FAQs

Why static key should be applied to seldomly changed features?

Two reasons:

  • The modification of static key will impose a potential security risk since we need to bypass DEP. The DEP will be restored after modification done.
  • The modification is less efficient since many syscalls are involved.

Why static keys must only be modified in a single-thread environment?

In userland, it is very complicated to modify an instruction which may be executed by another thread. Linux kernel community once proposed a text_poke syscall, but is still not available nowadays. BTW, Linus doesn't seem to like it, and his reasons do make sense.

Another reason is that we need to manipulate memory protection to bypass DEP, which may involves race condition on the protection itself in multi-thread environment. Mutex may be used to avoid data race, while if cargo resolves multi-version static-key crates dependencies, the mutexes would be duplicated for each version, and this approach is thus useless. This shall be resolved when RFC 1977: public & private dependencies is stabilized. rust-lang/cargo#2363 is also a reference.

Why is nightly Rust required?

We use inline assembly internally, where asm_goto and asm_const feature is required. As long as these two features are stablized, we can use stable Rust then.

Why static_branch_likely! and static_branch_unlikely! are implemented as macros?

Because when passing a static variable to inline assembly as sym argument, it requires the argument to be a static path. We cannot construct such path in a function.

What OS-specific features are required to extend to new OSs?

  • Symbols indicating a custom section's start and stop

    Used for sorting static keys section and iterating stop signs.

  • Approaches to keep custom section retained from linker's GC

  • Approaches to bypass DEP

    Used to update static branch instructions.

What arch-specific features are required to extend to new architectures?

  • A nop instruction with same length as jmp (or can divide the length, e.g. 2-byte nop and 4-byte jmp)
  • Approaches to clear instruction cache in Linux. (Should be added to Evian-Zhang/clear-cache)
  • Inline assembly supported by Rust

Can I use this crate in no_std?

Yes.

How can I use this crate in bare metal?

You should modify your linker script to add __start and __stop prefix symbols for marking start and end address of corresponding sections. For more details, see Evian-Zhang/static-keys#6.