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

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Date:   Tue, 31 Jul 2018 19:32:28 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
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.

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


I'll do that unless anyone screams loudly, because it would be nice to
get this into 4.19.

cheers