> Hmm, I’m not familiar with cdqe. A quick search reveals that it sign-extends the lower 4 bytes into the full 8 bytes. Ah ha! A flash of insight illuminates my grasping waddles. Could it be that kmalloc() is implicitly defined at the call site, so the compiler assumes the return type is a 32-bit int, chucking out the top half of the pointer value?
This really shows the glory, and the horror, of low-level systems programming. You make a seemingly-simple change, which surfaces a bug, which leads to gruesome crashes - the kind where the black box ends up in pieces - which you finally somehow trace back to an odd assembly instruction, which happened because you forgot to include a header so the C compiler simply hallucinated a function prototype! Because header files weren't free in the '70s, apparently!
It requires the kind of mind that can diagnose a leaky head gasket from a customer pantomiming goosh-goosh sounds.