From 3c8563abcf91eda340aa77d238954dd00fa0cb34 Mon Sep 17 00:00:00 2001 From: Ralf Jung Date: Sat, 6 Aug 2022 17:18:59 -0400 Subject: [PATCH] make NOP dyn casts not require anything about the vtable --- .../rustc_codegen_cranelift/src/unsize.rs | 1 + compiler/rustc_codegen_ssa/src/base.rs | 1 + .../rustc_const_eval/src/interpret/cast.rs | 7 ++++- .../const-eval/ub-wide-ptr.32bit.stderr | 26 +++++++++++++------ .../const-eval/ub-wide-ptr.64bit.stderr | 26 +++++++++++++------ src/test/ui/consts/const-eval/ub-wide-ptr.rs | 6 ++--- 6 files changed, 46 insertions(+), 21 deletions(-) diff --git a/compiler/rustc_codegen_cranelift/src/unsize.rs b/compiler/rustc_codegen_cranelift/src/unsize.rs index 052ca0a082b3c..dd9d891ddbdee 100644 --- a/compiler/rustc_codegen_cranelift/src/unsize.rs +++ b/compiler/rustc_codegen_cranelift/src/unsize.rs @@ -29,6 +29,7 @@ pub(crate) fn unsized_info<'tcx>( let old_info = old_info.expect("unsized_info: missing old info for trait upcasting coercion"); if data_a.principal_def_id() == data_b.principal_def_id() { + // A NOP cast that doesn't actually change anything, should be allowed even with invalid vtables. return old_info; } diff --git a/compiler/rustc_codegen_ssa/src/base.rs b/compiler/rustc_codegen_ssa/src/base.rs index a840b27097492..4c6be3f910827 100644 --- a/compiler/rustc_codegen_ssa/src/base.rs +++ b/compiler/rustc_codegen_ssa/src/base.rs @@ -151,6 +151,7 @@ pub fn unsized_info<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>( let old_info = old_info.expect("unsized_info: missing old info for trait upcasting coercion"); if data_a.principal_def_id() == data_b.principal_def_id() { + // A NOP cast that doesn't actually change anything, should be allowed even with invalid vtables. return old_info; } diff --git a/compiler/rustc_const_eval/src/interpret/cast.rs b/compiler/rustc_const_eval/src/interpret/cast.rs index c97c31eb9dadf..14eb2a1537b1f 100644 --- a/compiler/rustc_const_eval/src/interpret/cast.rs +++ b/compiler/rustc_const_eval/src/interpret/cast.rs @@ -298,7 +298,12 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> { self.write_immediate(val, dest) } (&ty::Dynamic(ref data_a, ..), &ty::Dynamic(ref data_b, ..)) => { - let (old_data, old_vptr) = self.read_immediate(src)?.to_scalar_pair()?; + let val = self.read_immediate(src)?; + if data_a.principal() == data_b.principal() { + // A NOP cast that doesn't actually change anything, should be allowed even with mismatching vtables. + return self.write_immediate(*val, dest); + } + let (old_data, old_vptr) = val.to_scalar_pair()?; let old_vptr = old_vptr.to_pointer(self)?; let (ty, old_trait) = self.get_ptr_vtable(old_vptr)?; if old_trait != data_a.principal() { diff --git a/src/test/ui/consts/const-eval/ub-wide-ptr.32bit.stderr b/src/test/ui/consts/const-eval/ub-wide-ptr.32bit.stderr index 345ead48151df..9431fb33c5329 100644 --- a/src/test/ui/consts/const-eval/ub-wide-ptr.32bit.stderr +++ b/src/test/ui/consts/const-eval/ub-wide-ptr.32bit.stderr @@ -278,26 +278,36 @@ LL | const TRAIT_OBJ_CONTENT_INVALID: &dyn Trait = unsafe { mem::transmute::<_, ╾allocN─╼ ╾allocN─╼ │ ╾──╼╾──╼ } -error[E0080]: evaluation of constant value failed - --> $DIR/ub-wide-ptr.rs:147:62 +error[E0080]: it is undefined behavior to use this value + --> $DIR/ub-wide-ptr.rs:147:1 | LL | const RAW_TRAIT_OBJ_VTABLE_NULL: *const dyn Trait = unsafe { mem::transmute((&92u8, 0usize)) }; - | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ out-of-bounds pointer use: null pointer is a dangling pointer (it has no provenance) + | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value: encountered null pointer, but expected a vtable pointer + | + = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior. + = note: the raw bytes of the constant (size: 8, align: 4) { + ╾allocN─╼ 00 00 00 00 │ ╾──╼.... + } -error[E0080]: evaluation of constant value failed - --> $DIR/ub-wide-ptr.rs:150:65 +error[E0080]: it is undefined behavior to use this value + --> $DIR/ub-wide-ptr.rs:149:1 | LL | const RAW_TRAIT_OBJ_VTABLE_INVALID: *const dyn Trait = unsafe { mem::transmute((&92u8, &3u64)) }; - | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ using allocN as vtable pointer but it does not point to a vtable + | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value: encountered allocN, but expected a vtable pointer + | + = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior. + = note: the raw bytes of the constant (size: 8, align: 4) { + ╾allocN─╼ ╾allocN─╼ │ ╾──╼╾──╼ + } error[E0080]: could not evaluate static initializer - --> $DIR/ub-wide-ptr.rs:157:5 + --> $DIR/ub-wide-ptr.rs:155:5 | LL | mem::transmute::<_, &dyn Trait>((&92u8, 0usize)) | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ out-of-bounds pointer use: null pointer is a dangling pointer (it has no provenance) error[E0080]: could not evaluate static initializer - --> $DIR/ub-wide-ptr.rs:161:5 + --> $DIR/ub-wide-ptr.rs:159:5 | LL | mem::transmute::<_, &dyn Trait>((&92u8, &3u64)) | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ using allocN as vtable pointer but it does not point to a vtable diff --git a/src/test/ui/consts/const-eval/ub-wide-ptr.64bit.stderr b/src/test/ui/consts/const-eval/ub-wide-ptr.64bit.stderr index 501932cb95c63..15ef703024aff 100644 --- a/src/test/ui/consts/const-eval/ub-wide-ptr.64bit.stderr +++ b/src/test/ui/consts/const-eval/ub-wide-ptr.64bit.stderr @@ -278,26 +278,36 @@ LL | const TRAIT_OBJ_CONTENT_INVALID: &dyn Trait = unsafe { mem::transmute::<_, ╾──────allocN───────╼ ╾──────allocN───────╼ │ ╾──────╼╾──────╼ } -error[E0080]: evaluation of constant value failed - --> $DIR/ub-wide-ptr.rs:147:62 +error[E0080]: it is undefined behavior to use this value + --> $DIR/ub-wide-ptr.rs:147:1 | LL | const RAW_TRAIT_OBJ_VTABLE_NULL: *const dyn Trait = unsafe { mem::transmute((&92u8, 0usize)) }; - | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ out-of-bounds pointer use: null pointer is a dangling pointer (it has no provenance) + | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value: encountered null pointer, but expected a vtable pointer + | + = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior. + = note: the raw bytes of the constant (size: 16, align: 8) { + ╾──────allocN───────╼ 00 00 00 00 00 00 00 00 │ ╾──────╼........ + } -error[E0080]: evaluation of constant value failed - --> $DIR/ub-wide-ptr.rs:150:65 +error[E0080]: it is undefined behavior to use this value + --> $DIR/ub-wide-ptr.rs:149:1 | LL | const RAW_TRAIT_OBJ_VTABLE_INVALID: *const dyn Trait = unsafe { mem::transmute((&92u8, &3u64)) }; - | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ using allocN as vtable pointer but it does not point to a vtable + | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value: encountered allocN, but expected a vtable pointer + | + = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior. + = note: the raw bytes of the constant (size: 16, align: 8) { + ╾──────allocN───────╼ ╾──────allocN───────╼ │ ╾──────╼╾──────╼ + } error[E0080]: could not evaluate static initializer - --> $DIR/ub-wide-ptr.rs:157:5 + --> $DIR/ub-wide-ptr.rs:155:5 | LL | mem::transmute::<_, &dyn Trait>((&92u8, 0usize)) | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ out-of-bounds pointer use: null pointer is a dangling pointer (it has no provenance) error[E0080]: could not evaluate static initializer - --> $DIR/ub-wide-ptr.rs:161:5 + --> $DIR/ub-wide-ptr.rs:159:5 | LL | mem::transmute::<_, &dyn Trait>((&92u8, &3u64)) | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ using allocN as vtable pointer but it does not point to a vtable diff --git a/src/test/ui/consts/const-eval/ub-wide-ptr.rs b/src/test/ui/consts/const-eval/ub-wide-ptr.rs index a0377ab1efd23..2da694a8bc43c 100644 --- a/src/test/ui/consts/const-eval/ub-wide-ptr.rs +++ b/src/test/ui/consts/const-eval/ub-wide-ptr.rs @@ -145,11 +145,9 @@ const TRAIT_OBJ_CONTENT_INVALID: &dyn Trait = unsafe { mem::transmute::<_, &bool // # raw trait object const RAW_TRAIT_OBJ_VTABLE_NULL: *const dyn Trait = unsafe { mem::transmute((&92u8, 0usize)) }; -//~^ ERROR evaluation of constant value failed -//~| null pointer +//~^ ERROR it is undefined behavior to use this value const RAW_TRAIT_OBJ_VTABLE_INVALID: *const dyn Trait = unsafe { mem::transmute((&92u8, &3u64)) }; -//~^ ERROR evaluation of constant value failed -//~| does not point to a vtable +//~^ ERROR it is undefined behavior to use this value const RAW_TRAIT_OBJ_CONTENT_INVALID: *const dyn Trait = unsafe { mem::transmute::<_, &bool>(&3u8) } as *const dyn Trait; // ok because raw // Const eval fails for these, so they need to be statics to error.