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

bash

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

bash

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

ldd

shows

not found

, the shared library is missing from

LD_LIBRARY_PATH

/etc/ld.so.conf

Fix:

bash

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

3. Symbols:

nm

bash

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

bash

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

bash

# 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

bash

# 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)

bash

# 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.

bash

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

9. Common diagnosis flows

"undefined symbol at runtime"

bash

# 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"

bash

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

NPX Install

Tags

SKILL.md Content

ELF Inspection

Purpose

Triggers

Workflow

1. Quick overview:
`file`
and
`size`

2. Dynamic dependencies:
`ldd`

3. Symbols:
`nm`

4. Sections:
`readelf`

5. Disassembly and source:
`objdump`

6. Binary hardening check

7. Section size analysis (binary bloat)

8. Build ID

9. Common diagnosis flows

Related skills