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From the kernel's POV, there's no undefined behavior in user code. (If the kernel knew a program had violated C's memory rules, it could kill it and we wouldn't have endemic security vulnerabilities.) Likewise, in safe Rust, the access to that array might be well defined with respect to Rust's view of the world (just like even UB in C programs is well defined from the kernel POV), but it can still cause havoc at a higher level of abstraction --- your application. And it's hard to predict what kind of breakage at the application layer might result.