nac3ld: do not allocate image space for NOBITS sections

nac3ld/src/lib.rs

@@ -178,6 +178,7 @@ pub struct Linker<'a> {
     section_map: HashMap<usize, usize>,
     image: Vec<u8>,
     load_offset: u32,
+    image_offset: u32,
     rela_dyn_relas: Vec<Elf32_Rela>,
 }
 
@@ -196,15 +197,26 @@ impl<'a> Linker<'a> {
 
         // Maintain alignment requirement specified in sh_addralign
         let align = shdr.sh_addralign;
-        let padding = (align - (self.load_offset % align)) % align;
-        self.load_offset += padding;
+        let load_padding = (align - (self.load_offset % align)) % align;
+        let image_padding = (align - (self.image_offset % align)) % align;
 
-        elf_shdr.sh_addr =
-            if (shdr.sh_flags as usize & SHF_ALLOC) == SHF_ALLOC { self.load_offset } else { 0 };
-        elf_shdr.sh_offset = self.load_offset;
+        let section_load_offset = if (shdr.sh_flags as usize & SHF_ALLOC) == SHF_ALLOC {
+            self.load_offset + load_padding
+        } else {
+            0
+        };
+        let section_image_offset = self.image_offset + image_padding;
+
+        elf_shdr.sh_addr = section_load_offset;
+        elf_shdr.sh_offset = section_image_offset;
         self.elf_shdrs.push(SectionRecord { shdr: elf_shdr, name: sh_name_str, data });
 
-        self.load_offset += shdr.sh_size;
+        if (shdr.sh_flags as usize & SHF_ALLOC) == SHF_ALLOC {
+            self.load_offset = section_load_offset + shdr.sh_size;
+        }
+        if shdr.sh_type as usize != SHT_NOBITS {
+            self.image_offset = section_image_offset + shdr.sh_size;
+        }
 
         self.elf_shdrs.len() - 1
     }
@@ -754,6 +766,7 @@ impl<'a> Linker<'a> {
             section_map,
             image,
             load_offset: elf_sh_data_off as u32,
+            image_offset: elf_sh_data_off as u32,
             rela_dyn_relas,
         };
 
@@ -1302,8 +1315,7 @@ impl<'a> Linker<'a> {
                 let bss_elf_index = linker.load_section(
                     shdr,
                     section_name,
-                    data[shdr.sh_offset as usize..(shdr.sh_offset + shdr.sh_size) as usize]
-                        .to_vec(),
+                    vec![0; 0], // NOBITS section has no data
                 );
                 linker.section_map.insert(bss_section_index, bss_elf_index);
 
@@ -1401,11 +1413,13 @@ impl<'a> Linker<'a> {
             linker.implement_eh_frame_hdr()?;
         }
 
-        // Load all section data into the image
+        // Load all non-NOBITS section data into the image
         for rec in &linker.elf_shdrs[1..] {
+            if rec.shdr.sh_type as usize != SHT_NOBITS {
                 linker.image.extend(vec![0; (rec.shdr.sh_offset as usize) - linker.image.len()]);
                 linker.image.extend(&rec.data);
             }
+        }
 
         // Load all section headers to the image
         let alignment = (4 - (linker.image.len() % 4)) % 4;