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Message-ID: <20180514110717.GE7753@e103592.cambridge.arm.com>
Date: Mon, 14 May 2018 12:07:18 +0100
From: Dave Martin <Dave.Martin@....com>
To: Mark Rutland <mark.rutland@....com>
Cc: linux-arm-kernel@...ts.infradead.org, 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
Subject: Re: [PATCH 08/18] arm64: convert raw syscall invocation to C
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, 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
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