I would like a command-line-tool to fail if it opens a particular file for writing.
Is there a way I can modify the environment (maybe via cgroups) of the command-line tool, so that the command/process gets (for example) “permission denied”?
chmod a-w file does not work. The process seems to unlink() and then re-create the file.
I know that I can watch the syscalls of a process with strace. But is there a way to alter some calls, so that the process gets a different result?
Background: unittesting
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Answer
strace has an option called -e inject or simply --inject which can be used to alter system calls of the tracee. (See manpage here)
In particular, in can be combined with the -P option to only trace syscalls accessing a specified path.