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@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,
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.
> 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?
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.
> > + 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.
Thanks,
Mark.
