PageAlreadyMapped: Failed to load code

[abrassel]

I have the following program:

#![no_std]
#![no_main]

use core::panic::PanicInfo;

static HELLO: &[u8] = b"Hello World!";

#[panic_handler]
fn panic(_: &PanicInfo) -> ! {
    loop {}
}

#[no_mangle]
pub extern "C" fn _start() -> ! {
    let vga_buffer = 0xb8000 as *mut u8;
    for (i, &byte) in HELLO.iter().enumerate() {
        unsafe {
            *vga_buffer.offset(i as isize * 2) = byte;
            *vga_buffer.offset(i as isize * 2 + 1) = 0xb;
        }
    }

    loop {}
}

I run it with

qemu-system-x86_64 -drive format=raw,file=target
/x86_64-blog-os/debug/bootimage-phil-os.bi

and get the following error:

Failed to map page... PageAlreadyMapped

From looking around, it seems like this error is related to having two "main" functions. But I don't think that that is the case for me.

[bjorn3]

What are the .cargo/config.toml, Cargo.toml and the cargo invocation you used to build it?

[abrassel]

https://github.com/abrassel/phil-os is the exact source.

[bjorn3]

Maybe try with the following target json file?

{
    "llvm-target": "x86_64-unknown-linux-gnu",
    "data-layout": "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128",
    "arch": "x86_64",
    "target-endian": "little",
    "target-pointer-width": "64",
    "target-c-int-width": "32",
    "os": "linux",
    "executables": true,
    "linker-flavor": "gcc",
    "pre-link-args": ["-m64"],
    "morestack": false
}

This matches https://os.phil-opp.com/minimal-rust-kernel/#target-specification

[abrassel]

Interestingly, rustc complains about linker-flavor.

error: failed to run `rustc` to learn about target-specific information

Caused by:
  process didn't exit successfully: `/home/abrassel/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/rustc - --crate-name ___ --print=file-names --target /home/abrassel/Documents/phil-os/x86_64-blog-os.json --crate-type bin --crate-type rlib --crate-type dylib --crate-type cdylib --crate-type staticlib --crate-type proc-macro --print=sysroot --print=split-debuginfo --print=crate-name --print=cfg -Wwarnings` (exit status: 1)
  --- stderr
  error: Error loading target specification: pre-link-args: expected a JSON object with fields per linker-flavor.. Run `rustc --print target-list` for a list of built-in targets

[abrassel]

I did this instead (dropping the -m64 flag) and adding a couple of other required fields:

{
    "llvm-target": "x86_64-unknown-linux-gnu",
    "data-layout": "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128",
    "arch": "x86_64",
    "target-endian": "little",
    "target-pointer-width": "64",
    "target-c-int-width": "32",
    "os": "linux",
    "executables": true,
    "linker-flavor": "gcc",
    "panic-strategy": "abort"
}

now, the complaint is:

error: linking with `cc` failed: exit status: 1
  |
  = note:  "cc" "/tmp/rustclYSL1G/symbols.o" "<5 object files omitted>" "-Wl,--as-needed" "-Wl,-Bstatic" "/home/abrassel/Documents/phil-os/target/x86_64-blog-os/debug/deps/{librustc_std_workspace_core-498ee787797a7283.rlib,libcore-0114943c562e945d.rlib,libcompiler_builtins-055b757525724935.rlib}.rlib" "-Wl,-Bdynamic" "-Wl,--eh-frame-hdr" "-Wl,-z,noexecstack" "-L" "<sysroot>/lib/rustlib/x86_64-blog-os/lib" "-o" "/home/abrassel/Documents/phil-os/target/x86_64-blog-os/debug/deps/phil_os-621cb2ffa6c28bdb" "-Wl,--gc-sections" "-no-pie" "-nodefaultlibs"
  = note: some arguments are omitted. use `--verbose` to show all linker arguments
  = note: /usr/bin/ld: /usr/lib/gcc/x86_64-linux-gnu/11/../../../x86_64-linux-gnu/crt1.o: in function `_start':
          (.text+0x21): undefined reference to `__libc_start_main'
          collect2: error: ld returned 1 exit status

[abrassel]

excellent, I permuted a bit more. I changed the linking flags, and got it working after I did the following:

{
    "llvm-target": "x86_64-unknown-linux-gnu",
    "data-layout": "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128",
    "arch": "x86_64",
    "target-endian": "little",
    "target-pointer-width": "64",
    "target-c-int-width": "32",
    "os": "linux",
    "executables": true,
    "linker": "rust-lld",
    "linker-flavor": "ld.lld",
    "panic-strategy": "abort"
}