1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
use rustc_hir::def_id::{LocalDefId, LOCAL_CRATE};
use rustc_middle::mir::*;
use rustc_middle::query::{LocalCrate, Providers};
use rustc_middle::ty::{self, layout, TyCtxt};
use rustc_middle::{bug, span_bug};
use rustc_session::lint::builtin::FFI_UNWIND_CALLS;
use rustc_target::spec::abi::Abi;
use rustc_target::spec::PanicStrategy;
use tracing::debug;

use crate::errors;

// Check if the body of this def_id can possibly leak a foreign unwind into Rust code.
fn has_ffi_unwind_calls(tcx: TyCtxt<'_>, local_def_id: LocalDefId) -> bool {
    debug!("has_ffi_unwind_calls({local_def_id:?})");

    // Only perform check on functions because constants cannot call FFI functions.
    let def_id = local_def_id.to_def_id();
    let kind = tcx.def_kind(def_id);
    if !kind.is_fn_like() {
        return false;
    }

    let body = &*tcx.mir_built(local_def_id).borrow();

    let body_ty = tcx.type_of(def_id).skip_binder();
    let body_abi = match body_ty.kind() {
        ty::FnDef(..) => body_ty.fn_sig(tcx).abi(),
        ty::Closure(..) => Abi::RustCall,
        ty::CoroutineClosure(..) => Abi::RustCall,
        ty::Coroutine(..) => Abi::Rust,
        ty::Error(_) => return false,
        _ => span_bug!(body.span, "unexpected body ty: {:?}", body_ty),
    };
    let body_can_unwind = layout::fn_can_unwind(tcx, Some(def_id), body_abi);

    // Foreign unwinds cannot leak past functions that themselves cannot unwind.
    if !body_can_unwind {
        return false;
    }

    let mut tainted = false;

    for block in body.basic_blocks.iter() {
        if block.is_cleanup {
            continue;
        }
        let Some(terminator) = &block.terminator else { continue };
        let TerminatorKind::Call { func, .. } = &terminator.kind else { continue };

        let ty = func.ty(body, tcx);
        let sig = ty.fn_sig(tcx);

        // Rust calls cannot themselves create foreign unwinds.
        // We assume this is true for intrinsics as well.
        if let Abi::RustIntrinsic | Abi::Rust | Abi::RustCall | Abi::RustCold = sig.abi() {
            continue;
        };

        let fn_def_id = match ty.kind() {
            ty::FnPtr(..) => None,
            &ty::FnDef(def_id, _) => {
                // Rust calls cannot themselves create foreign unwinds (even if they use a non-Rust ABI).
                // So the leak of the foreign unwind into Rust can only be elsewhere, not here.
                if !tcx.is_foreign_item(def_id) {
                    continue;
                }
                Some(def_id)
            }
            _ => bug!("invalid callee of type {:?}", ty),
        };

        if layout::fn_can_unwind(tcx, fn_def_id, sig.abi()) {
            // We have detected a call that can possibly leak foreign unwind.
            //
            // Because the function body itself can unwind, we are not aborting this function call
            // upon unwind, so this call can possibly leak foreign unwind into Rust code if the
            // panic runtime linked is panic-abort.

            let lint_root = body.source_scopes[terminator.source_info.scope]
                .local_data
                .as_ref()
                .assert_crate_local()
                .lint_root;
            let span = terminator.source_info.span;

            let foreign = fn_def_id.is_some();
            tcx.emit_node_span_lint(
                FFI_UNWIND_CALLS,
                lint_root,
                span,
                errors::FfiUnwindCall { span, foreign },
            );

            tainted = true;
        }
    }

    tainted
}

fn required_panic_strategy(tcx: TyCtxt<'_>, _: LocalCrate) -> Option<PanicStrategy> {
    if tcx.is_panic_runtime(LOCAL_CRATE) {
        return Some(tcx.sess.panic_strategy());
    }

    if tcx.sess.panic_strategy() == PanicStrategy::Abort {
        return Some(PanicStrategy::Abort);
    }

    for def_id in tcx.hir().body_owners() {
        if tcx.has_ffi_unwind_calls(def_id) {
            // Given that this crate is compiled in `-C panic=unwind`, the `AbortUnwindingCalls`
            // MIR pass will not be run on FFI-unwind call sites, therefore a foreign exception
            // can enter Rust through these sites.
            //
            // On the other hand, crates compiled with `-C panic=abort` expects that all Rust
            // functions cannot unwind (whether it's caused by Rust panic or foreign exception),
            // and this expectation mismatch can cause unsoundness (#96926).
            //
            // To address this issue, we enforce that if FFI-unwind calls are used in a crate
            // compiled with `panic=unwind`, then the final panic strategy must be `panic=unwind`.
            // This will ensure that no crates will have wrong unwindability assumption.
            //
            // It should be noted that it is okay to link `panic=unwind` into a `panic=abort`
            // program if it contains no FFI-unwind calls. In such case foreign exception can only
            // enter Rust in a `panic=abort` crate, which will lead to an abort. There will also
            // be no exceptions generated from Rust, so the assumption which `panic=abort` crates
            // make, that no Rust function can unwind, indeed holds for crates compiled with
            // `panic=unwind` as well. In such case this function returns `None`, indicating that
            // the crate does not require a particular final panic strategy, and can be freely
            // linked to crates with either strategy (we need such ability for libstd and its
            // dependencies).
            return Some(PanicStrategy::Unwind);
        }
    }

    // This crate can be linked with either runtime.
    None
}

pub(crate) fn provide(providers: &mut Providers) {
    *providers = Providers { has_ffi_unwind_calls, required_panic_strategy, ..*providers };
}