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When a ftrace filter has a module function, and that module is removed, the filter still has its address as being enabled. This can cause interesting side effects. Nothing dangerous, but unwanted functions can be traced because of it. # cd /sys/kernel/tracing # echo ':mod:snd_seq' > set_ftrace_filter # cat set_ftrace_filter snd_use_lock_sync_helper [snd_seq] check_event_type_and_length [snd_seq] snd_seq_ioctl_pversion [snd_seq] snd_seq_ioctl_client_id [snd_seq] snd_seq_ioctl_get_queue_tempo [snd_seq] update_timestamp_of_queue [snd_seq] snd_seq_ioctl_get_queue_status [snd_seq] snd_seq_set_queue_tempo [snd_seq] snd_seq_ioctl_set_queue_tempo [snd_seq] snd_seq_ioctl_get_queue_timer [snd_seq] seq_free_client1 [snd_seq] [..] # rmmod snd_seq # cat set_ftrace_filter # modprobe kvm # cat set_ftrace_filter kvm_set_cr4 [kvm] kvm_emulate_hypercall [kvm] kvm_set_dr [kvm] This is because removing the snd_seq module after it was being filtered, left the address of the snd_seq functions in the hash. When the kvm module was loaded, some of its functions were loaded at the same address as the snd_seq module. This would enable them to be filtered and traced. Now we don't want to clear the hash completely. That would cause removing a module where only its functions are filtered, to cause the tracing to enable all functions, as an empty filter means to trace all functions. Instead, just set the hash ip address to zero. Then it will never match any function. Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.