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

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Message-ID: <072401d428b4$4c0f4340$e42dc9c0$@d-silva.org>
Date:   Tue, 31 Jul 2018 19:52:35 +1000
From:   "Alastair D'Silva" <alastair@...ilva.org>
To:     "'Michael Ellerman'" <mpe@...erman.id.au>,
        "'Murilo Opsfelder Araujo'" <muriloo@...ux.ibm.com>,
        "'LEROY Christophe'" <christophe.leroy@....fr>
Cc:     <linux-kernel@...r.kernel.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>,
        "'Joe Perches'" <joe@...ches.com>,
        "'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()

> -----Original Message-----
> From: Michael Ellerman <mpe@...erman.id.au>
> Sent: Tuesday, 31 July 2018 7:32 PM
> To: Murilo Opsfelder Araujo <muriloo@...ux.ibm.com>; LEROY Christophe
> <christophe.leroy@....fr>
> 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>; Joe Perches <joe@...ches.com>;
> 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> writes:
> > On Mon, Jul 30, 2018 at 06:30:47PM +0200, LEROY Christophe wrote:
> >> Murilo Opsfelder Araujo <muriloo@...ux.ibm.com> a écrit :
> >> > 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.
> >
> > One of the benefits is simplifying the code by removing some checks.
> > Another is deduplicating almost identical format strings in favor of a unified
> one.
> >
> > After reading Joe's comment [1], %px seems to be the format we're
> looking for.
> > An extract from Documentation/core-api/printk-formats.rst:
> >
> >   "%px is functionally equivalent to %lx (or %lu). %px is preferred because it
> >   is more uniquely grep'able."
> >
> > So I guess we don't need to worry about the format (%016lx vs. %08lx),
> > let's just use %px, as per the guideline.
> 
> I don't think I like %px.

Me neither, semantically, it's for pointers, and the data being displayed is not a pointer.

> It makes the format string cleaner, but it means we have to cast everything
> to void * which is ugly as heck.
> 
> I actually don't think the leading zeroes are helpful at all in the signal
> message, ie. we should just use %lx there.
> 
> They are useful in show_regs() because we want everything to line up.
> 
> So I think I'll drop patch 3 and use 0x%lx in show_signal_msg(), meaning we
> end up with, eg:
> 
>   [   73.414535] segv[3759]: segfault (11) at 0x0 nip 0x10000420 lr 0xfe61854
> code 0x1 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

Or better yet, "%#lx" - the hash adds the appropriate prefix in the right case for the format.

-- 
Alastair D'Silva           mob: 0423 762 819
skype: alastair_dsilva     msn: alastair@...ilva.org
blog: http://alastair.d-silva.org    Twitter: @EvilDeece