Understanding /usr/src/linux Symlink Importance

The Missing /usr/src/linux Directory

1. Why the Path Doesn’t Exist

Modern Linux distributions (especially RHEL/CentOS) no longer create /usr/src/linux by default
Kernel headers are installed to version-specific paths like:
```
/usr/src/kernels/$(uname -r)/
```

2. Why This Breaks Module Building

Many build systems (including LTP and kernel module builds) historically expected:
```
/usr/src/linux/include/linux/module.h
```
Without this symlink:
- Compiler cannot find kernel headers
- Module builds fail with “linux/module.h not found”
- LTP kernel tests cannot be compiled

3. Why It’s Critical for LTP

LTP’s kernel tests require:
- Exact kernel headers matching running kernel
- Standard header paths expected by build system
Without proper headers:
- All kernel-space tests will fail
- Key functionality (syscalls, drivers, filesystems) won’t be tested
- configure may disable critical test cases

4. Historical Context

Traditional Linux builds used /usr/src/linux symlink
Modern distributions use versioned paths for:
- Parallel kernel support
- Cleaner package management
- Multiple kernel versions coexistence

5. Impact on Module Building

Kernel modules must be built against exact headers:
- Version mismatch causes:
  - Module loading failures
  - Kernel panics
  - Undefined symbol errors
/usr/src/linux symlink ensures:
- Consistent build paths
- Version correctness
- Build system compatibility

Creating the Symlink (Solution)

1. Verify Kernel Headers Exist

ls /usr/src/kernels/$(uname -r)/include/linux/module.h

2. Create Symlink

sudo ln -s /usr/src/kernels/$(uname -r) /usr/src/linux

3. Verify

ls -l /usr/src/linux/include/linux/module.h
# Should show valid symlink chain

4. Permanent Fix

For persistent solution across kernel updates:

echo "ln -sf /usr/src/kernels/\$(uname -r) /usr/src/linux" | sudo tee /etc/kernel/postinst.d/linux-symlink
sudo chmod +x /etc/kernel/postinst.d/linux-symlink