在我的android 8.1.0系统上，我尝试直接写入块设备。(emmc上的分区)
我可以给P24写信，但不知何故不能给P25写信，我不明白为什么。在logcat或dmesg日志中没有任何内容。No avc: denied.
# whoami
root
# ls -lZ mmcblk1p24 mmcblk1p25
brwxrwxrwx 1 root root u:object_r:block_device:s0 259, 8 2011-11-11 11:11 mmcblk1p24
brwxrwxrwx 1 root root u:object_r:block_device:s0 259, 9 2011-11-11 11:11 mmcblk1p25
# dd if=/dev/zero of=mmcblk1p24 bs=1 count=1
1+0 records in
1+0 records out
1 bytes transferred in 0.001 secs (1000 bytes/sec)
# dd if=/dev/zero of=mmcblk1p25 bs=1 count=1
dd: mmcblk1p25: Operation not permitted
1+0 records in
0+0 records out
0 bytes transferred in 0.001 secs (0 bytes/sec)
# busybox lsattr mmcblk1p24 mmcblk1p25
lsattr: reading mmcblk1p24: Not supported
lsattr: reading mmcblk1p25: Not supported
找到了。块设备的ioctl BLKRO位设置为
# blockdev --getro /dev/block/mmcblk1p24
0
# blockdev --getro /dev/block/mmcblk1p25
1
# blockdev --setrw /dev/block/mmcblk1p25
# blockdev --getro /dev/block/mmcblk1p25
0