Possible bug in rustc with `x86_64-unknown-none` target

Ethindp · January 1, 2022, 6:45pm

So I don't know if this is a bug with rust codegen or if this is a bug in my code. I recently upgraded to rustc 1.59.0-nightly (b60e32c82 2021-12-30) and switched my kernel to use the x86_64-uknown-none target. I also upgraded some dependencies (I don't think I upgraded log though...). But when it boots now, when it tries to set the logger via log::set_logger or log::set_logger_racy, it immediately triple-faults. Setting the logger is the very first thing the kernel does (its first function call) after taking control of the system and entering kmain, so I'm extremely confused on why this is happening. The disassembly of the code is equally as strange:

0000000000019b10 <kmain>:
   19b10:	55                   	push   %rbp
   19b11:	41 57                	push   %r15
   19b13:	41 56                	push   %r14
   19b15:	41 55                	push   %r13
   19b17:	41 54                	push   %r12
   19b19:	53                   	push   %rbx
   19b1a:	48 81 ec 88 00 00 00 	sub    $0x88,%rsp
   19b21:	48 89 7c 24 40       	mov    %rdi,0x40(%rsp)
   19b26:	fa                   	cli    
   19b27:	4c 8d 35 12 50 ff ff 	lea    -0xafee(%rip),%r14        # eb40 <anon.c31ad24ece4221bbd8799acf4a463560.11.llvm.16454607279314885932+0x77>
   19b2e:	48 8d 35 b3 9f 05 00 	lea    0x59fb3(%rip),%rsi        # 73ae8 <anon.c31ad24ece4221bbd8799acf4a463560.12.llvm.16454607279314885932+0x78>
   19b35:	4c 89 f7             	mov    %r14,%rdi
   19b38:	ff 15 7a 3f 06 00    	call   *0x63f7a(%rip)        # 7dab8 <_DYNAMIC+0x170> // triple fault occurs here

Is this a bug in Rustc or something else?

bjorn3 · January 1, 2022, 11:26pm

Looks like the target spec uses the pic instead of pie relocation model. In that case the GOT needs to be initialized before any code that may attempt to use it runs. This should probably changed to use the pie reloc model.

github.com

rust-lang/rust/blob/c145692254e86974941f2c92c643a23df0f13e82/compiler/rustc_target/src/spec/x86_64_unknown_none.rs#L21

    
      
          
          
pub fn target() -> Target {
              let opts = TargetOptions {
                  cpu: "x86-64".to_string(),
                  max_atomic_width: Some(64),
                  // don't use probe-stack=inline-asm until rust#83139 and rust#84667 are resolved
                  stack_probes: StackProbeType::Call,
                  position_independent_executables: true,
                  static_position_independent_executables: true,
                  relro_level: RelroLevel::Full,
                  relocation_model: RelocModel::Pic,
                  linker_flavor: LinkerFlavor::Lld(LldFlavor::Ld),
                  linker: Some("rust-lld".to_owned()),
                  features:
                      "-mmx,-sse,-sse2,-sse3,-ssse3,-sse4.1,-sse4.2,-3dnow,-3dnowa,-avx,-avx2,+soft-float"
                          .to_string(),
                  executables: true,
                  disable_redzone: true,
                  panic_strategy: PanicStrategy::Abort,
                  code_model: Some(CodeModel::Kernel),
                  ..Default::default()

Ethindp · January 2, 2022, 1:40am

Oh... Whoops? I'll force the change until PIC is added in the bootloader since I don't know how to initialize the GOT myself. Can I even do that?

Ethindp · January 2, 2022, 2:06am

Also, how do you override the relocation model? I tried to set rustflags in .cargo/config.toml but it doesn't appear like its being respected. I don't want to make my own custom target definition -- I just want to override this one setting.