Segfault in a compiler-generated Drop impl

[emberian]

I've tried reducing this code in various ways, and unfortunately the original seems to already be the smallest that fails! Removing variants from Term causes it to not exhibit itself, as does removing cases from the outer match in small_eval. It reproduces on latest nightly only if debuginfo=2, but on older nightlies (@Aatch reports a repro on 2015-09-22) or 1.3.0 it repros for debuginfo=1. On all versions, doing optimizations causes the issue to disappear.

use self::Term::*;

#[derive(Clone)]
pub enum Term {
    True,
    False,
    If(Box<Term>, Box<Term>, Box<Term>),
    Succ(Box<Term>),
    Pred(Box<Term>),
    IsZero(Box<Term>),
}

fn isnumeric(v: &Term) -> bool {
    match *v {
        Pred(ref f) | Succ(ref f) => isnumeric(f),
        _ => false
    }
}

// a small-step evaluator
pub fn small_eval(v: &Term) -> Term {
    match *v {
        If(ref con, ref the, ref els) => match **con {
            True => *the.clone(),
            False => *els.clone(),
            _ => If(Box::new(small_eval(con)), the.clone(), els.clone()),
        },
        Succ(ref s) => Succ(Box::new(small_eval(s))),
        Pred(ref s) => match **s {
            Succ(ref v) if isnumeric(v) => {
                *v.clone()
            },
            _ => Pred(Box::new(small_eval(s))),
        },
        IsZero(ref s) => match **s {
            Succ(ref s) if isnumeric(s) => False,
            _ => IsZero(Box::new(small_eval(s))),
        },
        _ => v.clone()
    }
}

fn main() {
    let t = If(Box::new(True), Box::new(True), Box::new(True));
    small_eval(&t);
}

I suspect that an alloca (forced by debuginfo) is getting trampled somewhere.

GDB backtrace (from rustc 1.5.0-nightly (eafe106 2015-10-15)):

(gdb) run
Starting program: /tmp/foo 
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/usr/lib/libthread_db.so.1".

Program received signal SIGSEGV, Segmentation fault.
0x0000555555558b14 in Term::drop.3510::ha59bcdf39bbe7497 ()
(gdb) where
#0  0x0000555555558b14 in Term::drop.3510::ha59bcdf39bbe7497 ()
#1  0x0000555555558af2 in Box$LT$Term$GT$::drop.3508::h48a2557aadc5bcd2 ()
#2  0x00005555555592c1 in foo::small_eval (v=0x7fffffffdb08) at foo.rs:25
#3  0x0000555555559a06 in foo::main () at foo.rs:49
#4  0x000055555555f525 in sys_common::unwind::try::try_fn::h16668888951888304395 ()
#5  0x000055555555d1b9 in __rust_try ()
#6  0x000055555555f1c0 in rt::lang_start::ha93496369c7c61cfF5w ()
#7  0x0000555555559b1a in main ()

[emberian]

Doing If(Box::new(False), ...) causes it to not segfault, but other cases do. So I believe that it is the that gets clobbered.

[dotdash]

Reproduces for me only without any debuginfo. Both level 1 and 2 make the problem disappear, as do optimizations.

[dotdash]

use self::Term::*;

#[derive(Clone)]
pub enum Term {
    True,
    False,
    If(Box<Term>),
}

// a small-step evaluator
pub fn small_eval(v: Term) -> Term {
    match v {
        If(ref con) => match **con {
            True => True,
            False => *con.clone(),
            _ => True,
        },
        _ => True,
    }
}

fn main() {
    small_eval(If(Box::new(True)));
}

still segfaults for me.

[dotdash]

Minimized even further:

use self::Term::*;

#[derive(Clone)]
pub enum Term {
    True,
    False,
    If(Box<Term>),
}

// a small-step evaluator
pub fn small_eval(v: Term) -> Term {
    match v {
        If(ref con) => match **con {
            True => True,
            _ => *con.clone(),
        },
        _ => True,
    }
}

fn main() {
    small_eval(If(Box::new(True)));
}

The crash happens because the drop for the Box<Term> returned by the clone call is executed even though that match arm isn't taken.

[dotdash]

Adding braces around *con.clone() makes the error disappear, and moves the drop to the correct block. Maybe a missing AST scope?

cc @nikomatsakis

[emberian]

@dotdash The smallest example you posted repros in the same manner (that is, only no-op without debuginfo).

[dotdash]

Eliminated the need for the nested match.

use self::Term::*;

#[derive(Clone)]
pub enum Term {
    Dummy,
    A(Box<Term>),
    B(Box<Term>),
}

// a small-step evaluator
pub fn small_eval(v: Term) -> Term {
    match v {
        A(t) => *t.clone(),
        B(t) => *t.clone(),
        _ => Dummy,
    }
}

fn main() {
    small_eval(Dummy);
}

@nikomatsakis You said that you think that we might be missing a drop initializer, but I don't think that's the case. It's not the match binding that's being dropped, but the expression in the match arm. Initializing that to a dropped value wouldn't do anything because the initialization would also happen in the match arm, which is never executed.

[pnkfelix]

(revised title based on @dotdash observations)

[dotdash]

I cannot reproduce this anymore, and in any case, I suspect it would be fixed by #30823