rustc segfaults while translating inline assembly to write memory location

[Burgos]

#![feature(asm)]

fn test() -> () {
    unsafe {
        let test: i32;
        // using `str r0, [%0]` or any other instruction, it still crashes.
        asm!("mov %0, r0" : "=m"(test) ::);
    }   
}

Results in:

test2.rs:3:1: 8:2 warning: function is never used: `test`, #[warn(dead_code)] on by default
test2.rs:3 fn test() -> () {
test2.rs:4     unsafe {
test2.rs:5         let test: i32;
test2.rs:6         asm!("ldr %0, r0" : "=m"(test) ::);
test2.rs:7     }
test2.rs:8 }
test2.rs:5:13: 5:17 warning: variable `test` is assigned to, but never used, #[warn(unused_variables)] on by default
test2.rs:5         let test: i32;
                       ^~~~
test2.rs:6:34: 6:38 warning: value assigned to `test` is never read, #[warn(unused_assignments)] on by default
test2.rs:6         asm!("ldr %0, r0" : "=m"(test) ::);
                                            ^~~~
test2.rs:6:9: 6:44 note: in this expansion of asm!
Segmentation fault

Backtrace:

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0xb33ffb40 (LWP 8295)]
0xb573ad67 in llvm::SelectionDAGBuilder::visitInlineAsm(llvm::ImmutableCallSite) ()
   from /home/burgos/rust/arm-unknown-linux-gnueabi/lib/librustc_llvm-71b07a99.so
(gdb) bt
#0  0xb573ad67 in llvm::SelectionDAGBuilder::visitInlineAsm(llvm::ImmutableCallSite) ()
   from /home/burgos/rust/arm-unknown-linux-gnueabi/lib/librustc_llvm-71b07a99.so
#1  0xb574e899 in llvm::SelectionDAGBuilder::visitCall(llvm::CallInst const&) ()
   from /home/burgos/rust/arm-unknown-linux-gnueabi/lib/librustc_llvm-71b07a99.so
#2  0xb5726cf4 in llvm::SelectionDAGBuilder::visit(unsigned int, llvm::User const&) ()
   from /home/burgos/rust/arm-unknown-linux-gnueabi/lib/librustc_llvm-71b07a99.so
#3  0xb5750ed5 in llvm::SelectionDAGBuilder::visit(llvm::Instruction const&) ()
   from /home/burgos/rust/arm-unknown-linux-gnueabi/lib/librustc_llvm-71b07a99.so
#4  0xb575dd4d in llvm::SelectionDAGISel::SelectBasicBlock(llvm::ilist_iterator<llvm::Instruction const>, llvm::ilist_iterator<llvm::Instruction const>, bool&) ()
   from /home/burgos/rust/arm-unknown-linux-gnueabi/lib/librustc_llvm-71b07a99.so
#5  0xb576144f in llvm::SelectionDAGISel::SelectAllBasicBlocks(llvm::Function const&) ()
   from /home/burgos/rust/arm-unknown-linux-gnueabi/lib/librustc_llvm-71b07a99.so
#6  0xb57623db in llvm::SelectionDAGISel::runOnMachineFunction(llvm::MachineFunction&) ()
   from /home/burgos/rust/arm-unknown-linux-gnueabi/lib/librustc_llvm-71b07a99.so
#7  0xb54ffae9 in (anonymous namespace)::X86DAGToDAGISel::runOnMachineFunction(llvm::MachineFunction&) ()
   from /home/burgos/rust/arm-unknown-linux-gnueabi/lib/librustc_llvm-71b07a99.so
#8  0xb58ae64e in llvm::MachineFunctionPass::runOnFunction(llvm::Function&) ()
   from /home/burgos/rust/arm-unknown-linux-gnueabi/lib/librustc_llvm-71b07a99.so
#9  0xb5ff6fec in llvm::FPPassManager::runOnFunction(llvm::Function&) ()
   from /home/burgos/rust/arm-unknown-linux-gnueabi/lib/librustc_llvm-71b07a99.so
#10 0xb5ff7378 in llvm::FPPassManager::runOnModule(llvm::Module&) ()
   from /home/burgos/rust/arm-unknown-linux-gnueabi/lib/librustc_llvm-71b07a99.so
#11 0xb5ff7695 in llvm::legacy::PassManagerImpl::run(llvm::Module&) ()
   from /home/burgos/rust/arm-unknown-linux-gnueabi/lib/librustc_llvm-71b07a99.so
#12 0xb5ff78a6 in llvm::legacy::PassManager::run(llvm::Module&) ()
   from /home/burgos/rust/arm-unknown-linux-gnueabi/lib/librustc_llvm-71b07a99.so
#13 0xb4ea06b3 in LLVMRustWriteOutputFile (Target=0xb3d4b370, PMR=0xb3d21870, M=0xb3d0dc88, 
    path=0xb34219c0 "test2.0.o", FileType=llvm::TargetMachine::CGFT_ObjectFile)
    at /home/burgos/arm/rustc-1.6.0/src/rustllvm/PassWrapper.cpp:252
#14 0xb73d1b07 in back::write::write_output_file::h989c25c3cd2a3d4cnPc ()
   from /home/burgos/rust/arm-unknown-linux-gnueabi/lib/librustc_trans-71b07a99.so
#15 0xb73d3f2c in back::write::optimize_and_codegen::_$LT$closure$GT$::closure.49106 ()
   from /home/burgos/rust/arm-unknown-linux-gnueabi/lib/librustc_trans-71b07a99.so
#16 0xb73dcf8d in back::write::execute_work_item::hba4e87c8ecf4087c6Ed ()
   from /home/burgos/rust/arm-unknown-linux-gnueabi/lib/librustc_trans-71b07a99.so
#17 0xb73d5625 in back::write::run_passes::hb688c8c39e303091Wqd ()
   from /home/burgos/rust/arm-unknown-linux-gnueabi/lib/librustc_trans-71b07a99.so
#18 0xb7e4d890 in driver::phase_5_run_llvm_passes::h2c8f91b2f1b966e8aUa ()
   from /home/burgos/rust/arm-unknown-linux-gnueabi/lib/librustc_driver-71b07a99.so
#19 0xb7df9d35 in driver::compile_input::h9c0ab200de97eadbhca ()
   from /home/burgos/rust/arm-unknown-linux-gnueabi/lib/librustc_driver-71b07a99.so
#20 0xb7deb795 in run_compiler::h7e44862e31a6afdaOwc ()
   from /home/burgos/rust/arm-unknown-linux-gnueabi/lib/librustc_driver-71b07a99.so
#21 0xb7de7e7b in sys_common::unwind::try::try_fn::try_fn::h8832516399314236240 ()
   from /home/burgos/rust/arm-unknown-linux-gnueabi/lib/librustc_driver-71b07a99.so
#22 0xb7b36a85 in __rust_try () from /home/burgos/rust/arm-unknown-linux-gnueabi/lib/libstd-71b07a99.so
#23 0xb7b2dd58 in sys_common::unwind::try::inner_try::ha18b63b3bff0cd283Ps ()
   from /home/burgos/rust/arm-unknown-linux-gnueabi/lib/libstd-71b07a99.so
#24 0xb7de8200 in boxed::_$LT$impl$GT$::call_box::call_box::h2869009502916178978 ()
   from /home/burgos/rust/arm-unknown-linux-gnueabi/lib/librustc_driver-71b07a99.so
#25 0xb7b3edca in sys::thread::_$LT$impl$GT$::new::thread_start::h1c9cba6d903a4e18oax ()
   from /home/burgos/rust/arm-unknown-linux-gnueabi/lib/libstd-71b07a99.so
#26 0xb41f9f70 in start_thread () from /lib/i386-linux-gnu/libpthread.so.0
---Type <return> to continue, or q <return> to quit--- 
#27 0xb79cfbee in clone () from /lib/i386-linux-gnu/libc.so.6

rustc:

~/rust/arm-unknown-linux-gnueabi/bin/rustc --version
rustc 1.6.0-dev

[Burgos]

I've changed the title, as I've managed to reduce test case.

[Burgos]

rustc was built with just --target=arm-unknown-linux-gnueabi

[Burgos]

I'm compiling beta 1.7, will report if it is resolved.

[Burgos]

Can confirm that this happens with 1.7.0-dev as well.

[Burgos]

Debug build shows segfault as a result of failed assertion:

rustc: /home/burgos/rustc-beta/src/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp:6012: void llvm::SelectionDAGBuilder::visitInlineAsm(llvm::ImmutableCallSite): Assertion `(OpInfo.isMultipleAlternative || (OpInfo.Type == InlineAsm::isInput)) && "Can only indirectify direct input operands!"' failed.

Which leads to the following issues:

#29382
#16337
#16383
#836

[Burgos]

This one looks promising, I will try it out: effcd29

[MagaTailor]

On my ARM machine there's no problem using a nightly. You were using an i386 host cross-compiler, right?

It duly segfaults under 1.5 though.

[Burgos]

On my ARM machine there's no problem using a nightly. You were using an i386 host cross-compiler, right?

Yeah, I haven't tried with nightly, just with v1.6 and v1.7-beta. I will try the nightly as well, perhaps tomorrow.

You were using an i386 host cross-compiler, right?

Correct.

Thanks!

[Burgos]

This one looks promising, I will try it out: effcd29

Yes, moving m with =* to in set workaround the assertion. Still, the original example should not assert. I will try it on nightly, it the assertion is fixed as @petevine reports, I will close this issue.

[Burgos]

Still segfaults rustc 1.8.0-dev :-(.

[MagaTailor]

Getting interesting! The rustc I was using was built Feb, 4th from a source snapshot.

Other differences between how yours and mine were built (apart from native vs cross) are:
--disable-jemalloc and this ARM specific llvm patch.

[Burgos]

Interesting, thanks! I will try first with the llvm's patch!

[MagaTailor]

No, wait - theses details are completely irrelevant.

I get the segfault too - the reason I was able to compile was using rustc -O transparently.

[pinkisemils]

Doesn't even compile on a nightly for 1.13.

[aidanhs]

Probably a similar issue so I'll report here:

#![feature(asm)]
fn main() {
    unsafe { asm!("" : : "X"(main)) };
}

https://is.gd/wn6Vnv

Dec  8 02:31:56 a-3-linux kernel: [136862.765624] rustc[30254]: segfault at 18 ip 00007f421469f1b9 sp 00007f42105f1870 error 4 in librustc_llvm-1357b93f.so[7f421364c000+2e4d000]

[istankovic]

Still segfaults with rustc 1.26.0-nightly (adf2135ad 2018-03-17).

[oberien]

Still segfaults on rustc 1.28.0-nightly (1d4dbf4 2018-06-11).

Regarding @aidanhs example: If you change "X"(main) to "X"(main as usize), it doesn't segfault.

[dancrossnyc]

Adding another voice to the chorus pointing out that this is still broken. My example code:

#![feature(asm)]

pub fn xswap(word: &mut u64, value: u64) -> u64 {
    let result: u64;
    let wp = word as *mut u64;
    unsafe {
        asm!("lock; xchgq %0, %1"
             : "=r"(result), "=m"(*wp)
             : "0"(value), "m"(*wp)
             :: "volatile");
    }
    result
}

fn main() {
    let mut a = 0u64;
    let b = xswap(&mut a, 1);
    println!("a = {}, b = {}", a, b);
}

This makes it difficult to give a pedagogical examples of writing interrupt-safe locks in a no_std environment (the spin crate, for example, does not disable interrupts when holding a lock). Replacing the body of the function with an intrinsic call works, but is not great as an example for students.

Of course, it could be that my assembler is messed up, in which case I'd welcome a correction. :-)

[Amanieu]

This issue does not apply to the new asm! (RFC 2850) which doesn't support memory operands.

The legacy llvm_asm! is deprecated and is no longer maintained.