Warn against clippy::cast_possible_truncation in Wasmtime

This commit explicitly enables the `clippy::cast_possible_truncation`
lint in Clippy for just the `wasmtime::runtime` module. This does not
enable it for the entire workspace since it's a very noisy lint and in
general has a low signal value. For the domain that `wasmtime::runtime`
is working in, however, this is a much more useful lint. We in general
want to be very careful about casting between `usize`, `u32`, and `u64`
and the purpose of this module-targeted lint is to help with just that.
I was inspired to do this after reading over bytecodealliance#9206 where especially when
refactoring code and changing types I think it would be useful to have
locations flagged as "truncation may happen here" which previously
weren't truncating.

The failure mode for this lint is that panics might be introduced where
truncation is explicitly intended. Most of the time though this isn't
actually desired so the more practical consequence of this lint is to
probably slow down wasmtime ever so slightly and bloat it ever so
slightly by having a few more checks in a few places. This is likely
best addressed in a more comprehensive manner, however, rather than
specifically for just this one case. This problem isn't unique to just
casts, but to many other forms of `.unwrap()` for example.

Author: alexcrichton

crates/wasmtime/src/runtime.rs

@@ -1,3 +1,31 @@
+// Wasmtime's runtime has lots of fiddly bits where we're doing operations like
+// casting between wasm i32/i64 and host `usize` values. There's also in general
+// just lots of pieces of low-level manipulation of memory and internals of VM
+// runtime state. To help keep all the integer casts correct be a bit more
+// strict than the default settings to help weed out bugs ahead of time.
+//
+// This inevitably leads to wordier code than might otherwise be used because,
+// for example, `u64 as usize` is warned against and will be an error on CI.
+// This happens pretty frequently and needs to be replaced with `val.try_into()`
+// or `usize::try_from(val)` where the error is handled. In some cases the
+// correct thing to do is to `.unwrap()` the error to indicate a fatal mistake,
+// but in some cases the correct thing is to propagate the error.
+//
+// Some niche cases that explicitly want truncation are recommended to have a
+// function along the lines of
+//
+//     #[allow(clippy::cast_possible_truncation)]
+//     fn truncate_i32_to_i8(a: i32) -> i8 { a as i8 }
+//
+// as this explicitly indicates the intent of truncation is desired. Other
+// locations should use fallible conversions.
+//
+// If performance is absolutely critical then it's recommended to use `#[allow]`
+// with a comment indicating why performance is critical as well as a short
+// explanation of why truncation shouldn't be happening at runtime. This
+// situation should be pretty rare though.
+#![warn(clippy::cast_possible_truncation)]
+
 #[macro_use]
 pub(crate) mod func;
 

crates/wasmtime/src/runtime/component/func/typed.rs

@@ -837,7 +837,7 @@ macro_rules! integers {
 
         unsafe impl Lift for $primitive {
             #[inline]
-            #[allow(trivial_numeric_casts)]
+            #[allow(trivial_numeric_casts, clippy::cast_possible_truncation)]
             fn lift(_cx: &mut LiftContext<'_>, ty: InterfaceType, src: &Self::Lower) -> Result<Self> {
                 debug_assert!(matches!(ty, InterfaceType::$ty));
                 Ok(src.$get() as $primitive)
@@ -1110,8 +1110,8 @@ unsafe impl Lower for str {
         debug_assert!(offset % (Self::ALIGN32 as usize) == 0);
         let (ptr, len) = lower_string(cx, self)?;
         // FIXME: needs memory64 handling
-        *cx.get(offset + 0) = (ptr as i32).to_le_bytes();
-        *cx.get(offset + 4) = (len as i32).to_le_bytes();
+        *cx.get(offset + 0) = u32::try_from(ptr).unwrap().to_le_bytes();
+        *cx.get(offset + 4) = u32::try_from(len).unwrap().to_le_bytes();
         Ok(())
     }
 }
@@ -1457,8 +1457,8 @@ where
         };
         debug_assert!(offset % (Self::ALIGN32 as usize) == 0);
         let (ptr, len) = lower_list(cx, elem, self)?;
-        *cx.get(offset + 0) = (ptr as i32).to_le_bytes();
-        *cx.get(offset + 4) = (len as i32).to_le_bytes();
+        *cx.get(offset + 0) = u32::try_from(ptr).unwrap().to_le_bytes();
+        *cx.get(offset + 4) = u32::try_from(len).unwrap().to_le_bytes();
         Ok(())
     }
 }

crates/wasmtime/src/runtime/component/resource_table.rs

@@ -161,7 +161,7 @@ impl ResourceTable {
     fn push_(&mut self, e: TableEntry) -> Result<u32, ResourceTableError> {
         if let Some(free) = self.pop_free_list() {
             self.entries[free] = Entry::Occupied { entry: e };
-            Ok(free as u32)
+            Ok(free.try_into().unwrap())
         } else {
             let ix = self
                 .entries

crates/wasmtime/src/runtime/component/values.rs

@@ -187,11 +187,11 @@ impl Val {
                 let u32_count = cx.types.canonical_abi(&ty).flat_count(usize::MAX).unwrap();
                 let ty = &cx.types[i];
                 let mut flags = Vec::new();
-                for i in 0..u32_count {
+                for i in 0..u32::try_from(u32_count).unwrap() {
                     push_flags(
                         ty,
                         &mut flags,
-                        (i as u32) * 32,
+                        i * 32,
                         u32::lift(cx, InterfaceType::U32, next(src))?,
                     );
                 }
@@ -480,8 +480,8 @@ impl Val {
                 let ty = &cx.types[ty];
                 let (ptr, len) = lower_list(cx, ty.element, values)?;
                 // FIXME: needs memory64 handling
-                *cx.get(offset + 0) = (ptr as i32).to_le_bytes();
-                *cx.get(offset + 4) = (len as i32).to_le_bytes();
+                *cx.get(offset + 0) = u32::try_from(ptr).unwrap().to_le_bytes();
+                *cx.get(offset + 4) = u32::try_from(len).unwrap().to_le_bytes();
                 Ok(())
             }
             (InterfaceType::List(_), _) => unexpected(ty, self),
@@ -947,7 +947,7 @@ fn get_enum_discriminant(ty: &TypeEnum, n: &str) -> Result<u32> {
     ty.names
         .get_index_of(n)
         .ok_or_else(|| anyhow::anyhow!("enum variant name `{n}` is not valid"))
-        .map(|i| i as u32)
+        .map(|i| i.try_into().unwrap())
 }
 
 fn get_variant_discriminant<'a>(
@@ -958,7 +958,7 @@ fn get_variant_discriminant<'a>(
         .cases
         .get_full(name)
         .ok_or_else(|| anyhow::anyhow!("unknown variant case: `{name}`"))?;
-    Ok((i as u32, ty))
+    Ok((i.try_into().unwrap(), ty))
 }
 
 fn next<'a>(src: &mut Iter<'a, ValRaw>) -> &'a ValRaw {

crates/wasmtime/src/runtime/coredump.rs

@@ -137,16 +137,18 @@ impl WasmCoreDump {
                 // to balance these conflicting desires, we break the memory up
                 // into reasonably-sized chunks and then trim runs of zeroes
                 // from the start and end of each chunk.
-                const CHUNK_SIZE: u32 = 4096;
-                for (i, chunk) in mem
-                    .data(&store)
-                    .chunks_exact(CHUNK_SIZE as usize)
-                    .enumerate()
-                {
+                const CHUNK_SIZE: usize = 4096;
+                for (i, chunk) in mem.data(&store).chunks_exact(CHUNK_SIZE).enumerate() {
                     if let Some(start) = chunk.iter().position(|byte| *byte != 0) {
                         let end = chunk.iter().rposition(|byte| *byte != 0).unwrap() + 1;
-                        let offset = (i as u32) * CHUNK_SIZE + (start as u32);
-                        let offset = wasm_encoder::ConstExpr::i32_const(offset as i32);
+                        let offset = i * CHUNK_SIZE + start;
+                        let offset = if ty.is_64() {
+                            let offset = u64::try_from(offset).unwrap();
+                            wasm_encoder::ConstExpr::i64_const(offset as i64)
+                        } else {
+                            let offset = u32::try_from(offset).unwrap();
+                            wasm_encoder::ConstExpr::i32_const(offset as i32)
+                        };
                         data.active(memory_idx, &offset, chunk[start..end].iter().copied());
                     }
                 }

crates/wasmtime/src/runtime/debug.rs

@@ -63,8 +63,11 @@ fn relocate_dwarf_sections(bytes: &mut [u8], code_region: (*const u8, usize)) ->
     }
 
     for (offset, value) in relocations {
-        let (loc, _) = object::from_bytes_mut::<U64Bytes<NE>>(&mut bytes[offset as usize..])
-            .map_err(|()| anyhow!("invalid dwarf relocations"))?;
+        let (loc, _) = offset
+            .try_into()
+            .ok()
+            .and_then(|offset| object::from_bytes_mut::<U64Bytes<NE>>(&mut bytes[offset..]).ok())
+            .ok_or_else(|| anyhow!("invalid dwarf relocations"))?;
         loc.set(NE, value);
     }
     Ok(())
@@ -126,7 +129,8 @@ fn convert_object_elf_to_loadable_file<E: Endian>(
     let text_range = match sections.section_by_name(e, b".text") {
         Some((i, text)) => {
             let range = text.file_range(e);
-            let off = header.e_shoff.get(e) as usize + i.0 * header.e_shentsize.get(e) as usize;
+            let e_shoff = usize::try_from(header.e_shoff.get(e)).unwrap();
+            let off = e_shoff + i.0 * header.e_shentsize.get(e) as usize;
 
             let section: &mut SectionHeader64<E> =
                 object::from_bytes_mut(&mut bytes[off..]).unwrap().0;
@@ -160,6 +164,8 @@ fn convert_object_elf_to_loadable_file<E: Endian>(
     let header: &mut FileHeader64<E> = object::from_bytes_mut(bytes).unwrap().0;
     header.e_type.set(e, ET_DYN);
     header.e_phoff.set(e, ph_off as u64);
-    header.e_phentsize.set(e, e_phentsize as u16);
-    header.e_phnum.set(e, e_phnum as u16);
+    header
+        .e_phentsize
+        .set(e, u16::try_from(e_phentsize).unwrap());
+    header.e_phnum.set(e, u16::try_from(e_phnum).unwrap());
 }

crates/wasmtime/src/runtime/instantiate.rs

@@ -274,9 +274,12 @@ impl CompiledModule {
                 .meta
                 .dwarf
                 .binary_search_by_key(&(id as u8), |(id, _)| *id)
-                .map(|i| {
+                .ok()
+                .and_then(|i| {
                     let (_, range) = &self.meta.dwarf[i];
-                    &self.code_memory().dwarf()[range.start as usize..range.end as usize]
+                    let start = range.start.try_into().ok()?;
+                    let end = range.end.try_into().ok()?;
+                    self.code_memory().dwarf().get(start..end)
                 })
                 .unwrap_or(&[]);
             Ok(EndianSlice::new(data, gimli::LittleEndian))

crates/wasmtime/src/runtime/trap.rs

@@ -4,9 +4,7 @@ use crate::prelude::*;
 use crate::store::StoreOpaque;
 use crate::{AsContext, Module};
 use core::fmt;
-use wasmtime_environ::{
-    demangle_function_name, demangle_function_name_or_index, EntityRef, FilePos,
-};
+use wasmtime_environ::{demangle_function_name, demangle_function_name_or_index, FilePos};
 
 /// Representation of a WebAssembly trap and what caused it to occur.
 ///
@@ -451,7 +449,7 @@ impl FrameInfo {
             text_offset,
         );
         let index = compiled_module.module().func_index(index);
-        let func_index = index.index() as u32;
+        let func_index = index.as_u32();
         let func_name = compiled_module.func_name(index).map(|s| s.to_string());
 
         // In debug mode for now assert that we found a mapping for `pc` within

crates/wasmtime/src/runtime/vm/component.rs

@@ -23,6 +23,8 @@ use sptr::Strict;
 use wasmtime_environ::component::*;
 use wasmtime_environ::{HostPtr, PrimaryMap, VMSharedTypeIndex};
 
+#[allow(clippy::cast_possible_truncation)] // it's intended this is truncated on
+                                           // 32-bit platforms
 const INVALID_PTR: usize = 0xdead_dead_beef_beef_u64 as usize;
 
 mod libcalls;

crates/wasmtime/src/runtime/vm/cow.rs

@@ -72,10 +72,13 @@ impl MemoryImage {
         data: &[u8],
         mmap: Option<&MmapVec>,
     ) -> Result<Option<MemoryImage>> {
+        let assert_page_aligned = |val: usize| {
+            assert_eq!(val % (page_size as usize), 0);
+        };
         // Sanity-check that various parameters are page-aligned.
         let len = data.len();
         assert_eq!(offset % u64::from(page_size), 0);
-        assert_eq!((len as u32) % page_size, 0);
+        assert_page_aligned(len);
         let linear_memory_offset = match usize::try_from(offset) {
             Ok(offset) => offset,
             Err(_) => return Ok(None),
@@ -101,10 +104,10 @@ impl MemoryImage {
             let data_start = data.as_ptr() as usize;
             let data_end = data_start + data.len();
             assert!(start <= data_start && data_end <= end);
-            assert_eq!((start as u32) % page_size, 0);
-            assert_eq!((data_start as u32) % page_size, 0);
-            assert_eq!((data_end as u32) % page_size, 0);
-            assert_eq!((mmap.original_offset() as u32) % page_size, 0);
+            assert_page_aligned(start);
+            assert_page_aligned(data_start);
+            assert_page_aligned(data_end);
+            assert_page_aligned(mmap.original_offset());
 
             #[cfg(feature = "std")]
             if let Some(file) = mmap.original_file() {
@@ -167,7 +170,8 @@ impl ModuleMemoryImages {
             _ => return Ok(None),
         };
         let mut memories = PrimaryMap::with_capacity(map.len());
-        let page_size = crate::runtime::vm::host_page_size() as u32;
+        let page_size = crate::runtime::vm::host_page_size();
+        let page_size = u32::try_from(page_size).unwrap();
         for (memory_index, init) in map {
             // mmap-based-initialization only works for defined memories with a
             // known starting point of all zeros, so bail out if the mmeory is