example: add lazy parse example

examples/lazy_parse.rs

@@ -0,0 +1,87 @@
+/// Demonstrates parsing elf file in a lazy manner by reading only the needed parts.
+/// Lets's say we just want to know the interpreter for an elf file.
+/// Steps:
+///     1. cd tests/bins/elf/gnu_hash/ && gcc -o hello helloworld.c
+///     2. cargo run --example=lazy_parse
+use goblin::container::{Container, Ctx};
+use goblin::elf::*;
+use std::ffi::CStr;
+use std::fs::File;
+use std::io::{Read, Seek, SeekFrom};
+
+const ELF64_HDR_SIZE: usize = 64;
+
+fn main() -> Result<(), &'static str> {
+    let mut file = File::open("tests/bins/elf/gnu_hash/hello").map_err(|_| "open file error")?;
+    let file_len = file.metadata().map_err(|_| "get metadata error")?.len();
+
+    // init the content vec
+    let mut contents = vec![0; file_len as usize];
+
+    // read in header only
+    file.read_exact(&mut contents[..ELF64_HDR_SIZE])
+        .map_err(|_| "read header error")?;
+
+    // parse header
+    let header = Elf::parse_header(&contents).map_err(|_| "parse elf header error")?;
+    if header.e_phnum == 0 {
+        return Err("ELF doesn't have any program segments");
+    }
+
+    // read in program header table
+    let program_hdr_table_size = header.e_phnum * header.e_phentsize;
+    file.seek(SeekFrom::Start(header.e_phoff))
+        .map_err(|_| "seek error")?;
+    file.read_exact(
+        &mut contents[ELF64_HDR_SIZE..ELF64_HDR_SIZE + (program_hdr_table_size as usize)],
+    )
+    .map_err(|_| "read program header table error")?;
+
+    // dummy Elf with only header
+    let mut elf = Elf::lazy_parse(header).map_err(|_| "cannot parse ELF file")?;
+
+    let ctx = Ctx {
+        le: scroll::Endian::Little,
+        container: Container::Big,
+    };
+
+    // parse and assemble the program headers
+    elf.program_headers = ProgramHeader::parse(
+        &contents,
+        header.e_phoff as usize,
+        header.e_phnum as usize,
+        ctx,
+    )
+    .map_err(|_| "parse program headers error")?;
+
+    let mut intepreter_count = 0;
+    let mut intepreter_offset = 0;
+    for ph in &elf.program_headers {
+        // read in interpreter segment
+        if ph.p_type == program_header::PT_INTERP && ph.p_filesz != 0 {
+            intepreter_count = ph.p_filesz as usize;
+            intepreter_offset = ph.p_offset as usize;
+            file.seek(SeekFrom::Start(intepreter_offset as u64))
+                .map_err(|_| "seek error")?;
+            file.read_exact(&mut contents[intepreter_offset..intepreter_offset + intepreter_count])
+                .map_err(|_| "read interpreter segment error")?;
+            break;
+        }
+    }
+
+    // assemble the interpreter
+    elf.interpreter = if intepreter_count == 0 {
+        None
+    } else {
+        let cstr: &CStr = CStr::from_bytes_with_nul(
+            &contents[intepreter_offset..intepreter_offset + intepreter_count],
+        )
+        .map_err(|_| "invalid interpreter path")?;
+        cstr.to_str().ok()
+    };
+
+    // print result
+    println!("interpreter is {:?}", elf.interpreter);
+
+    Ok(())
+}