Thanks for the reply. I was trying to evaluate the same via Flamegraphs and what I noticed was that :
1. Despite deleting all rules (auditctl -D), there were still calls to audit_filter_syscall() on each syscall. I assume this is because syscall auditing is enabled and despite no rules, there still will be some performance impact and calls to syscall filtering functions on each syscall. 2. For a single watch rule as well without any syscall rules, I could see calls to audit_filter_syscall() followed by audit_filter_rules() for unrelated syscalls such as futex() and recvmsg() - not present in include/asm-generic/audit_*.h Why would these functions be called for a single watch rule for syscalls unrelated to the permissions? On Tue, Feb 14, 2023 at 8:29 AM Steve Grubb <sgr...@redhat.com> wrote: > 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 > > >
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