Re: Programmatic Back Trace

9 Sep 2003

      On 03.09.2003 at 2:22 PM, Chris Thomas <chris@m-audio.com> wrote:
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I have a need (well, a strong desire) for this myself right now.
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http://developer.apple.com/technotes/tn2002/tn2063.html has
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information which may be pertinent: it lists the source files used to
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generate the textual kernel panic, ie:
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Line 6 xnu/osfmk/ppc/model_dep.c: dump_backtrace
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It's not immediately clear to me whether or not this code is useful
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outside of an exception context.
Thanks for the information; model_dep.c was quite helpful.  The

dump_backtrace function does basically what I need.  I was able to

easily integrate its logic into my library and now have the ability to

capture kernel backtraces.

Thanks again!

geoff
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On Sep 3, 2003, at 9:30 AM, Geoffrey Schmit wrote:
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On 03.09.2003 at 11:06 AM, Jim Magee <jmagee@apple.com> wrote:
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On Sep 3, 2003, at 10:23 AM, Geoffrey Schmit wrote:
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I'm currently stuck trying to find a function that is accessible by
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a kernel extension and that is equivalent to vm_read_overwrite in
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that it will return an error if the specified address is
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inaccessible rather than throwing a SIGSEGV signal.
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So, a couple of questions:
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Is there an example that captures a back trace in a kernel
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extension? If so, where is it?
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Are you trying to get a user-level backtrace or a kernel backtrace
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from your kernel extension? And more importantly, why are you doing
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this from a kernel extension in the first place?
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I'm primarily interested in getting a kernel backtrace from my kernel
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extension. I'm porting a library that provides various services to
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other kernel extensions we develop. One of these services is to
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capture
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a backtrace. For example, we may capture a backtrace when a kernel
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extension detects an error in order to facilitate the debugging of
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that error condition. As another example, we may capture a backtrace
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when a kernel extension acquires a resource in order to later
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determine who acquired that resource if the resource is leaked.
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Ideally, it would be beneficial for the backtrace to eventually
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include the stack of the user-mode thread that initiated the
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transition to the kernel (in scenarios where this is the case).
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However, that is probably
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pushing my luck....
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I'm not at all interested in developing a kernel extension to capture
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user-level backtraces from arbitrary processes.
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In-kernel, if you access memory you don't have mapped, you're
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dead (panic). You don't get a signal to respond to. And, besides,
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you can't directly access any user-level memory at all. On
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PowerPC, user- memory isn't mapped into the kernel's view of
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memory, since the kernel gets it own 4GB address space. i386 is
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slightly different, but changing more towards the PowerPC model
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than the other way around.
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So, my problem is probably that as I walk the stack I eventually
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encounter an address from a user-mode thread and cause a panic. Is
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there a way to test if the address belongs to the kernel or otherwise
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determine that I should stop walking the stack?
--

Geoffrey Schmit

Senior Software Engineer

National Instruments

geoffrey.schmit@ni.com

www.ni.com

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