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; \
}
#define iterate_kvec(i, n, __v, __p, skip, STEP) { \
--
2.18.0
