On Wed, Nov 19, 2014 at 08:46:19AM +0000, AKASHI Takahiro wrote:
> On 11/18/2014 11:04 PM, Will Deacon wrote:
> > On Tue, Nov 18, 2014 at 01:10:34AM +0000, AKASHI Takahiro wrote:
> >>
> >> + if (((int)regs->syscallno == -1) && (orig_syscallno == -1)) {
> >> + /*
> >> + * user-issued syscall(-1):
> >> + * RESTRICTION: We always return ENOSYS whatever value is
> >> + * stored in x0 (a return value) at this point.
> >> + * Normally, with ptrace off, syscall(-1) returns -ENOSYS.
> >> + * With ptrace on, however, if a tracer didn't pay any
> >> + * attention to user-issued syscall(-1) and just let it go
> >> + * without a hack here, it would return a value in x0 as in
> >> + * other system call cases. This means that this system call
> >> + * might succeed and see any bogus return value.
> >> + * This should be definitely avoided.
> >> + */
> >> + regs->regs[0] = -ENOSYS;
> >> + }
> >
> > I'm still really uncomfortable with this, and it doesn't seem to match what
> > arch/arm/ does either.
>
> Yeah, I know but
> as I mentioned before, syscall(-1) will be signaled on arm, and so we don't
> have to care about a return value :)
What does x86 do?
> > Doesn't it also prevent a tracer from skipping syscall(-1)?
>
> Syscall(-1) will return -ENOSYS whether or not a syscallno is explicitly
> replaced with -1 by a tracer, and, in this sense, it is *skipped*.
Ok, but now userspace sees -ENOSYS for a skipped system call in that case,
whereas it would usually see whatever the trace put in x0, right?
Will
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