write/elf: fix writing of strtab when symtab is empty

Previously, if the symtab was empty then we were writing a strtab
that had 0 bytes of data. This gives a linker error:
SHT_STRTAB string table section [index 4] is empty

Also, if there is a symtab then there must be a strtab,
otherwise the error is:
invalid sh_type for string table section [index 0]: expected SHT_STRTAB, but got SHT_NULL

src/write/elf/writer.rs

@@ -658,7 +658,7 @@ impl<'a> Writer<'a> {
     ///
     /// This range is used for a section named `.strtab`.
     ///
-    /// This function does nothing if no strings were defined.
+    /// This function does nothing if a string table is not required.
     /// This must be called after [`Self::add_string`].
     pub fn reserve_strtab(&mut self) {
         debug_assert_eq!(self.strtab_offset, 0);
@@ -684,13 +684,19 @@ impl<'a> Writer<'a> {
 
     /// Reserve the section index for the string table.
     ///
+    /// You should check [`Self::strtab_needed`] before calling this
+    /// unless you have other means of knowing if this section is needed.
+    ///
     /// This must be called before [`Self::reserve_section_headers`].
     pub fn reserve_strtab_section_index(&mut self) -> SectionIndex {
         self.reserve_strtab_section_index_with_name(&b".strtab"[..])
     }
 
     /// Reserve the section index for the string table.
     ///
+    /// You should check [`Self::strtab_needed`] before calling this
+    /// unless you have other means of knowing if this section is needed.
+    ///
     /// This must be called before [`Self::reserve_section_headers`].
     pub fn reserve_strtab_section_index_with_name(&mut self, name: &'a [u8]) -> SectionIndex {
         debug_assert_eq!(self.strtab_index, SectionIndex(0));
@@ -732,6 +738,8 @@ impl<'a> Writer<'a> {
         debug_assert_eq!(self.symtab_offset, 0);
         debug_assert_eq!(self.symtab_num, 0);
         self.symtab_num = 1;
+        // The symtab must link to a strtab.
+        self.need_strtab = true;
         SymbolIndex(0)
     }
 
@@ -752,6 +760,8 @@ impl<'a> Writer<'a> {
         debug_assert_eq!(self.symtab_shndx_offset, 0);
         if self.symtab_num == 0 {
             self.symtab_num = 1;
+            // The symtab must link to a strtab.
+            self.need_strtab = true;
         }
         let index = self.symtab_num;
         self.symtab_num += 1;
@@ -1052,13 +1062,19 @@ impl<'a> Writer<'a> {
 
     /// Reserve the section index for the dynamic string table.
     ///
+    /// You should check [`Self::dynstr_needed`] before calling this
+    /// unless you have other means of knowing if this section is needed.
+    ///
     /// This must be called before [`Self::reserve_section_headers`].
     pub fn reserve_dynstr_section_index(&mut self) -> SectionIndex {
         self.reserve_dynstr_section_index_with_name(&b".dynstr"[..])
     }
 
     /// Reserve the section index for the dynamic string table.
     ///
+    /// You should check [`Self::dynstr_needed`] before calling this
+    /// unless you have other means of knowing if this section is needed.
+    ///
     /// This must be called before [`Self::reserve_section_headers`].
     pub fn reserve_dynstr_section_index_with_name(&mut self, name: &'a [u8]) -> SectionIndex {
         debug_assert_eq!(self.dynstr_index, SectionIndex(0));

tests/round_trip/elf.rs

@@ -42,6 +42,20 @@ fn symtab_shndx() {
     }
 }
 
+#[test]
+fn empty_symtab() {
+    let object = write::Object::new(BinaryFormat::Elf, Architecture::X86_64, Endianness::Little);
+    let bytes = object.write().unwrap();
+
+    let object = read::File::parse(&*bytes).unwrap();
+    assert_eq!(object.format(), BinaryFormat::Elf);
+    assert_eq!(object.architecture(), Architecture::X86_64);
+    let symtab = object.section_by_name(".symtab").unwrap();
+    assert_eq!(symtab.size(), 24);
+    let strtab = object.section_by_name(".strtab").unwrap();
+    assert_eq!(strtab.size(), 1);
+}
+
 #[test]
 fn aligned_sections() {
     let mut object =