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Re: Finding symbols in loaded kexts

On Aug 5, 2007, at 6:21 PM, Ryan Govostes wrote:
Is it possible for one kext to locate a symbol in another kext? I've tried APETools, dlcompat, kld, and a couple other tricks without success, but I'll admit (and it should be obvious) that I only have a vague notion of what I'm doing.

Obviously this is not for anything production and I accept responsibility for the horrible condition I'll likely leave my kernel in if I screw up!

I did notice that kextstat is able to get a list of loaded kexts and their addresses, but I could not replicate its functionality from within the kernel. Surely there is a way to do so, but I'm not familiar with the functions used.

[below, I refer to the kext that contains the symbol you are after as the "target" kext, and the one that is doing the lookup as the "probe" kext.

When you say "find a symbol", I assume you trying get the kernel- virtual address of the storage/entry point for a symbol defined in the target kext by running code only in your probe kext. Is the symbol you are after global? (Use the nm command to examine the binary of your target kext. if the symbol of interest has a type specifier that is capital, then it is global.) Does the target kext allow other kexts to link against it? (does it have the "OSBundleCompatibleVersion" property.)

If the answer to both of these it yes, then the solution is quite simple: Just have your probe kext declare a dependency on the target kext. You can then declare an extern reference to the symbol in question, and when the probe kext loads, it'll have its reference patched to the same address as the global symbol in the target kext. This is typically used to share data and make direct function calls between kexts, but you can also take the address of such symbols if that is what you want.

If the target kext does not allow linking, then you can fake it by adding the "OSBundleCompatibleVersion" property with a value that is less-than-or-equal the CFBundleVersion value. Obviously, this is just a hack, and you shouldn't go and modify somebody else's kext in production code (you indicated this was just for your own experimentation).

If the symbol you want is not global (lower-case type specifier in the nm output), or has been stripped out of the target kext entirely, then there isn't really anything you can do.

Hope this helps,
 - Dean
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 >Finding symbols in loaded kexts (From: Ryan Govostes <email@hidden>)



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