On Mon, 1 Feb 2016, James Hogan wrote:
> > > diff --git a/arch/mips/include/asm/syscall.h b/arch/mips/include/asm/syscall.h
> > > index 6499d93ae68d..47bc45a67e9b 100644
> > > --- a/arch/mips/include/asm/syscall.h
> > > +++ b/arch/mips/include/asm/syscall.h
> > > @@ -101,10 +101,8 @@ static inline void syscall_get_arguments(struct task_struct *task,
> > > /* O32 ABI syscall() - Either 64-bit with O32 or 32-bit */
> > > if ((config_enabled(CONFIG_32BIT) ||
> > > test_tsk_thread_flag(task, TIF_32BIT_REGS)) &&
> > > - (regs->regs[2] == __NR_syscall)) {
> > > + (regs->regs[2] == __NR_syscall))
> > > i++;
> > > - n++;
> > > - }
> > >
> > > while (n--)
> > > ret |= mips_get_syscall_arg(args++, task, regs, i++);
> >
> > What I think it really needs to do is to *decrease* the number of
> > arguments, as we're throwing the syscall number away as not an argument to
> > itself. So this looks like a typo to me, the expression was meant to be
> > `n--' rather than `n++'. With the number of arguments unchanged, as in
> > your proposed change, we're still reaching one word too far.
>
> No, the caller asked for n args, thats what it should get. It doesn't
> care whether the syscall was indirected or not.
>
> The syscall doesn't have 1 less arg as a result of indirection. E.g.
> What about system calls with 6 arguments, and hence 7 arguments
> including syscall number argument when redirected? We'd get an
> uninitialised 6th arg back when passing n=6.
Do you mean the caller ignores the extra argument holding the number of
the wrapped syscall in counting syscall arguments? Where does it take `n'
from?
> Note scall32-o32.S sys_syscall shifts 7 arguments starting at a1 (I've
> reordered code slightly):
> move a0, a1 # shift argument registers
> move a1, a2
> move a2, a3
> lw a3, 16(sp)
> lw t4, 20(sp)
> lw t5, 24(sp)
> lw t6, 28(sp)
>
> sw a0, PT_R4(sp) # .. and push back a0 - a3, some
> sw a1, PT_R5(sp) # syscalls expect them there
> sw a2, PT_R6(sp)
> sw a3, PT_R7(sp)
> sw t4, 16(sp)
> sw t5, 20(sp)
> sw t6, 24(sp)
>
> So it takes args 0..2 from a1..a3, and 3..6 from stack.
Sure, no need to explain it as far as I'm concerned, I didn't question
it.
Maciej
