On Mon, Feb 01, 2016 at 12:50:16PM +0000, Maciej W. Rozycki wrote:
> 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?
No, the caller never sees the extra argument containing syscall number,
because i is incremented, so if caller asks for 3 args start at arg 0,
it would now actually gets 3 args starting at arg 1.
> Where does it take `n' from?
From a quick audit:
sys_enter trace event, seccomp, & proc_pid_syscall gets n=6 args
starting at i=0.
Syscall tracing uses syscall metadata to fetch exactly the right number
of arguments starting at i=0 (this is where its particularly important
not to decrement n).
(This could probably be considered broken for the above cases of padded
64-bit args).
Also of interest is the kerneldoc comment describing this function in
include/asm-generic/syscall.h:
/**
* syscall_get_arguments - extract system call parameter values
* @task: task of interest, must be blocked
* @regs: task_pt_regs() of @task
* @i: argument index [0,5]
* @n: number of arguments; n+i must be [1,6].
* @args: array filled with argument values
*
* Fetches @n arguments to the system call starting with the @i'th argument
* (from 0 through 5). Argument @i is stored in @args[0], and so on.
* An arch inline version is probably optimal when @i and @n are constants.
*
* It's only valid to call this when @task is stopped for tracing on
* entry to a system call, due to %TIF_SYSCALL_TRACE or %TIF_SYSCALL_AUDIT.
* It's invalid to call this with @i + @n > 6; we only support system calls
* taking up to 6 arguments.
*/
void syscall_get_arguments(struct task_struct *task, struct pt_regs *regs,
unsigned int i, unsigned int n, unsigned long *args);
Cheers
James
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