Re: [PATCH] lib: iov_iter.c: fix a possible calculation error on remaining bytes

[Al Viro]

On Tue, Feb 18, 2020 at 03:41:12PM +0800, Macpaul Lin wrote:
> This issue was found when adbd trying to open functionfs with AIO mode.
> Usually, we need to set "setprop sys.usb.ffs.aio_compat 0" to enable
> adbd with AIO mode on Android.
> 
> When adbd is opening functionfs, it will try to read 24 bytes at the
> fisrt read I/O control. If this reading has been failed, adbd will
> try to send FUNCTIONFS_CLEAR_HALT to functionfs. When adbd is in AIO
> mode, functionfs will be acted with asyncronized I/O path. After the
> successful read transfer has been completed by gadget hardware, the
> following series of functions will be called.
>   ffs_epfile_async_io_complete() -> ffs_user_copy_worker() ->
>     copy_to_iter() -> _copy_to_iter() -> copyout() ->
>     iterate_and_advance() -> iterate_iovec()
> 
> Adding debug trace to these functions, it has been found that in
> iterate_iovec(), the calculation result of n will be turned into zero.
>    n = wanted - n; /* 0 == n = 24 - 24; */
> Which causes copyout() won't copy data to userspace since the length
> to be copied "v.iov_len" will be zero, which isn't correct. This also
> leads ffs_copy_to_iter() always return -EFAULT. Finally adbd cannot
> open functionfs and send FUNCTIONFS_CLEAR_HALT.
> 
> Signed-off-by: Macpaul Lin <macpaul.lin@xxxxxxxxxxxx>
> ---
>  lib/iov_iter.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/lib/iov_iter.c b/lib/iov_iter.c
> index fb29c02c6a3c..f9334144e259 100644
> --- a/lib/iov_iter.c
> +++ b/lib/iov_iter.c
> @@ -36,7 +36,8 @@
>  		skip = __v.iov_len;			\
>  		n -= __v.iov_len;			\
>  	}						\
> -	n = wanted - n;					\
> +	if (n != wanted)				\
> +		n = wanted - n;				\
>  }

	First of all, nothing in that line can possibly *cause*
copyout() to do anything - it's after the calls of step.  What's
more, this changes behaviour only when wanted would've been equal to
n, doesn't it?  Which translates into "no decrements of n have
happened at all", i.e. "nothing has been copied".  IOW, it's
a consequence of no copyout, not the cause of such.  You can
make copy_to_iter() lie and pretend if has copied everything
when it has copied nothing, but that won't change the underlying
bug.

	So I'm afraid your debugging is not finished - you
still need to find out what causes the copyout failures and/or
BS iov_iter padded by caller.