__rust_probestack needs CFI annotations to assist in backtracing

[da-x]

In a stack overflow situations, under Rust 1.36, gdb fails to backtrace. Also, various other backtracer mechanisms fail to work. The reason is that __rust_probestack meddles with the stack register, and gdb is missing the required information in order to find the stack frame which triggered the overflow. Even with -C debuginfo=1 this still happens, because the relevant assembly code is manually written without the required annotations.

Here how it looks like:

>>> bt
#0  0x0000555555576f73 in __rust_probestack () at /cargo/registry/src/github.com-1ecc6299db9ec823/compiler_builtins-0.1.14/src/probestack.rs:55
Backtrace stopped: Cannot access memory at address 0x7fffff7fee50

Example program test.rs:

fn main() {
    let arr : [u8; 0xf000000] = [0x1; 0xf000000];
    let mut sum : u64 = 0;

    for i in arr.iter() {
        sum += *i as u64;
    }

    println!("{}", sum);
}

Observing the assembly code, it is possible to figure out based on $rdi and $rsp what should be the offset to add to $rsp in order to bring gdb to the correct analysis.

>>> set $rsp = $rsp + 0x7fe000
>>> bt
#0  0x0000555555576f83 in __rust_probestack () at /cargo/registry/src/github.com-1ecc6299db9ec823/compiler_builtins-0.1.14/src/probestack.rs:55
#1  0x000055555555835a in test::main () at a.rs:1

One workaround that can be implemented by various backtracers (the internal, and the backtrace crate) is automatic hand-coded compensation, by figuring out the state of the __rust_probestack function.

Proof-of-concept (assuming ctx captured CPU state):

// `Pseudo`-code

#[cfg(target_arch = "x86_64")]
if symbol_name == "__rust_probestack" {
    let ptr = ctx.rip as *const u8;
    let back = unsafe { std::slice::from_raw_parts(ptr.offset(-6), 6) };
    let mut array = [0u8; 6];
    array.copy_from_slice(&back);

    let matched = {
        if array == [0x81, 0xec, 0x00, 0x10, 0x00, 0x00] {
            let diff = 0x1000 + (ctx.rax - ctx.r11);
            ctx.rsp += diff;
            true
        } else if array == [0x00, 0x77, 0xe4, 0x4c, 0x29, 0xdc] {
            ctx.rsp += ctx.rax;
            true
        } else {
            false
        }
    };

    if matched { // Unwind
        ctx.rip = unsafe { *(ctx.rsp as *const u64) };
    }
}

backtrace_from(&ctx);

While I am against this particular workaround in the general sense, one might find it useful as a stop-gap, at least until compiler-builtins is fixed.

Related issue: rust-lang/rust#51405

[alexcrichton]

This may be fixable by having standard management of the base pointer, but otherwise I'm not sure how we'd do this with LLVM's inline assembly so we could have a raw assembly file compiled in if necessary.

[alexcrichton]

Fixed now!