[ I first asked this question on stackoverflow by mistake. ]
I have this "old" Lenovo A2107 tablet which I rooted years ago (using a download from http://androidforums.com/threads/a2107-root-mods-and-rom-discussion.661261/, tho running its root.bat script by hand from my GNU/Linux machine) and that worked fine: I could "su" from the shell and then edit all the files I wanted.
Then I passed this tablet to my daughter for a couple years, and now I got it back.
/system/bin/su is still present, still the same date and size, still mode "rwsr-sr-x", still the same firmware (Android 4.0.3) so all looks fine, but when I try to run it from a shell, it just tells me "Permission denied". And of course, other applications that require root bump into the same problem.
Any idea what might be going on? What I could try to do to track it down?
[ See also https://stackoverflow.com/questions/32353148 where I posted the solution I encountered. ]