r-linux 0.1.0

//! System Calls on x86
//!
//! This implements the syscall entries for x86. One function for each
//! possible number of arguments is provided: syscall0 to syscall6.
//!
//! The implementation uses the x86-`int$0x80` software interrupt to enter the
//! kernel. It would be much faster to use the VDSO entry point, but it does
//! require access to `%gs` and the TLS mappings, and thus is left for future
//! improvements (if anyone cares enough for 32bit x86).
//!
//! Arguments are passed as:
//!     Nr: eax
//!     Args: ebx, ecx, edx, esi, edi, ebp
//! Return value is in:
//!     Ret: eax
//!
//! The entry-points are currently not marked as `readonly`. That is, the
//! system calls are allowed to modify memory. If necessary, alternative calls
//! with `readonly` (or maybe even `pure`) can be provided in the future.

#[cfg(target_arch = "x86")]
#[inline]
#[export_name = "r_linux_asm_syscall0"]
pub unsafe fn syscall0(
    nr: usize,
) -> usize {
    let mut r: usize;

    core::arch::asm!(
        "int $0x80",
        inlateout("eax") nr => r,
        options(nostack, preserves_flags)
    );

    r
}

#[cfg(target_arch = "x86")]
#[inline]
#[export_name = "r_linux_asm_syscall1"]
pub unsafe fn syscall1(
    nr: usize,
    arg0: usize,
) -> usize {
    let mut r: usize;

    core::arch::asm!(
        "int $0x80",
        inlateout("eax") nr => r,
        in("ebx") arg0,
        options(nostack, preserves_flags)
    );

    r
}

#[cfg(target_arch = "x86")]
#[inline]
#[export_name = "r_linux_asm_syscall2"]
pub unsafe fn syscall2(
    nr: usize,
    arg0: usize,
    arg1: usize,
) -> usize {
    let mut r: usize;

    core::arch::asm!(
        "int $0x80",
        inlateout("eax") nr => r,
        in("ebx") arg0,
        in("ecx") arg1,
        options(nostack, preserves_flags)
    );

    r
}

#[cfg(target_arch = "x86")]
#[inline]
#[export_name = "r_linux_asm_syscall3"]
pub unsafe fn syscall3(
    nr: usize,
    arg0: usize,
    arg1: usize,
    arg2: usize,
) -> usize {
    let mut r: usize;

    core::arch::asm!(
        "int $0x80",
        inlateout("eax") nr => r,
        in("ebx") arg0,
        in("ecx") arg1,
        in("edx") arg2,
        options(nostack, preserves_flags)
    );

    r
}

#[cfg(target_arch = "x86")]
#[inline]
#[export_name = "r_linux_asm_syscall4"]
pub unsafe fn syscall4(
    nr: usize,
    arg0: usize,
    arg1: usize,
    arg2: usize,
    arg3: usize,
) -> usize {
    let mut r: usize;

    // LLVM reserves `esi` for inline-asm management (to make sure stack
    // management is not corrupted). However, it is completely save to use
    // `esi`, and it is not clobbered by the kernel. GCC allows using it for
    // inline-asm input, but unfortunately LLVM does not. Hence, we have to
    // manually swap it out with whatever was picked as alternative for arg3.
    //
    // Note that in most cases LLVM still picks `esi`, so this looks slightly
    // stupid running `xchg esi, esi`. Unfortunately, there is little we can
    // do about it, so we keep it as it is.
    core::arch::asm!(
        "xchg esi, {arg3}",
        "int $0x80",
        "xchg esi, {arg3}",
        arg3 = in(reg) arg3,
        inlateout("eax") nr => r,
        in("ebx") arg0,
        in("ecx") arg1,
        in("edx") arg2,
        options(nostack, preserves_flags)
    );

    r
}

#[cfg(target_arch = "x86")]
#[inline]
#[export_name = "r_linux_asm_syscall5"]
pub unsafe fn syscall5(
    nr: usize,
    arg0: usize,
    arg1: usize,
    arg2: usize,
    arg3: usize,
    arg4: usize,
) -> usize {
    let mut r: usize;

    // see syscall4() for `esi` handling
    core::arch::asm!(
        "xchg esi, {arg3}",
        "int $0x80",
        "xchg esi, {arg3}",
        arg3 = in(reg) arg3,
        inlateout("eax") nr => r,
        in("ebx") arg0,
        in("ecx") arg1,
        in("edx") arg2,
        in("edi") arg4,
        options(nostack, preserves_flags)
    );

    r
}

#[cfg(target_arch = "x86")]
#[inline]
#[export_name = "r_linux_asm_syscall6"]
pub unsafe fn syscall6(
    nr: usize,
    arg0: usize,
    arg1: usize,
    arg2: usize,
    arg3: usize,
    arg4: usize,
    arg5: usize,
) -> usize {
    let mut r: usize;

    // The last argument `arg5` needs to be passed in `ebp`. Again, LLVM does
    // allow us to use it as `in`-register. Hence, we just let LLVM pick a
    // register itself. Since there a none left, it will pick the right one,
    // anyway. But we try to be safe and assume both `arg3` and `arg5` might
    // be in other registers (or actually swapped). Hence, we just push the
    // values to the stack, then save `esi` and `ebp`, then load the values
    // into those registers and jump into the kernel. Afterwards, we restore
    // `esi` and `ebp` again, and restore the registers picked by LLVM.
    //
    // Note that the assembly will likely look stupid, since `arg3` usually
    // ends up being `esi` and `arg5` ends up being `ebp`. Unfortunately,
    // there is little we can do to detect that scenario. However, a 6-argument
    // syscall is likely not noticing the slight slowdown by this.
    core::arch::asm!(
        "push {arg3}",
        "push {arg5}",
        "push esi",
        "push ebp",
        "mov ebp, DWORD PTR [esp + 8]",
        "mov esi, DWORD PTR [esp + 12]",
        "int $0x80",
        "pop ebp",
        "pop esi",
        "pop {arg5}",
        "pop {arg3}",
        arg3 = in(reg) arg3,
        arg5 = in(reg) arg5,
        inlateout("eax") nr => r,
        in("ebx") arg0,
        in("ecx") arg1,
        in("edx") arg2,
        in("edi") arg4,
        options(preserves_flags)
    );

    r
}