Re: [BUG] arm64: an infinite loop in generic_perform_write()

[Al Viro <viro@zeniv.linux.org.uk>]

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().