Re: [PATCH v11 5/8] powerpc/64: make buildable without CONFIG_COMPAT

From: Nicholas Piggin <npiggin@gmail.com>
To: "Michal Suchánek" <msuchanek@suse.de>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Allison Randal <allison@lohutok.net>,
	Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
	Arnd Bergmann <arnd@arndb.de>,
	Thiago Jung Bauermann <bauerman@linux.ibm.com>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Claudio Carvalho <cclaudio@linux.ibm.com>,
	"David S. Miller" <davem@davemloft.net>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	Eric Richter <erichte@linux.ibm.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Gustavo Luiz Duarte <gustavold@linux.ibm.com>,
	Hari Bathini <hbathini@linux.ibm.com>,
	Jordan Niethe <jniethe5@gmail.com>, Jiri Olsa <jolsa@redhat.com>,
	Jonathan Cameron <Jonathan.Cameron@huawei.com>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	linuxppc-dev@lists.ozlabs.org,
	Mark Rutland <mark.rutland@arm.com>,
	Masahiro Yamada <masahiroy@kernel.org>,
	Mauro Carvalho Chehab <mchehab+huawei@kernel.org>,
	Michael Neuling <mikey@neuling.org>,
	Ingo Molnar <mingo@redhat.com>,
	Namhyung Kim <namhyung@kernel.org>,
	Nayna Jain <nayna@linux.ibm.com>,
	Paul Mackerras <paulus@samba.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Rob Herring <robh@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Valentin Schneider <valentin.schneider@arm.com>,
	Alexander Viro <viro@zeniv.linux.org.uk>
Subject: Re: [PATCH v11 5/8] powerpc/64: make buildable without CONFIG_COMPAT
Date: Fri, 03 Apr 2020 17:16:43 +1000	[thread overview]
Message-ID: <1585898018.8y4vw9c8hc.astroid@bobo.none> (raw)
In-Reply-To: <20200324193055.GG25468@kitsune.suse.cz>

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