linux-kernel - Re: [PATCH v11 5/8] powerpc/64: make buildable without CONFIG

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Date:   Fri, 03 Apr 2020 17:16:43 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Michal Suchánek <msuchanek@...e.de>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Allison Randal <allison@...utok.net>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Arnd Bergmann <arnd@...db.de>,
        Thiago Jung Bauermann <bauerman@...ux.ibm.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Claudio Carvalho <cclaudio@...ux.ibm.com>,
        "David S. Miller" <davem@...emloft.net>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Eric Richter <erichte@...ux.ibm.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Gustavo Luiz Duarte <gustavold@...ux.ibm.com>,
        Hari Bathini <hbathini@...ux.ibm.com>,
        Jordan Niethe <jniethe5@...il.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, Mark Rutland <mark.rutland@....com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Mauro Carvalho Chehab <mchehab+huawei@...nel.org>,
        Michael Neuling <mikey@...ling.org>,
        Ingo Molnar <mingo@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Nayna Jain <nayna@...ux.ibm.com>,
        Paul Mackerras <paulus@...ba.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Rob Herring <robh@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Valentin Schneider <valentin.schneider@....com>,
        Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH v11 5/8] powerpc/64: make buildable without CONFIG_COMPAT

Michal Suchánek's on March 25, 2020 5:30 am:
> On Tue, Mar 24, 2020 at 06:54:20PM +1000, Nicholas Piggin wrote:
>> Michal Suchanek's on March 19, 2020 10:19 pm:
>> > diff --git a/arch/powerpc/kernel/signal.c b/arch/powerpc/kernel/signal.c
>> > index 4b0152108f61..a264989626fd 100644
>> > --- a/arch/powerpc/kernel/signal.c
>> > +++ b/arch/powerpc/kernel/signal.c
>> > @@ -247,7 +247,6 @@ static void do_signal(struct task_struct *tsk)
>> >  	sigset_t *oldset = sigmask_to_save();
>> >  	struct ksignal ksig = { .sig = 0 };
>> >  	int ret;
>> > -	int is32 = is_32bit_task();
>> >  
>> >  	BUG_ON(tsk != current);
>> >  
>> > @@ -277,7 +276,7 @@ static void do_signal(struct task_struct *tsk)
>> >  
>> >  	rseq_signal_deliver(&ksig, tsk->thread.regs);
>> >  
>> > -	if (is32) {
>> > +	if (is_32bit_task()) {
>> >          	if (ksig.ka.sa.sa_flags & SA_SIGINFO)
>> >  			ret = handle_rt_signal32(&ksig, oldset, tsk);
>> >  		else
>> 
>> Unnecessary?
>> 
>> > diff --git a/arch/powerpc/kernel/syscall_64.c b/arch/powerpc/kernel/syscall_64.c
>> > index 87d95b455b83..2dcbfe38f5ac 100644
>> > --- a/arch/powerpc/kernel/syscall_64.c
>> > +++ b/arch/powerpc/kernel/syscall_64.c
>> > @@ -24,7 +24,6 @@ notrace long system_call_exception(long r3, long r4, long r5,
>> >  				   long r6, long r7, long r8,
>> >  				   unsigned long r0, struct pt_regs *regs)
>> >  {
>> > -	unsigned long ti_flags;
>> >  	syscall_fn f;
>> >  
>> >  	if (IS_ENABLED(CONFIG_PPC_IRQ_SOFT_MASK_DEBUG))
>> > @@ -68,8 +67,7 @@ notrace long system_call_exception(long r3, long r4, long r5,
>> >  
>> >  	local_irq_enable();
>> >  
>> > -	ti_flags = current_thread_info()->flags;
>> > -	if (unlikely(ti_flags & _TIF_SYSCALL_DOTRACE)) {
>> > +	if (unlikely(current_thread_info()->flags & _TIF_SYSCALL_DOTRACE)) {
>> >  		/*
>> >  		 * We use the return value of do_syscall_trace_enter() as the
>> >  		 * syscall number. If the syscall was rejected for any reason
>> > @@ -94,7 +92,7 @@ notrace long system_call_exception(long r3, long r4, long r5,
>> >  	/* May be faster to do array_index_nospec? */
>> >  	barrier_nospec();
>> >  
>> > -	if (unlikely(ti_flags & _TIF_32BIT)) {
>> > +	if (unlikely(is_32bit_task())) {
>> 
>> Problem is, does this allow the load of ti_flags to be used for both
>> tests, or does test_bit make it re-load?
>> 
>> This could maybe be fixed by testing if(IS_ENABLED(CONFIG_COMPAT) &&
> Both points already discussed here:

Agh, I'm hopeless.

I don't think it really resolves this issue. But probably don't have time
to look at generated asm, and might never because it won't really hit
LE unless we add a 32-bit ABI. It's pretty minor though either way.

Sorry for being difficult, I really do like your patches :)

Thanks,
Nick