[PATCH 3/9] iov_iter: refactor rw_copy_check_uvector and import_iovec

[Al Viro]

On Wed, Sep 23, 2020 at 08:05:41AM +0200, Christoph Hellwig wrote:

> +struct iovec *iovec_from_user(const struct iovec __user *uvec,
> +		unsigned long nr_segs, unsigned long fast_segs,

Hmm...  For fast_segs unsigned long had always been ridiculous
(4G struct iovec on caller stack frame?), but that got me wondering about
nr_segs and I wish I'd thought of that when introducing import_iovec().

The thing is, import_iovec() takes unsigned int there.  Which is fine
(hell, the maximal value that can be accepted in 1024), except that
we do pass unsigned long syscall argument to it in some places.

E.g. vfs_readv() quietly truncates vlen to 32 bits, and vlen can
come unchanged through sys_readv() -> do_readv() -> vfs_readv().
With unsigned long passed by syscall glue.

AFAICS, passing 4G+1 as the third argument to readv(2) on 64bit box
will be quietly treated as 1 these days.  Which would be fine, except
that before "switch {compat_,}do_readv_writev() to {compat_,}import_iovec()"
it used to fail with -EINVAL.

Userland, BTW, describes readv(2) iovcnt as int; process_vm_readv(),
OTOH, has these counts unsigned long from the userland POV...

I suppose we ought to switch import_iovec() to unsigned long for nr_segs ;-/
Strictly speaking that had been a userland ABI change, even though nothing
except regression tests checking for expected errors would've been likely
to notice.  And it looks like no regression tests covered that one...

Linus, does that qualify for your "if no userland has noticed the change,
it's not a breakage"?