Inline assembly warning gets turned into error

[jethrogb]

Warnings generated by LLVM in inline assembly are reported by rustc as errors.

For example:

#![feature(asm, llvm_asm)]
fn main() {
    unsafe{
        asm!("call *(%rax)", options(att_syntax));
        llvm_asm!("call *(%rax)");
    }
}

Compile with today's nightly or newer: rustc +nightly --target x86_64-fortanix-unknown-sgx test.rs. You'll get these errors:

error: Instruction may be vulnerable to LVI and requires manual mitigation
 --> test.rs:4:15
  |
4 |         asm!("call *(%rax)", options(att_syntax));
  |               ^
  |
note: instantiated into assembly here
 --> <inline asm>:1:2
  |
1 |     call *(%rax)
  |     ^

error: See https://software.intel.com/security-software-guidance/insights/deep-dive-load-value-injection#specialinstructions for more information

error: Instruction may be vulnerable to LVI and requires manual mitigation
 --> test.rs:5:9
  |
5 |         llvm_asm!("call *(%rax)");
  |         ^
  |
note: instantiated into assembly here
 --> <inline asm>:1:2
  |
1 |     call *(%rax)
  |     ^

error: aborting due to 3 previous errors

However, when using clang, these are reported as warnings. For example:

asm("call *(%rax)");

Compile with a recent Clang nightly: clang-11 -mlvi-hardening -mllvm -x86-experimental-lvi-inline-asm-hardening -c test.c. You'll get this warning:

<inline asm>:1:1: warning: Instruction may be vulnerable to LVI and requires manual mitigation [-Winline-asm]
call *(%rax)
^
note: See https://software.intel.com/security-software-guidance/insights/deep-dive-load-value-injection#specialinstructions for more information
1 warning generated.

Note this issue has nothing to do with SGX/LVI, it's just a convenient way to trigger the assembler warnings.

[jethrogb]

cc @Amanieu