Stop including a count in the .wasmtime.epochchecks section. And align better.

The count is unnecessary because ELF sections know how long they are.

Also, align to 4 bytes to make the reading simpler.

Author: erikrose

crates/environ/src/compile/epoch_checks.rs

@@ -61,16 +61,7 @@ impl EpochCheckSection {
             SectionKind::ReadOnlyData,
         );
 
-        // Append length.
-        obj.append_section_data(
-            section,
-            &u32::try_from(self.return_offsets.len())
-                .unwrap()
-                .to_le_bytes(),
-            1,
-        );
-
         // Append offsets.
-        obj.append_section_data(section, object::bytes_of_slice(&self.return_offsets), 1);
+        obj.append_section_data(section, object::bytes_of_slice(&self.return_offsets), 4);
     }
 }

crates/environ/src/obj.rs

@@ -104,17 +104,13 @@ pub const ELF_WASMTIME_TRAPS: &str = ".wasmtime.traps";
 /// The contents are examined at runtime by the signal handler to determine
 /// whether a segfault is due to an epoch ending (vs. a legitimate crash).
 ///
-/// This section has a custom binary encoding:
+/// This section is a sorted array of 32-bit unsigned little-endian integers
+/// which represent offsets from the beginning of the text section to the
+/// instruction following the load which triggers epoch-ending segfaults. TODO:
+/// We may point elsewhere if it's more useful to the signal handler. Be careful
+/// if this could end up pointing off the end of the text section.
 ///
-/// * The section starts with a 32-bit little-endian integer which tell the
-///   number of items in the following array.
-/// * Next comes a sorted array of 32-bit unsigned little-endian integers which
-///   represent offsets from the beginning of the text section to the
-///   instruction following the load which triggers epoch-ending segfaults.
-///   TODO: We may point elsewhere if it's more useful to the signal handler. Be
-///   careful if this could end up pointing off the end of the text section.
-///
-/// The 32-bit encodings herein mean that >=4gb text sections are not supported.
+/// The 32-bit encodings herein mean >=4gb text sections are not supported.
 pub const ELF_WASMTIME_EPOCH_CHECKS: &str = ".wasmtime.epochchecks";
 
 /// A custom binary-encoded section of the wasmtime compilation

tests/all/epoch_mmu.rs

@@ -35,14 +35,7 @@ fn epoch_check_offsets() {
             "{ELF_WASMTIME_EPOCH_CHECKS} section should be present"
         ));
     let data = section.data().unwrap();
-    assert!(data.len() >= 4, "section should at least contain a count");
-    let count = u32::from_le_bytes(data[..4].try_into().unwrap()) as usize;
-    assert_eq!(
-        data.len(),
-        4 + count * 4,
-        "section should be the right size to hold the offsets it claims to contain"
-    );
-    let offsets: Vec<u32> = data[4..]
+    let offsets: Vec<u32> = data
         .chunks_exact(4)
         .map(|c| u32::from_le_bytes(c.try_into().unwrap()))
         .collect();