tool: emit empty strtab when serialising ELF

tool/microkit/src/elf.rs

@@ -108,6 +108,7 @@ const PF_R: u32 = 0x4;
 
 /// ELF section-header type (`sh_type`)
 const SHT_PROGBITS: u32 = 0x1;
+const SHT_STRTAB: u32 = 0x3;
 
 /// ELF section-header flags (`sh_flags`)
 const SHF_WRITE: u64 = 0x1;
@@ -494,14 +495,15 @@ impl ElfFile {
         self.segments.iter().filter(|s| s.loadable).collect()
     }
 
-    /// Re-create a minimal ELF file with all the segments.
+    /// Re-create a minimal ELF file with all the program and section headers.
     pub fn reserialise(&self, out: &std::path::Path) -> Result<u64, String> {
         let ehsize = size_of::<ElfHeader64>();
 
         let phnum = self.loadable_segments().len();
         let phentsize = size_of::<ElfProgramHeader64>();
 
-        let shnum = self.loadable_segments().len() + 1; // First entry is reserved
+        // First entry is reserved, last entry is dummy strtab
+        let shnum = self.loadable_segments().len() + 2;
         let shentsize = size_of::<ElfSectionHeader64>();
 
         let mut elf_file = match File::create(out) {
@@ -539,7 +541,7 @@ impl ElfFile {
             phnum: phnum as u16,
             shentsize: shentsize as u16,
             shnum: shnum as u16,
-            shstrndx: 0,
+            shstrndx: (shnum - 1) as u16,
         };
         elf_file
             .write_all(unsafe {
@@ -617,6 +619,33 @@ impl ElfFile {
                     )
                 });
         }
+        let strtab_seg = ElfSectionHeader64 {
+            name: 0,
+            type_: SHT_STRTAB,
+            flags: 0,
+            addr: 0,
+            offset: data_off_watermark, // points to a null byte that we will write later after all the data
+            size: 1,
+            link: 0,
+            info: 0,
+            addralign: 0,
+            entsize: 0,
+        };
+
+        // Uncomment this if more data needs to be written.
+        // data_off_watermark += 1;
+        // Have to comment out to keep clippy happy, else:
+        // "warning: value assigned to `data_off_watermark` is never read", or if we try to allow with `#[allow(unused_assignments)]`:
+        // "error[E0658]: attributes on expressions are experimental"
+
+        elf_file
+            .write_all(unsafe { struct_to_bytes(&strtab_seg) })
+            .unwrap_or_else(|_| {
+                panic!(
+                    "Failed to write ELF string table section header for '{}'",
+                    out.display()
+                )
+            });
 
         // Finally the data for each segment will follow
         for (i, seg) in self
@@ -634,6 +663,16 @@ impl ElfFile {
             });
         }
 
+        // Then the null byte for string table
+        elf_file
+            .write_all(vec![0u8; 1].as_slice())
+            .unwrap_or_else(|_| {
+                panic!(
+                    "Failed to write ELF string table byte for '{}'",
+                    out.display()
+                )
+            });
+
         elf_file.flush().unwrap();
 
         Ok(metadata(out).unwrap().len())