Date: Tue, 15 May 2007 14:18:37 +1000 From: Lawrence Stewart <lstewart@room52.net> To: freebsd-hackers@freebsd.org Subject: Writing a plain text file to disk from kernel space Message-ID: <4649349D.4060101@room52.net>
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Hi all, I'm in the process of writing a kernel module to instrument some parts of the network stack in FreeBSD 6.2. Everything works perfectly, except that writing the logging info to syslog using printf is too inefficient for this particular task. Given that log messages need to be generated for each packet entering/leaving the network stack, my testing thus far has showed that syslog groans under the load and uses far too much CPU time. So, I'm looking for a way to manually open up a file from within kernel space and dump characters into it. I've come across the following threads that are related to my problem: http://lists.freebsd.org/pipermail/freebsd-hackers/2007-May/020330.html http://lists.freebsd.org/pipermail/freebsd-fs/2005-September/001306.html Neither thread has provided the "ah-ha" information/discovery I'm needing to get this working, and so I'd be very grateful to anyone that can shed some light on how to piece this together in the most efficient way possible. To summarise the things I've experimented with thus far: 1. Following the "open" syscall down into kernel land and investigating how the underlying kernel functions work. The "kern_open" function in "kern/vfs_syscalls.c" seems interesting. However, all of interesting functions that I've found take a thread struct pointer as the first argument. I have not been able to figure out where to get a thread struct pointer from. So perhaps if someone can shed light on how to go about creating a thread struct suitable for passing to these functions, I may be able to figure it out. I've tried creating a new kthread using kthread_create, and got this working, but this still didn't give me access to a thread struct pointer. I've also tried accessing the thread queue for process 0 but didn't have any luck there either (though I could have been accessing it incorrectly!). 2. Following the kernel panic code where it writes a dump out to disk. This uses very scary low level structs and functions that I couldn't wrap my head around... appeared to be below the vfs layer in the kernel, so I suspect its to far below where I want to be at. 3. I followed the ktrace code, but that seemed to lead to a dead end of passing logging data up to userspace via the ddb subsystem, which is not what I want to do. 4. I followed coredump code, and this looked very interesting. It operates directly on vnodes, but once again, the elusive thread struct pointer is needed. The "kern/vfs_*" files seem to contain the functions relating to all the action of creating/writing/reading files (vnodes)... is there a good document/tutorial somewhere that describes the layering and various abstractions in the FreeBSD kernel fileystem related code? TCP Illustrated has been invaluable in describing the layout and structure of the various network stack components/functions/structs in the kernel. Something similar for filesystem related tasks would be most useful. I suspect that once I understand the mechanics of where thread struct pointers come from and how to get my hands one one from within the kernel, I'll be able to get the rest going. Many thanks for your time and help. Regards, Lawrence Stewart
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