Re: [PATCH 08/18] arm64: convert raw syscall invocation to C

[Dave Martin]

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@xxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Will Deacon <will.deacon@xxxxxxx>
> ---
>  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, why __sys_trace_return
existed, and why its disappearance doesn't break anything?

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.)

>  	.popsection				// .entry.text
>  
>  #ifdef CONFIG_UNMAP_KERNEL_AT_EL0
> diff --git a/arch/arm64/kernel/syscall.c b/arch/arm64/kernel/syscall.c
> new file mode 100644
> index 000000000000..58d7569f47df
> --- /dev/null
> +++ b/arch/arm64/kernel/syscall.c
> @@ -0,0 +1,29 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <linux/nospec.h>
> +#include <linux/ptrace.h>
> +
> +long do_ni_syscall(struct pt_regs *regs);
> +
> +typedef long (*syscall_fn_t)(unsigned long, unsigned long,
> +			     unsigned long, unsigned long,
> +			     unsigned long, unsigned long);
> +
> +static void __invoke_syscall(struct pt_regs *regs, syscall_fn_t syscall_fn)
> +{
> +	regs->regs[0] = syscall_fn(regs->regs[0], regs->regs[1],
> +				   regs->regs[2], regs->regs[3],
> +				   regs->regs[4], regs->regs[5]);
> +}
> +
> +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?

"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.

> +		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?

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.

This is still cosmetic, but reduces unnecessary special-case-ness
for ni_syscall.

Cheers
---Dave