On Thu, Jun 24, 2021 at 05:38:35PM +0100, Robin Murphy wrote:
> On 2021-06-24 17:27, Al Viro wrote:
> > On Thu, Jun 24, 2021 at 02:22:27PM +0100, Robin Murphy wrote:
> >
> > > FWIW I think the only way to make the kernel behaviour any more robust here
> > > would be to make the whole uaccess API more expressive, such that rather
> > > than simply saying "I only got this far" it could actually differentiate
> > > between stopping due to a fault which may be recoverable and worth retrying,
> > > and one which definitely isn't.
> >
> > ... and propagate that "more expressive" information through what, 3 or 4
> > levels in the call chain?
> >
> > From include/linux/uaccess.h:
> >
> > * If raw_copy_{to,from}_user(to, from, size) returns N, size - N bytes starting
> > * at to must become equal to the bytes fetched from the corresponding area
> > * starting at from. All data past to + size - N must be left unmodified.
> > *
> > * If copying succeeds, the return value must be 0. If some data cannot be
> > * fetched, it is permitted to copy less than had been fetched; the only
> > * hard requirement is that not storing anything at all (i.e. returning size)
> > * should happen only when nothing could be copied. In other words, you don't
> > * have to squeeze as much as possible - it is allowed, but not necessary.
> >
> > arm64 instances violate the aforementioned hard requirement. Please, fix
> > it there; it's not hard. All you need is an exception handler in .Ltiny15
> > that would fall back to (short) byte-by-byte copy if the faulting address
> > happened to be unaligned. Or just do one-byte copy, not that it had been
> > considerably cheaper than a loop. Will be cheaper than propagating that extra
> > information up the call chain, let alone paying for extra ->write_begin()
> > and ->write_end() for single byte in generic_perform_write().
>
> And what do we do if we then continue to fault with an external abort
> because whatever it is that warranted being mapped as Device-type memory in
> the first place doesn't support byte accesses?
If it does not support byte access, it would've failed on fault-in.
