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.
