Hello, On Monday, February 13, 2023 4:24:02 PM EST Amjad Gabbar wrote: > I wanted some help in better understanding the workflow of file system > auditing(watch rules) vs Syscall Auditing(syscall rules). I know in general > file system auditing does not have the same performance impact as syscall > auditing, even though both make use of syscall exits for their evaluation. > > > From the manpage - "Unlike most syscall auditing rules, watches do not > impact performance based on the number of rules sent to the kernel." > > From a previous thread, I found this excerpt regarding file watch rules vs > sycall rules - > > "The reason it doesn't have performance impact like normal syscall rules is > because it gets moved to a list that is not evaluated every syscall. A > normal syscall rule will get evaluated for every syscall because it has to > see if the syscall number is of interest and then it checks the next > rule." > > Based on this I had a couple of questions: > > For normal syscall rules, the evaluation happens as __audit_syscall_exit > <https://elixir.bootlin.com/linux/v6.1.10/C/ident/__audit_syscall_exit> > calls audit_filter_syscall > (https://elixir.bootlin.com/linux/v6.1.10/source/kernel/auditsc.c#L841) > > Here, we check if the syscall is of interest or not in the audit_in_mask > <https://elixir.bootlin.com/linux/v6.1.10/C/ident/audit_in_mask> function. > Only if the syscall is of interest do we proceed with examining the task > and return on the first rule match. > > 1. What is the process or code path for watch rules? audit_filter_syscall > <https://elixir.bootlin.com/linux/v6.1.10/C/ident/audit_filter_syscall> is > called for watch rules as well. Then how is it that these are not called > for every syscall? Could you point me to the code where the evaluation > happens only once?
There is a file, kernel/audit_watch.c, that implements the interface between audit and fsnotify. You would want to learn how fsnotify works to understand how it avoids the syscall filter. > 2. Also, do file watches only involve the open system call family (open, > openat etc). The man page implies the same, so just wanted to confirm. > > I assume -w /etc -p wa is the same as -a always,exit -S open -S openat -F > dir=/etc? It depends on the flag passed for perm as to what syscall it wants. See: include/asm-generic/audit_*.h -Steve -- Linux-audit mailing list Linux-audit@redhat.com https://listman.redhat.com/mailman/listinfo/linux-audit
