Re: [PATCH v4] usb: gadget: f_fs: try to fix AIO issue under ARM 64 bit TAGGED mode

[Catalin Marinas]

On Wed, Feb 26, 2020 at 08:01:52PM +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
> first 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
> copyout(), access_ok() will check if the user space address is valid
> to write. However if CONFIG_ARM64_TAGGED_ADDR_ABI is enabled, adbd
> always passes user space address start with "0x3C" to gadget's AIO
> blocks. This tagged address will cause access_ok() check always fail.
> Which causes later calculation in iterate_iovec() turn zero.
> Copyout() won't copy data to user space since the length to be copied
> "v.iov_len" will be zero. Finally leads ffs_copy_to_iter() always return
> -EFAULT, causes adbd cannot open functionfs and send
> FUNCTIONFS_CLEAR_HALT.
> 
> Signed-off-by: Macpaul Lin <macpaul.lin@xxxxxxxxxxxx>
> Cc: Peter Chen <peter.chen@xxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Miles Chen <miles.chen@xxxxxxxxxxxx>
> ---
> Changes for v4:
>   - Abandon solution v3 by adding "TIF_TAGGED_ADDR" flag to gadget driver.
>     According to Catalin's suggestion, change the solution by untagging 
>     user space address passed by AIO in gadget driver.

Well, this was suggested in case you have a strong reason not to do the
untagging in adbd. As I said, tagged pointers in user space were
supported long before we introduced CONFIG_ARM64_TAGGED_ADDR_ABI. How
did adb cope with such tagged pointers before? It was not supposed to
pass them to the kernel.

> diff --git a/drivers/usb/gadget/function/f_fs.c b/drivers/usb/gadget/function/f_fs.c
> index ce1d023..192935f 100644
> --- a/drivers/usb/gadget/function/f_fs.c
> +++ b/drivers/usb/gadget/function/f_fs.c
> @@ -715,7 +715,20 @@ static void ffs_epfile_io_complete(struct usb_ep *_ep, struct usb_request *req)
>  
>  static ssize_t ffs_copy_to_iter(void *data, int data_len, struct iov_iter *iter)
>  {
> -	ssize_t ret = copy_to_iter(data, data_len, iter);
> +	ssize_t ret;
> +
> +#if defined(CONFIG_ARM64)
> +	/*
> +	 * Replace tagged address passed by user space application before
> +	 * copying.
> +	 */
> +	if (IS_ENABLED(CONFIG_ARM64_TAGGED_ADDR_ABI) &&
> +		(iter->type == ITER_IOVEC)) {
> +		*(unsigned long *)&iter->iov->iov_base =
> +			(unsigned long)untagged_addr(iter->iov->iov_base);
> +	}
> +#endif
> +	ret = copy_to_iter(data, data_len, iter);

Here you should probably drop all the #ifdefs and IS_ENABLED checks
since untagged_addr() is defined globally as a no-op (and overridden by
arm64 and sparc).

Please don't send another patch until we understand (a) whether this is
a user-space problem to fix or (b) if we fix it in the kernel, is this
the only/right place? If we settle for the in-kernel untagging, do we
explicitly untag the addresses in such kernel threads or we default to
TIF_TAGGED_ADDR for all kernel threads, in case they ever call use_mm()
(or we could even hook something in use_mm() to set this TIF flag
temporarily).

Looking for feedback from the Android folk and a better analysis of the
possible solution.

-- 
Catalin