Re: [perfmon2] I.1 - System calls - ioctl

Arnd Bergmann
2009-07-13T17:54:24Z

On Monday 13 July 2009, Peter Zijlstra wrote:
> On Mon, 2009-07-13 at 19:30 +0200, Arnd Bergmann wrote:
> > > +struct perf_counter_target {
> > > + __u32 id;
> > > + __u64 val;
> > > +};
> >
> > This structure is not compatible between 32 and 64 bit user space on x86,
> > because everything except i386 adds implicit padding between id and val.
>
> Humm, __u64 doesn't have natural alignment? That would break more than
> just this I think -- it sure surprises me.

Yes, nobody expects this, so it is a frequent source of bugs in the ABI.
Look for compat_u64 and __packed in the definition of compat ioctl and
syscall interfaces for how we had to work around this elsewhere.

> > Other than that, making it extensible sounds reasonable. How about just
> > using a '__u64 *target' and a bit in the 'flags' argument?
>
> Would there still be a point in having it a pointer in that case?, but
> yeah, that might work too?

passing u64 bit arguments directly to system calls is a bit complicated,
because some 32 bit architectures can only pass them in certain
register pairs, see the trouble we go through for llseek, sync_file_range
or preadv.

If you can directly pass an 'unsigned long' instead, that would work fine
though.