Elf Inspection

ELF Inspection

Purpose

Guide agents through inspecting Linux ELF binaries: symbol tables, section layout, dynamic linking, debug info, and diagnosing linker errors.

Triggers

• "What libraries does this binary depend on?"
• "Why is this binary so large?"
• "I have an undefined reference or symbol not found at runtime"
• "How do I check if debug info is in this binary?"
• "How do I find what symbols a library exports?"
• "How do I check if a binary is PIE / has RELRO?"

Workflow

1. Quick overview: file and size

file prog                    # type, arch, linkage, stripped or not
size prog                    # section sizes: text, data, bss
size --format=sysv prog      # detailed per-section breakdown

2. Dynamic dependencies: ldd

ldd ./prog                   # show all shared lib dependencies
ldd -v ./prog                # verbose: include symbol versions

# Check why a library is loaded
ldd ./prog | grep libssl

# For a library (not an executable)
ldd ./libfoo.so

If ldd shows not found, the shared library is missing from LD_LIBRARY_PATH or /etc/ld.so.conf.

Fix:

export LD_LIBRARY_PATH=/path/to/libs:$LD_LIBRARY_PATH
# Or install the library and run ldconfig
sudo ldconfig

3. Symbols: nm

nm prog                       # all symbols (T=text, D=data, U=undefined, etc.)
nm -D ./libfoo.so             # dynamic symbols only
nm -C prog                    # demangle C++ symbols
nm --defined-only prog        # only defined symbols
nm -u prog                    # only undefined (needed) symbols
nm -S prog                    # include symbol size

# Search for a symbol
nm -D /usr/lib/libssl.so | grep SSL_read

Symbol type codes:

• T / t — text (code): global / local
• D / d — data (initialised): global / local
• B / b — BSS (uninitialised): global / local
• R / r — read-only data: global / local
• U — undefined (needs to be provided at link time)
• W / w — weak symbol

4. Sections: readelf

readelf -h prog               # ELF header (arch, type, entry point)
readelf -S prog               # all sections
readelf -l prog               # program headers (segments)
readelf -d prog               # dynamic section (like ldd but raw)
readelf -s prog               # symbol table
readelf -r prog               # relocations
readelf -n prog               # notes (build ID, ABI tag)
readelf --debug-dump=info prog | head -100  # DWARF info
readelf -a prog               # all of the above

5. Disassembly and source: objdump

# Disassemble all code sections
objdump -d prog
objdump -d -M intel prog      # Intel syntax

# Disassemble + intermix source (needs -g at compile time)
objdump -d -S prog

# Disassemble specific symbol
objdump -d prog | awk '/^[0-9a-f]+ <main>:/,/^$/'

# All sections (including data)
objdump -D prog

# Header info
objdump -f prog
objdump -p prog               # private headers (including needed libs)

6. Binary hardening check

# Check for PIE, RELRO, stack canary, NX
# Use checksec (install separately)
checksec --file=prog

# Manual checks:
readelf -h prog | grep Type           # ET_DYN = PIE, ET_EXEC = non-PIE
readelf -d prog | grep GNU_RELRO      # RELRO present
readelf -d prog | grep BIND_NOW       # full RELRO
readelf -s prog | grep __stack_chk    # stack protector
readelf -l prog | grep GNU_STACK      # NX bit (RW = no exec, RWE = exec stack)

7. Section size analysis (binary bloat)

# Detailed section sizes
size --format=sysv prog | sort -k2 -nr | head -20

# Per-object contribution (with -Wl,--print-map or bloaty)
# Bloaty (install separately): https://github.com/google/bloaty
bloaty prog

# Check stripped vs not
file prog
strip --strip-all -o prog.stripped prog
ls -lh prog prog.stripped

8. Build ID

Build IDs uniquely identify a binary/library build, enabling debuginfod lookups.

readelf -n prog | grep 'Build ID'
# or
file prog | grep BuildID

9. Common diagnosis flows

"undefined symbol at runtime"

# Which library was expected to provide it?
nm -D libfoo.so | grep mysymbol
# Is the library in the runtime path?
ldd ./prog | grep libfoo
# Check LD_PRELOAD / LD_LIBRARY_PATH

"binary is too large"

size --format=sysv prog | sort -k2 -nr | head
nm -S --defined-only prog | sort -k2 -nr | head -20
objdump -d prog | awk '/^[0-9a-f]+ </{fn=$2} /^[0-9a-f]/{count[fn]++} END{for(f in count) print count[f], f}' | sort -nr | head -20

For a quick reference, see references/cheatsheet.md.

Related skills

• Use skills/binaries/linkers-lto for linker flags and LTO
• Use skills/binaries/binutils for ar, strip, objcopy, addr2line
• Use skills/debuggers/core-dumps for build ID and debuginfod usage

ELF Inspection Cheatsheet

Source: <https://man7.org/linux/man-pages/man1/readelf.1.html> Source: <https://man7.org/linux/man-pages/man1/objdump.1.html> Source: <https://man7.org/linux/man-pages/man1/nm.1.html> Source: <https://docs.redhat.com/en/documentation/red_hat_enterprise_linux/8/html-single/developing_c_and_cpp_applications_in_rhel_8/index>

Quick reference

---

nm symbol types

---

readelf sections

Key sections in a typical ELF:

---

Hardening checks

# PIE: ET_DYN = PIE executable (position-independent)
readelf -h prog | grep 'Type:'

# RELRO
readelf -l prog | grep 'GNU_RELRO'
readelf -d prog | grep BIND_NOW   # full RELRO requires BIND_NOW

# NX (non-executable stack)
readelf -l prog | grep 'GNU_STACK'
# Flags should be 'RW' not 'RWE'

# Stack protector
nm prog | grep __stack_chk_fail

# FORTIFY_SOURCE
nm prog | grep __memcpy_chk

# checksec (comprehensive)
checksec --file=prog --output=json

---

Shared library SONAME

# Check SONAME
readelf -d libfoo.so | grep SONAME
objdump -p libfoo.so | grep SONAME

# Create symlinks correctly
ln -sf libfoo.so.1.2.3 libfoo.so.1
ln -sf libfoo.so.1 libfoo.so