The Challenge

At Batna, I was tasked with solving one of the most critical problems in embedded systems: how do you securely and reliably update thousands of devices in the field without physical access? This wasn't just about pushing code—it was about building a system that could:

Understanding the Requirements

Device Constraints

The embedded Linux devices we were working with had:

Security Requirements

Reliability Requirements

System Architecture

I designed a three-tier architecture:

┌─────────────────────────────────────────┐
│      Update Server (Cloud)              │
│  - Package generation                   │
│  - Signature management                 │
│  - Update distribution                  │
│  - Device tracking                      │
└──────────────┬──────────────────────────┘
               │ HTTPS/TLS
               ▼
┌─────────────────────────────────────────┐
│      Update Agent (Device)             │
│  - Update checking                     │
│  - Download management                 │
│  - Verification                        │
│  - Installation orchestration          │
└──────────────┬──────────────────────────┘
               │
               ▼
┌─────────────────────────────────────────┐
│      System Layer (Linux)               │
│  - Dual-boot partitions                │
│  - Bootloader integration              │
│  - Kernel and rootfs updates            │
└─────────────────────────────────────────┘

Implementation Details

1. Package Generation Pipeline

I built a Jenkins-based CI/CD pipeline that:

#!/bin/bash
Package generation script
Build system image

buildroot-make linux-rebuild
buildroot-make
Create update package

UPDATE_DIR="/tmp/update-$(date +%s)"
mkdir -p "$UPDATE_DIR"
Copy rootfs

cp -r output/images/rootfs.ext2 "$UPDATE_DIR/rootfs.ext2"
Create delta if previous version exists

if [ -f "previous/rootfs.ext2" ]; then
    bsdiff previous/rootfs.ext2 "$UPDATE_DIR/rootfs.ext2" "$UPDATE_DIR/rootfs.delta"
fi
Generate metadata

cat > "$UPDATE_DIR/metadata.json" <{
  "version": "$(git describe --tags)",
  "timestamp": $(date +%s),
  "size": $(stat -f%z "$UPDATE_DIR/rootfs.ext2"),
  "checksum": "$(sha256sum "$UPDATE_DIR/rootfs.ext2" | cut -d' ' -f1)",
  "kernel_version": "$(uname -r)"
}
EOF
Sign package

openssl dgst -sha256 -sign private_key.pem "$UPDATE_DIR/metadata.json" > "$UPDATE_DIR/signature.bin"
Compress and upload

tar czf "update-$(date +%s).tar.gz" -C "$UPDATE_DIR" .
scp "update-*.tar.gz" update-server:/releases/

2. Update Agent (Client-Side)

The update agent ran as a systemd service on each device:

// Update agent main loop
int main(int argc, char *argv[]) {
    // Initialize logging
    init_logging();
    
    // Check for updates periodically
    while (1) {
        update_info_t *update = check_for_updates();
        
        if (update != NULL) {
            log_info("Update available: %s", update->version);
            
            // Download update
            if (download_update(update) == 0) {
                // Verify signature
                if (verify_signature(update) == 0) {
                    // Install update
                    if (install_update(update) == 0) {
                        log_info("Update installed successfully");
                        reboot_system();
                    } else {
                        log_error("Update installation failed");
                        rollback_update();
                    }
                } else {
                    log_error("Signature verification failed");
                    remove_update_files();
                }
            } else {
                log_error("Update download failed");
            }
            
            free_update_info(update);
        }
        
        sleep(UPDATE_CHECK_INTERVAL);
    }
    
    return 0;
}

3. Secure Download with Resume

Network interruptions were common, so I implemented resumable downloads:

int download_update(update_info_t *update) {
    int fd = open(update->local_path, O_WRONLY | O_CREAT, 0644);
    if (fd < 0) {
        return -1;
    }
    
    // Check if partial download exists
    off_t offset = lseek(fd, 0, SEEK_END);
    
    CURL *curl = curl_easy_init();
    curl_easy_setopt(curl, CURLOPT_URL, update->download_url);
    curl_easy_setopt(curl, CURLOPT_WRITEDATA, fd);
    curl_easy_setopt(curl, CURLOPT_RESUME_FROM_LARGE, offset);
    curl_easy_setopt(curl, CURLOPT_SSL_VERIFYPEER, 1L);
    curl_easy_setopt(curl, CURLOPT_SSL_VERIFYHOST, 2L);
    
    CURLcode res = curl_easy_perform(curl);
    
    close(fd);
    curl_easy_cleanup(curl);
    
    return (res == CURLE_OK) ? 0 : -1;
}

4. Signature Verification

Every update package was signed with RSA-2048:

int verify_signature(update_info_t *update) {
    // Load public key
    FILE *pubkey_file = fopen("/etc/update/public_key.pem", "r");
    EVP_PKEY *pubkey = PEM_read_PUBKEY(pubkey_file, NULL, NULL, NULL);
    fclose(pubkey_file);
    
    // Read signature
    unsigned char signature[256];
    FILE *sig_file = fopen(update->signature_path, "rb");
    fread(signature, 1, 256, sig_file);
    fclose(sig_file);
    
    // Read metadata
    unsigned char metadata_hash[32];
    SHA256_CTX sha256;
    SHA256_Init(&sha256);
    
    FILE *meta_file = fopen(update->metadata_path, "rb");
    unsigned char buffer[4096];
    size_t bytes;
    while ((bytes = fread(buffer, 1, 4096, meta_file)) > 0) {
        SHA256_Update(&sha256, buffer, bytes);
    }
    fclose(meta_file);
    
    SHA256_Final(metadata_hash, &sha256);
    
    // Verify signature
    EVP_MD_CTX *md_ctx = EVP_MD_CTX_new();
    EVP_DigestVerifyInit(md_ctx, NULL, EVP_sha256(), NULL, pubkey);
    EVP_DigestVerifyUpdate(md_ctx, metadata_hash, 32);
    int result = EVP_DigestVerifyFinal(md_ctx, signature, 256);
    
    EVP_MD_CTX_free(md_ctx);
    EVP_PKEY_free(pubkey);
    
    return (result == 1) ? 0 : -1;
}

5. Dual-Boot Partition Strategy

To enable safe rollbacks, I implemented a dual-boot partition scheme:

Device Storage Layout:
├── /dev/mmcblk0p1  (Boot partition - 16MB)
├── /dev/mmcblk0p2  (Rootfs A - 1GB)  ← Active
├── /dev/mmcblk0p3  (Rootfs B - 1GB)  ← Standby
├── /dev/mmcblk0p4  (Data partition - remaining)
└── /dev/mmcblk0p5  (Recovery partition - 512MB)

Update process:

6. Kernel Optimizations

I performed deep kernel optimizations to meet hardware requirements:

Memory Management:

// Reduced kernel memory footprint
CONFIG_HIGHMEM=n
CONFIG_X86_PAE=n
CONFIG_VMSPLIT_3G=y
// Optimized slab allocator
CONFIG_SLUB=y
CONFIG_SLUB_CPU_PARTIAL=y

I/O Optimizations:

// Tuned I/O scheduler for eMMC
CONFIG_MQ_IOSCHED_DEADLINE=y
// Reduced buffer sizes
CONFIG_BLK_DEV_RAM_SIZE=4096

Network Stack:

// Optimized TCP for low-bandwidth
CONFIG_TCP_CONGESTION_DEFAULT="bbr"
CONFIG_TCP_MEM="4096 8192 16384"

Testing and Validation

Test Scenarios

I created comprehensive test scenarios:

Test Infrastructure

# Automated testing script
import subprocess
import time
def test_update_flow():
    # Deploy test device
    device = deploy_test_device()
    
    # Trigger update
    trigger_update(device, "v1.0.0")
    
    # Simulate network interruption
    time.sleep(5)
    disconnect_network(device)
    time.sleep(10)
    reconnect_network(device)
    
    # Verify update completed
    version = get_device_version(device)
    assert version == "v1.0.0"
    
    # Test rollback
    trigger_update(device, "v1.0.1")
    simulate_power_failure(device)
    
    # Verify rollback
    version = get_device_version(device)
    assert version == "v1.0.0"

Results and Impact

Metrics

Challenges Overcome

Lessons Learned

Conclusion

Building the OTA update system at Batna was a deep dive into systems engineering. It required understanding everything from kernel internals to network protocols, from cryptographic signatures to bootloader mechanics. The system I built successfully updated thousands of devices in the field with a 99.2% success rate and zero data loss.

The experience taught me that systems engineering is about more than just writing code—it's about understanding the entire stack, from hardware to software, and designing for reliability, security, and maintainability.

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Interested in embedded systems, OTA updates, or kernel optimization? Let's connect!