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//! Minimal, dependency-free ELF inspection.
//!
//! Pure file reading only (no subprocess), so it is module-boundary-legal to
//! live in core. The single probe here detects whether a binary is
//! dynamically linked by parsing just enough of the ELF header and program
//! headers to find a `PT_INTERP` segment.
use std::path::Path;
/// Check whether the binary at `path` is dynamically linked by reading its ELF
/// program headers and looking for a `PT_INTERP` segment (type 3, the dynamic
/// linker).
///
/// Returns `Ok(false)` for a genuinely-absent path, a non-ELF file (macOS
/// Mach-O, Windows PE, or random bytes), a file too short to contain the header
/// fields for its own ELF class, or a statically linked ELF; `Ok(true)` when a
/// `PT_INTERP` segment is present.
///
/// Returns `Err` when a file that *does* exist cannot be read — an open failure
/// other than not-found, or a short/failed read of a confirmed ELF. A binary a
/// caller cannot inspect is a defect that must surface, never silently
/// "statically linked": swallowing it would, for example, drop
/// `autoPatchelfHook` from a Nix derivation and ship a broken install.
pub fn is_dynamically_linked(path: &Path) -> std::io::Result<bool> {
use std::io::Read;
let mut file = match std::fs::File::open(path) {
Ok(f) => f,
// A genuinely-absent path is not our concern (callers guard on
// `.exists()` / only feed registered artifacts); any OTHER open
// failure on a file we were asked to inspect is a real error, not
// "statically linked".
Err(e) if e.kind() == std::io::ErrorKind::NotFound => return Ok(false),
Err(e) => return Err(e),
};
// Read up to a full 64-bit ELF header. `read` may return fewer bytes than
// requested; the buffer stays zero-padded beyond what was read, so the
// magic check below rejects anything that could not supply the first 4
// bytes. A read *error* (not a short read) is a defect and propagates.
let mut buf = [0u8; 64];
let n = file.read(&mut buf)?;
// Verify ELF magic: 0x7f 'E' 'L' 'F'. A read shorter than 4 bytes leaves
// zero-padding here and cannot match.
if &buf[0..4] != b"\x7fELF" {
return Ok(false);
}
let is_64bit = buf[4] == 2;
let is_le = buf[5] == 1; // 1 = little-endian, 2 = big-endian
// The header fields we parse differ by ELF class: a 32-bit header is read
// up to byte 46 (`e_phnum`), a 64-bit header up to byte 58. A file too
// short to hold the fields for its own class is not an ELF we can inspect;
// treat it as "not dynamically linked" rather than misreading zero-padding
// as real header data. (A valid 32-bit header is 52 bytes, a 64-bit header
// 64 — real binaries always satisfy this; only truncated inputs fail it.)
let min_len = if is_64bit { 58 } else { 46 };
if n < min_len {
return Ok(false);
}
let read_u16 = |b: &[u8]| -> u16 {
if is_le {
u16::from_le_bytes([b[0], b[1]])
} else {
u16::from_be_bytes([b[0], b[1]])
}
};
let read_u32 = |b: &[u8]| -> u32 {
if is_le {
u32::from_le_bytes([b[0], b[1], b[2], b[3]])
} else {
u32::from_be_bytes([b[0], b[1], b[2], b[3]])
}
};
let read_u64 = |b: &[u8]| -> u64 {
if is_le {
u64::from_le_bytes([b[0], b[1], b[2], b[3], b[4], b[5], b[6], b[7]])
} else {
u64::from_be_bytes([b[0], b[1], b[2], b[3], b[4], b[5], b[6], b[7]])
}
};
// Parse program header offset, entry size, and count.
let (ph_offset, ph_entry_size, ph_count) = if is_64bit {
// 64-bit ELF: e_phoff at 32 (8 bytes), e_phentsize at 54, e_phnum at 56.
let offset = read_u64(&buf[32..40]);
let entry_size = read_u16(&buf[54..56]);
let count = read_u16(&buf[56..58]);
(offset, entry_size as u64, count)
} else {
// 32-bit ELF: e_phoff at 28 (4 bytes), e_phentsize at 42, e_phnum at 44.
let offset = read_u32(&buf[28..32]) as u64;
let entry_size = read_u16(&buf[42..44]);
let count = read_u16(&buf[44..46]);
(offset, entry_size as u64, count)
};
if ph_count == 0 || ph_entry_size == 0 {
return Ok(false);
}
// Read all program headers. A confirmed-ELF header pointing at program
// headers we cannot seek/read is a corrupt or truncated artifact — a defect
// that propagates rather than masquerading as "statically linked".
let total_size = ph_entry_size * ph_count as u64;
let mut ph_buf = vec![0u8; total_size as usize];
use std::io::Seek;
file.seek(std::io::SeekFrom::Start(ph_offset))?;
file.read_exact(&mut ph_buf)?;
// Scan for PT_INTERP (type 3), the presence of a dynamic linker.
const PT_INTERP: u32 = 3;
for i in 0..ph_count as usize {
let entry_start = i * ph_entry_size as usize;
let p_type = read_u32(&ph_buf[entry_start..entry_start + 4]);
if p_type == PT_INTERP {
return Ok(true);
}
}
Ok(false)
}
#[cfg(test)]
mod tests {
use super::is_dynamically_linked;
/// A genuinely-absent path is `Ok(false)` (open() NotFound), never an
/// error: callers guard on existence or only feed registered artifacts.
#[test]
fn missing_file_returns_false() {
assert!(
!is_dynamically_linked(std::path::Path::new(
"/nonexistent/path/to/binary/that/cannot/exist"
))
.expect("a missing file is Ok(false), not an error")
);
}
/// A file too short to hold an ELF header returns false (magic fails).
#[test]
fn short_file_returns_false() {
let tmp = tempfile::tempdir().unwrap();
let p = tmp.path().join("tiny");
std::fs::write(&p, b"abc").unwrap();
assert!(!is_dynamically_linked(&p).unwrap());
}
/// A path that EXISTS but errors on read is a real defect, not `Ok(false)`:
/// a directory opens as a File on Unix but errors (EISDIR) on read. A
/// silent `false` here would mask a build artifact we merely failed to
/// inspect and, e.g., ship a broken `nix` install.
#[test]
#[cfg(unix)]
fn unreadable_path_is_error() {
let tmp = tempfile::tempdir().unwrap();
assert!(is_dynamically_linked(tmp.path()).is_err());
}
/// A file without ELF magic (Mach-O / PE / random bytes) returns false.
#[test]
fn non_elf_returns_false() {
let tmp = tempfile::tempdir().unwrap();
let p = tmp.path().join("not-elf");
// 64 bytes of nonzero non-ELF data.
let bytes: Vec<u8> = (0..64u8).collect();
std::fs::write(&p, bytes).unwrap();
assert!(!is_dynamically_linked(&p).unwrap());
}
/// Hand-rolled minimal 64-bit ELF with a single PT_INTERP program header
/// returns true.
#[test]
fn elf64_with_pt_interp_returns_true() {
let tmp = tempfile::tempdir().unwrap();
let p = tmp.path().join("elf64-dyn");
// 64-byte ELF header followed by one 56-byte program header, p_type=3.
let phoff: u64 = 64;
let phentsize: u16 = 56;
let phnum: u16 = 1;
let mut bytes = Vec::with_capacity(64 + phentsize as usize);
bytes.extend_from_slice(b"\x7fELF"); // magic
bytes.push(2); // 64-bit
bytes.push(1); // little-endian
bytes.push(1); // EI_VERSION
bytes.extend_from_slice(&[0u8; 9]); // OSABI + padding
bytes.extend_from_slice(&[0u8; 2]); // e_type
bytes.extend_from_slice(&[0u8; 2]); // e_machine
bytes.extend_from_slice(&[0u8; 4]); // e_version
bytes.extend_from_slice(&[0u8; 8]); // e_entry
bytes.extend_from_slice(&phoff.to_le_bytes()); // e_phoff (32..40)
bytes.extend_from_slice(&[0u8; 8]); // e_shoff
bytes.extend_from_slice(&[0u8; 4]); // e_flags
bytes.extend_from_slice(&[0u8; 2]); // e_ehsize
bytes.extend_from_slice(&phentsize.to_le_bytes()); // e_phentsize (54..56)
bytes.extend_from_slice(&phnum.to_le_bytes()); // e_phnum (56..58)
bytes.extend_from_slice(&[0u8; 6]); // pad to 64
debug_assert_eq!(bytes.len(), 64);
// Program header: p_type=3 (PT_INTERP), 4-byte LE.
bytes.extend_from_slice(&3u32.to_le_bytes());
bytes.extend_from_slice(&vec![0u8; phentsize as usize - 4]);
std::fs::write(&p, &bytes).unwrap();
assert!(
is_dynamically_linked(&p).unwrap(),
"PT_INTERP must be detected"
);
}
/// 64-bit ELF whose only program header is PT_LOAD (1) returns false — the
/// file is statically linked.
#[test]
fn elf64_without_pt_interp_returns_false() {
let tmp = tempfile::tempdir().unwrap();
let p = tmp.path().join("elf64-static");
let phoff: u64 = 64;
let phentsize: u16 = 56;
let phnum: u16 = 1;
let mut bytes = Vec::with_capacity(64 + phentsize as usize);
bytes.extend_from_slice(b"\x7fELF");
bytes.push(2);
bytes.push(1);
bytes.push(1);
bytes.extend_from_slice(&[0u8; 9]);
bytes.extend_from_slice(&[0u8; 2]);
bytes.extend_from_slice(&[0u8; 2]);
bytes.extend_from_slice(&[0u8; 4]);
bytes.extend_from_slice(&[0u8; 8]);
bytes.extend_from_slice(&phoff.to_le_bytes());
bytes.extend_from_slice(&[0u8; 8]);
bytes.extend_from_slice(&[0u8; 4]);
bytes.extend_from_slice(&[0u8; 2]);
bytes.extend_from_slice(&phentsize.to_le_bytes());
bytes.extend_from_slice(&phnum.to_le_bytes());
bytes.extend_from_slice(&[0u8; 6]);
debug_assert_eq!(bytes.len(), 64);
// p_type = 1 (PT_LOAD), not 3.
bytes.extend_from_slice(&1u32.to_le_bytes());
bytes.extend_from_slice(&vec![0u8; phentsize as usize - 4]);
std::fs::write(&p, &bytes).unwrap();
assert!(!is_dynamically_linked(&p).unwrap());
}
/// 32-bit ELF with PT_INTERP returns true — pins the `is_64bit=false`
/// branch (phoff/phnum read from 32-bit offsets). The header is exactly 52
/// bytes, proving the class-aware min-length gate admits a valid 32-bit
/// ELF that the old 64-byte gate would have wrongly rejected.
#[test]
fn elf32_with_pt_interp_returns_true() {
let tmp = tempfile::tempdir().unwrap();
let p = tmp.path().join("elf32-dyn");
// For 32-bit ELF: e_entry is 4 bytes (24..28), e_phoff 4 bytes at
// 28..32, e_phentsize at 42..44, e_phnum at 44..46.
let phoff: u32 = 52;
let phentsize: u16 = 32;
let phnum: u16 = 1;
let mut bytes = Vec::with_capacity(52 + phentsize as usize);
bytes.extend_from_slice(b"\x7fELF"); // 0..4
bytes.push(1); // 32-bit class (4)
bytes.push(1); // little-endian (5)
bytes.push(1); // EI_VERSION (6)
bytes.extend_from_slice(&[0u8; 9]); // osabi + padding (7..16)
bytes.extend_from_slice(&[0u8; 2]); // e_type (16..18)
bytes.extend_from_slice(&[0u8; 2]); // e_machine (18..20)
bytes.extend_from_slice(&[0u8; 4]); // e_version (20..24)
bytes.extend_from_slice(&[0u8; 4]); // e_entry — 32-bit is 4 bytes (24..28)
bytes.extend_from_slice(&phoff.to_le_bytes()); // e_phoff (28..32)
bytes.extend_from_slice(&[0u8; 4]); // e_shoff (32..36)
bytes.extend_from_slice(&[0u8; 4]); // e_flags (36..40)
bytes.extend_from_slice(&[0u8; 2]); // e_ehsize (40..42)
bytes.extend_from_slice(&phentsize.to_le_bytes()); // e_phentsize (42..44)
bytes.extend_from_slice(&phnum.to_le_bytes()); // e_phnum (44..46)
bytes.extend_from_slice(&[0u8; 6]); // pad to 52
debug_assert_eq!(bytes.len(), 52);
bytes.extend_from_slice(&3u32.to_le_bytes()); // PT_INTERP
bytes.extend_from_slice(&vec![0u8; phentsize as usize - 4]);
std::fs::write(&p, &bytes).unwrap();
assert!(is_dynamically_linked(&p).unwrap());
}
/// Big-endian ELF with PT_INTERP returns true — exercises the
/// `is_le=false` branches of the byte readers.
#[test]
fn elf64_big_endian_with_pt_interp_returns_true() {
let tmp = tempfile::tempdir().unwrap();
let p = tmp.path().join("elf64-be-dyn");
let phoff: u64 = 64;
let phentsize: u16 = 56;
let phnum: u16 = 1;
let mut bytes = Vec::with_capacity(64 + phentsize as usize);
bytes.extend_from_slice(b"\x7fELF");
bytes.push(2);
bytes.push(2); // big-endian
bytes.push(1);
bytes.extend_from_slice(&[0u8; 9]);
bytes.extend_from_slice(&[0u8; 2]);
bytes.extend_from_slice(&[0u8; 2]);
bytes.extend_from_slice(&[0u8; 4]);
bytes.extend_from_slice(&[0u8; 8]);
bytes.extend_from_slice(&phoff.to_be_bytes());
bytes.extend_from_slice(&[0u8; 8]);
bytes.extend_from_slice(&[0u8; 4]);
bytes.extend_from_slice(&[0u8; 2]);
bytes.extend_from_slice(&phentsize.to_be_bytes());
bytes.extend_from_slice(&phnum.to_be_bytes());
bytes.extend_from_slice(&[0u8; 6]);
debug_assert_eq!(bytes.len(), 64);
bytes.extend_from_slice(&3u32.to_be_bytes());
bytes.extend_from_slice(&vec![0u8; phentsize as usize - 4]);
std::fs::write(&p, &bytes).unwrap();
assert!(is_dynamically_linked(&p).unwrap());
}
}