >   getname_flags(const char __user *filename, int flags, int *empty)
> >  
> > > > > Variable filename points to a filename located in user space memory.
> > > > > Looking at
> > > > > commit 88903c464321c ("tracing/probe: Add ustring type for user-space string")
> > > > > the kprobe should indicate that user space memory is accessed.
> >  
> > > > > The following patch specifies user space memory access first and if this
> > > > > fails use type 'string' in case 'ustring' is not supported.
> >  
> > > > What are you fixing?
> >  
> > > > I haven't seen any example of this test failing, and right now testing
> > > > it with:
> >  
> > > > [root@...co ~]# uname -a
> > > > Linux quaco 5.6.0-rc1+ #1 SMP Wed Feb 12 15:42:16 -03 2020 x86_64 x86_64 x86_64 GNU/Linux
> > > > [root@...co ~]#
> >  
> > > This bug doesn't happen on x86 or other archs on which user-address space and
> > > kernel address space is same. On some arch (ppc64 in this case?) user-address
> > > space is partially or completely same as kernel address space. (Yes, they switch
> > > the world when running into the kernel) In this case, we need to use different
> > > data access functions for each spaces. That is why I introduced "ustring" type
> > > for kprobe event.
> > > As far as I can see, Thomas's patch is sane.
> > 
> > Well, without his patch, on x86, the test he is claiming to be fixing
> > works well, with his patch it stops working, see the rest of my reply.
> 
> OK, let me see.
> 
> 
> > diff --git a/tools/perf/tests/shell/lib/probe_vfs_getname.sh b/tools/perf/tests/shell/lib/probe_vfs_getname.sh
> > index 7cb99b433888..30c1eadbc5be 100644
> > --- a/tools/perf/tests/shell/lib/probe_vfs_getname.sh
> > +++ b/tools/perf/tests/shell/lib/probe_vfs_getname.sh
> > @@ -13,7 +13,9 @@ add_probe_vfs_getname() {
> >  	local verbose=$1
> >  	if [ $had_vfs_getname -eq 1 ] ; then
> >  		line=$(perf probe -L getname_flags 2>&1 | egrep 'result.*=.*filename;' | sed -r 's/[[:space:]]+([[:digit:]]+)[[:space:]]+result->uptr.*/\1/')
> > -		perf probe -q       "vfs_getname=getname_flags:${line} pathname=result->name:string" || \
> > +		perf probe -q       "vfs_getname=getname_flags:${line} pathname=result->uptr:ustring" || \
> > +		perf probe $verbose "vfs_getname=getname_flags:${line} pathname=filename:ustring" || \
> > +		perf probe -q       "vfs_getname=getname_flags:${line} pathname=result->uptr:string" || \
> >  		perf probe $verbose "vfs_getname=getname_flags:${line} pathname=filename:string"
> >  	fi
> >  }
> 
> This looks no good (depends on architecture or debuginfo). In fs/namei.c,
> 
> struct filename *
> getname_flags(const char __user *filename, int flags, int *empty)
> ...
>         kname = (char *)result->iname;
>         result->name = kname;
> ...
>         result->uptr = filename;
>         result->aname = NULL;
>         audit_getname(result);
>         return result;
> }
> 
> And the line number script, egreps below line.
> 
>         result->uptr = filename;
> 
> However, the probe on this line will hit *before* execute this line.
> Note that kprobes is a breakpoint, which breaks into this line execution,
> not after executed.
> 
> So, I thik at this point, result->uptr should be NULL, but filename and
> result->name already have assigned value.
> 
> Thus, the fix should be something like below.
> 
> > 		perf probe -q       "vfs_getname=getname_flags:${line} pathname=result->name:string" || \
> > - 		perf probe $verbose "vfs_getname=getname_flags:${line} pathname=filename:string"
> > +		perf probe $verbose "vfs_getname=getname_flags:${line} pathname=filename:ustring" || \
> 
> Thomas, is this OK for you too, or would you have any reason to trace
> result->uptr?

Ok, I retract my last e-mail, can you provide a final patch with the fix?

- Arnaldo

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