Message-ID: <20180514125351.GK7753@e103592.cambridge.arm.com>
Date:   Mon, 14 May 2018 13:53:52 +0100
From:   Dave Martin <Dave.Martin@....com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     marc.zyngier@....com, catalin.marinas@....com, will.deacon@....com,
        linux-kernel@...r.kernel.org, linux@...inikbrodowski.net,
        james.morse@....com, viro@...iv.linux.org.uk,
        linux-fsdevel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 08/18] arm64: convert raw syscall invocation to C

On Mon, May 14, 2018 at 12:41:10PM +0100, Mark Rutland wrote:
> On Mon, May 14, 2018 at 12:07:18PM +0100, Dave Martin wrote:
> > On Mon, May 14, 2018 at 10:46:30AM +0100, Mark Rutland wrote:
> > > As a first step towards invoking syscalls with a pt_regs argument,
> > > convert the raw syscall invocation logic to C. We end up with a bit more
> > > register shuffling, but the unified invocation logic means we can unify
> > > the tracing paths, too.
> > > 
> > > This only converts the invocation of the syscall. The rest of the
> > > syscall triage and tracing is left in assembly for now, and will be
> > > converted in subsequent patches.
> > > 
> > > Signed-off-by: Mark Rutland <mark.rutland@....com>
> > > Cc: Catalin Marinas <catalin.marinas@....com>
> > > Cc: Will Deacon <will.deacon@....com>
> > > ---
> > >  arch/arm64/kernel/Makefile  |  3 ++-
> > >  arch/arm64/kernel/entry.S   | 36 ++++++++++--------------------------
> > >  arch/arm64/kernel/syscall.c | 29 +++++++++++++++++++++++++++++
> > >  3 files changed, 41 insertions(+), 27 deletions(-)
> > >  create mode 100644 arch/arm64/kernel/syscall.c
> > > 
> > > diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile
> > > index bf825f38d206..c22e8ace5ea3 100644
> > > --- a/arch/arm64/kernel/Makefile
> > > +++ b/arch/arm64/kernel/Makefile
> > > @@ -18,7 +18,8 @@ arm64-obj-y		:= debug-monitors.o entry.o irq.o fpsimd.o		\
> > >  			   hyp-stub.o psci.o cpu_ops.o insn.o	\
> > >  			   return_address.o cpuinfo.o cpu_errata.o		\
> > >  			   cpufeature.o alternative.o cacheinfo.o		\
> > > -			   smp.o smp_spin_table.o topology.o smccc-call.o
> > > +			   smp.o smp_spin_table.o topology.o smccc-call.o	\
> > > +			   syscall.o
> > >  
> > >  extra-$(CONFIG_EFI)			:= efi-entry.o
> > >  
> > > diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> > > index 08ea3cbfb08f..d6e057500eaf 100644
> > > --- a/arch/arm64/kernel/entry.S
> > > +++ b/arch/arm64/kernel/entry.S
> > > @@ -873,7 +873,6 @@ ENDPROC(el0_error)
> > >   */
> > >  ret_fast_syscall:
> > >  	disable_daif
> > > -	str	x0, [sp, #S_X0]			// returned x0
> > >  	ldr	x1, [tsk, #TSK_TI_FLAGS]	// re-check for syscall tracing
> > >  	and	x2, x1, #_TIF_SYSCALL_WORK
> > >  	cbnz	x2, ret_fast_syscall_trace
> > > @@ -946,15 +945,11 @@ el0_svc_naked:					// compat entry point
> > >  
> > >  	tst	x16, #_TIF_SYSCALL_WORK		// check for syscall hooks
> > >  	b.ne	__sys_trace
> > > -	cmp     wscno, wsc_nr			// check upper syscall limit
> > > -	b.hs	ni_sys
> > > -	mask_nospec64 xscno, xsc_nr, x19	// enforce bounds for syscall number
> > > -	ldr	x16, [stbl, xscno, lsl #3]	// address in the syscall table
> > > -	blr	x16				// call sys_* routine
> > > -	b	ret_fast_syscall
> > > -ni_sys:
> > >  	mov	x0, sp
> > > -	bl	do_ni_syscall
> > > +	mov	w1, wscno
> > > +	mov	w2, wsc_nr
> > > +	mov	x3, stbl
> > > +	bl	invoke_syscall
> > >  	b	ret_fast_syscall
> > >  ENDPROC(el0_svc)
> > >  
> > > @@ -971,29 +966,18 @@ __sys_trace:
> > >  	bl	syscall_trace_enter
> > >  	cmp	w0, #NO_SYSCALL			// skip the syscall?
> > >  	b.eq	__sys_trace_return_skipped
> > > -	mov	wscno, w0			// syscall number (possibly new)
> > > -	mov	x1, sp				// pointer to regs
> > > -	cmp	wscno, wsc_nr			// check upper syscall limit
> > > -	b.hs	__ni_sys_trace
> > > -	ldp	x0, x1, [sp]			// restore the syscall args
> > > -	ldp	x2, x3, [sp, #S_X2]
> > > -	ldp	x4, x5, [sp, #S_X4]
> > > -	ldp	x6, x7, [sp, #S_X6]
> > > -	ldr	x16, [stbl, xscno, lsl #3]	// address in the syscall table
> > > -	blr	x16				// call sys_* routine
> > >  
> > > -__sys_trace_return:
> > > -	str	x0, [sp, #S_X0]			// save returned x0
> > > +	mov	x0, sp
> > > +	mov	w1, wscno
> > > +	mov w2, wsc_nr
> > > +	mov	x3, stbl
> > > +	bl	invoke_syscall
> > > +
> > >  __sys_trace_return_skipped:
> > >  	mov	x0, sp
> > >  	bl	syscall_trace_exit
> > >  	b	ret_to_user
> > >  
> > > -__ni_sys_trace:
> > > -	mov	x0, sp
> > > -	bl	do_ni_syscall
> > > -	b	__sys_trace_return
> > > -
> > 
> > Can you explain why ni_syscall is special here, 
> 
> This is for out-of-range syscall numbers, instances of ni_syscall in the
> syscall table are handled by the regular path. When the syscall number
> is out-of-range, we can't index the syscall table, and have to call
> ni_sys directly.
> 
> The c invoke_syscall() wrapper handles that case internally so that we
> don't have to open-code it everywhere.
> 
> > why __sys_trace_return existed, 
> 
> The __sys_trace_return label existed so that the special __ni_sys_trace
> path could return into a common tracing return path.
> 
> > and why its disappearance doesn't break anything?
> 
> Now that invoke_syscall() handles out-of-range syscall numbers, and we
> can remove the __ni_sys_trace path, nothing branches to
> __sys_trace_return.
> 
> Only the label has been removed, not the usual return path.

OK, I think I understand.  I think the name "__sys_trace_return" was
confusing me, as if this was something special that only relates to the
ni_syscall case.  If it was only ever intended as the merge point for
those two paths, then I can see that merging the paths for real enables
us to get rid of it.

> > Not saying there's a bug, just that I'm a little confuse -- I see no
> > real reason for ni_syscall being special, and this may be a good
> > opportunity to decruft it.  (See also comments below.)
> 
> Hopefully the above clarifies things?

Yes, I think so.

> I've updated the commit message with a description.
> 
> [...]
> 
> > > +asmlinkage void invoke_syscall(struct pt_regs *regs, int scno, int sc_nr,
> > > +			       syscall_fn_t syscall_table[])
> > > +{
> > > +	if (scno < sc_nr) {
> > 
> > What if (int)scno < 0?  Should those args both by unsigned ints?
> 
> Yes, they should -- I've fixed that up locally.
> 
> That is a *very* good point, thanks!
> 
> > "sc_nr" sounds too much like "syscall number" to me.  Might
> > "syscall_table_size" might be clearer?  Similarly, we could have
> > "stbl_size" or similar in the asm.  This is purely cosmetic,
> > though.
> 
> I'd tried to stick to the naming used in assembly to keep the conversion
> clearer for those familiar with the asm.
> 
> I agree the names aren't great.

Not a big deal.  If you feel you would like to rename them though, I
won't argue with it ;)

> > > +		syscall_fn_t syscall_fn;
> > > +		syscall_fn = syscall_table[array_index_nospec(scno, sc_nr)];
> > > +		__invoke_syscall(regs, syscall_fn);
> > > +	} else {
> > > +		regs->regs[0] = do_ni_syscall(regs);
> > 
> > Can we make __invoke_syscall() the universal syscall wrapper, and give
> > do_ni_syscall() the same interface as any other syscall body?
> 
> Not at this point in time, since the prototype (in core code) differs.
> 
> I agree that would be nicer, but there are a number of complications;
> more details below.
> 
> > Then you could factor this as
> > 
> > static syscall_fn_t syscall_fn(syscall_fn_t const syscall_table[],
> > 				(unsigned) int scno, (unsigned) int sc_nr)
> > {
> > 	if (sc_no >= sc_nr)
> > 		return sys_ni_syscall;
> > 
> > 	return syscall_table[array_index_nospec(scno, sc_nr)];
> > }
> > 
> > ...
> > 	__invoke_syscall(regs, syscall_fn(syscall_table, scno, sc_nr);
> > 
> > 
> > 
> > This is cosmetic too, of course.
> > 
> > do_ni_syscall() should be given a pt_regs-based wrapper like all the
> > rest.
> 
> I agree it would be nicer if it had a wrapper that took a pt_regs, even
> if it does nothing with it.
> 
> We can't use SYSCALL_DEFINE0() due to the fault injection muck, we'd
> need a ksys_ni_syscall() for our traps.c logic, and adding this
> uniformly would involve some arch-specific rework for x86, too, so I
> decided it was not worth the effort.

Does allowing error injection for ni_syscall actually matter?  Error
injection is an ABI break by itself.  It would arguably be a bit
strange though.

Cheers
---Dave

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