Incorrect login password in ubuntu 12.04

Incorrect logon to ubuntu 12.04 with a correct password: After installing arm-linux-gcc 4.3.3 (accurately, decompress tar ), modify the/etc/profile file according to the instructions in the development document, add a PATH to the PATH, and modify the PATH value in/etc/environment; then restart ubuntu as required by the document. The password is entered correctly. After a black screen is flashed for a second, the password is changed to the required state again, and the guest session cannot be entered. ctrl + shift + f2 enable tty2, the entered username and password are both incorrect (root login is not enabled). After checking a lot of information, the problem must be that an error occurred while modifying the environment variable above, so how can I modify the environment variables when tty2 cannot be used? As described in the Link, when you start the recovery mode, press shift and select the second recovery mode. The following interface is not the same as the link. The 12.04 interface is that The fsck and root options are on the same page, select fsck to remove the write restriction (just a few seconds after fsck is pressed), and then select root as the familiar terminal rhythm, vi/etc/profile, vi/etc/environment: restore the environment variable, and then log on again. OK. Success!