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