Linode's kernels aren't modified; you can reproduce them using
the mainline kernel source and the configuration file from /proc/config.gz, plus a quick edit to config.h to stick -linodeXX into the kernel version. Rackspace Cloud's kernels are also (probably) not modified and could be reproduced similarly. However, since they have no mechanism by which to boot arbitrary kernels, I haven't tried.
Of course, neither provide the kernel your distribution shipped with; they're often compiled with a particular configuration or modified to fit the distribution's needs. If you need to run such a nonstandard kernel

,
see this document.
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