[PATCH v2] usb: gadget: f_fs: try to fix AIO issue under ARM 64 bit TAGGED mode

From: Macpaul Lin
Date: Sun Feb 23 2020 - 10:49:25 EST

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
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 gdaget'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 userspace 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

Signed-off-by: Macpaul Lin <macpaul.lin@xxxxxxxxxxxx>
Changes for v2:
- Fix build error for 32-bit load. An #if defined(CONFIG_ARM64) still need
for avoiding undeclared defines.

drivers/usb/gadget/function/f_fs.c | 5 +++++
1 file changed, 5 insertions(+)

diff --git a/drivers/usb/gadget/function/f_fs.c b/drivers/usb/gadget/function/f_fs.c
index ce1d023..728c260 100644
--- a/drivers/usb/gadget/function/f_fs.c
+++ b/drivers/usb/gadget/function/f_fs.c
@@ -35,6 +35,7 @@
#include <linux/mmu_context.h>
#include <linux/poll.h>
#include <linux/eventfd.h>
+#include <linux/thread_info.h>

#include "u_fs.h"
#include "u_f.h"
@@ -826,6 +827,10 @@ static void ffs_user_copy_worker(struct work_struct *work)
if (io_data->read && ret > 0) {
mm_segment_t oldfs = get_fs();

+#if defined(CONFIG_ARM64)
+ set_thread_flag(TIF_TAGGED_ADDR);
ret = ffs_copy_to_iter(io_data->buf, ret, &io_data->data);