linux-kernel - Re: [PATCH v2 04/10] powerpc/traps: Use REG_FMT in show_signal

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Date:   Mon, 30 Jul 2018 18:30:47 +0200
From:   LEROY Christophe <christophe.leroy@....fr>
To:     Murilo Opsfelder Araujo <muriloo@...ux.ibm.com>
Cc:     linux-kernel@...r.kernel.org,
        Alastair D'Silva <alastair@...ilva.org>,
        Andrew Donnellan <andrew.donnellan@....ibm.com>,
        Balbir Singh <bsingharora@...il.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Cyril Bur <cyrilbur@...il.com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Michael Neuling <mikey@...ling.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Paul Mackerras <paulus@...ba.org>,
        Simon Guo <wei.guo.simon@...il.com>,
        Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>,
        "Tobin C . Harding" <me@...in.cc>, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2 04/10] powerpc/traps: Use REG_FMT in
 show_signal_msg()

Murilo Opsfelder Araujo <muriloo@...ux.ibm.com> a écrit :

> Hi, Christophe.
>
> On Fri, Jul 27, 2018 at 06:40:23PM +0200, LEROY Christophe wrote:
>> Murilo Opsfelder Araujo <muriloo@...ux.ibm.com> a écrit :
>>
>> > Simplify the message format by using REG_FMT as the register format.  This
>> > avoids having two different formats and avoids checking for MSR_64BIT.
>>
>> Are you sure it is what we want ?
>
> Yes.
>
>> Won't it change the behaviour for a 32 bits app running on a 64bits kernel ?
>
> In fact, this changes how many zeroes are prefixed when displaying  
> the registers
> (%016lx vs. %08lx format).  For example, 32-bits userspace, 64-bits kernel:

Indeed that's what I suspected. What is the real benefit of this  
change ? Why not keep the current format for 32bits userspace ? All  
those leading zeroes are pointless to me.

>
> before this series:
>   [66475.002900] segv[4599]: unhandled signal 11 at 00000000 nip  
> 10000420 lr 0fe61854 code 1
>
> after this series:
>   [   73.414535] segv[3759]: segfault (11) at 0000000000000000 nip  
> 0000000010000420 lr 000000000fe61854 code 1 in segv[10000000+10000]
>   [   73.414641] segv[3759]: code: 4e800421 80010014 38210010  
> 7c0803a6 4bffff30 9421ffd0 93e1002c 7c3f0b78
>   [   73.414665] segv[3759]: code: 39200000 913f001c 813f001c  
> 39400001 <91490000> 39200000 7d234b78 397f0030
>
> Have you spotted any other behaviour change?

Not as of today

Christophe

>
> Cheers
> Murilo