Whats the deal with the use std:: in the middle of linux_raw::io::syscalls.rs when target_arch = "aarch64"

[AsafFisher]

I am compiling a no_std code using this library, it works fine with x86_64 but when I set target_arch = "aarch64" it fails with:

error[E0433]: failed to resolve: use of undeclared crate or module `std`
   --> /home/a/.cargo/registry/src/github.com-1ecc6299db9ec823/rustix-0.34.6/src/imp/linux_raw/io/syscalls.rs:560:13
    |
560 |         use std::os::unix::io::AsRawFd;
    |             ^^^ use of undeclared crate or module `std`

rustix/src/imp/linux_raw/io/syscalls.rs

Line 396 in 23841ae

#[cfg(any(target_arch = "aarch64", target_arch = "riscv64"))]

[sunfishcode]

no_std support in rustix is currently only used for porting std to rustix, which I've only happened to test on x86_64 so far. It looks like you found a bug; the fix is to change std::os::unix::io::AsRawFd to crate::fd::AsRawFd.

I'm also curious; if you have Linux to make syscalls to, why do you need no_std?

[AsafFisher]

Hi! First of all I'll be happy to fix this and some other issues that I've found.

I am making a shellcode debugger (:
I want it to be cross-arch, cross environment and position independent.

Therefore I use this library as it is minimal and does not use libc.

Just in case you want some more details:
stdlib is not an option because the debugger should be robust to kernel mode / user mode
If you try to make a staticshellcode with the libc crate it will use a static lib of libc to link with. It will therefore embed a static GOT (yes the addresses will be already resolved and naturally the binary will contain absolute jumps which is bad for PIE shellcode ) so yah libc crate is not an option

With this library I am able to make a static and PIE(0 absolute calls) shellcode that can perform syscalls on different os's

BTW,
Whats the purpose of #[cfg(any(target_arch = "aarch64", target_arch = "riscv64"))]

rustix/src/imp/linux_raw/io/syscalls.rs

Line 396 in 23841ae

#[cfg(any(target_arch = "aarch64", target_arch = "riscv64"))]

[AsafFisher]

The point here is to optimize the check of the fd's in the user mode instead of performing the system call?
https://github.com/AsafFisher/rustix/blob/5cf42f790a9fad676e3fa912529845467790b0ff/src/imp/linux_raw/io/syscalls.rs#L401

[sunfishcode]

arch64 and riscv64 don't have a dup2 system call, so on those platforms we need to use dup3.

There's also a subtlety that dup2 and dup3 have different behavior when the old and new file descriptors are the same, so we have some logic to ensure that rustix::io::dup2 has the dup2 behavior even when using the dup3 system call.

[AsafFisher]

Yah! I looked it up today and saw it. Coolz

[sunfishcode]

This is now released in 0.34.7.