Message-Id: <c355217e-e834-99bf-f24c-b2cef71f515c@linux.ibm.com>
Date:   Fri, 14 Feb 2020 11:02:41 +0100
From:   Thomas Richter <tmricht@...ux.ibm.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:     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

On 2/14/20 1:45 AM, Masami Hiramatsu wrote:
> 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?
> 
> Thank you,
> 


Masami, thanks very much for your help!

I started from scratch and just installed linux 5.6.0rc1 without
any changes and got this failure:

[root@...lp76 perf]# ./perf test  66 67
66: Use vfs_getname probe to get syscall args filenames   : FAILED!
67: Check open filename arg using perf trace + vfs_getname: FAILED!
[root@...lp76 perf]#

Now I applied Masami's patch and this is the result

[root@...lp76 perf]# ./perf test  66 67
66: Use vfs_getname probe to get syscall args filenames   : Ok
67: Check open filename arg using perf trace + vfs_getname: Ok
[root@...lp76 perf  

This is what I wanted to achieve.
And I think I know what went wrong. I supplied an initial patch and when
I rewrote it for V3 I kprobed the wrong line.

Again thanks a lot and sorry for the noise...
-- 
Thomas Richter, Dept 3252, IBM s390 Linux Development, Boeblingen, Germany
--
Vorsitzender des Aufsichtsrats: Matthias Hartmann
Geschäftsführung: Dirk Wittkopp
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