elf: Handle SHN_XINDEX for strtab section index (#304)

This happens when the number of sections in an ELF object reaches 2^16,
at which point the u16 shstrndx takes the special value SHN_XINDEX.

Fix based on algorithm from:
github.com/gimli-rs/object/blob/c476071/src/read/elf/file.rs#L582-L600

src/elf/mod.rs

@@ -277,7 +277,14 @@ if_sylvan! {
 
             let section_headers = SectionHeader::parse(bytes, header.e_shoff as usize, header.e_shnum as usize, ctx)?;
 
-            let get_strtab = |section_headers: &[SectionHeader], section_idx: usize| {
+            let get_strtab = |section_headers: &[SectionHeader], mut section_idx: usize| {
+                if section_idx == section_header::SHN_XINDEX as usize {
+                    if section_headers.is_empty() {
+                        return Ok(Strtab::default())
+                    }
+                    section_idx = section_headers[0].sh_link as usize;
+                }
+
                 if section_idx >= section_headers.len() {
                     // FIXME: warn! here
                     Ok(Strtab::default())

tests/elf.rs

@@ -1,4 +1,4 @@
-use goblin::elf::section_header::SHT_GNU_HASH;
+use goblin::elf::section_header::{SHN_XINDEX, SHT_GNU_HASH};
 use goblin::elf::sym::{Sym, Symtab};
 use goblin::elf::symver::{VerdefSection, VerneedSection, VersymSection};
 use goblin::elf::Elf;
@@ -51,7 +51,12 @@ fn parse_text_section_size_lazy(base: &[u8]) -> Result<u64, &'static str> {
         SectionHeader::parse(base, header.e_shoff as usize, header.e_shnum as usize, ctx)
             .map_err(|_| "parse section headers error")?;
 
-    let strtab_idx = header.e_shstrndx as usize;
+    let strtab_idx = if header.e_shstrndx as u32 == SHN_XINDEX {
+        obj.section_headers[0].sh_link as usize
+    } else {
+        header.e_shstrndx as usize
+    };
+
     let strtab_shdr = &obj.section_headers[strtab_idx];
     let strtab = Strtab::parse(
         base,