Welcome to My Digital Garden

Overview When working with large graphics APIs like OpenGL ES and EGL, especially in complex systems like Android, developers face the challenge of managing hundreds of function declarations, pointers, and related metadata. A common and powerful solution is to use macro-based code generation with .in files. This approach, while adding some complexity, brings significant benefits in maintainability, consistency, and flexibility. This post explains what .in files are, how they’re used with macros, and why this pattern is so valuable in graphics stacks like OpenGL ES and EGL. We’ll also look at concrete examples and discuss the trade-offs involved. ...

Android and Linux systems rely on various communication methods for processes and threads to interact. Linux provides several IPC (Inter-Process Communication) mechanisms such as pipes, message queues, shared memory, sockets, semaphores, and signals. Android adds its own Binder IPC mechanism, which is widely used for communication between system components and apps. The Zygote process uses sockets for communication, while higher-level services (System Server, Media Server, Apps) primarily use Binder. For communication within the same process, especially between threads, Android uses the Handler message mechanism. ...

Android - System Architecture Linux Kernel Layer Manages core system services such as security, memory management, process management, network stack, and driver model. Provides an abstraction layer between the hardware and the rest of the software stack. Start the swapper process of the kernel (pid=0): idle process. It is used to initialize process management, memory management, and load Display, Camera Driver, Binder Driver and other related tasks. Start the kthreadd process (pid=2): Create kernel worker threads kworker, soft interrupt threads ksoftirqd, thermal and other kernel daemons Hardware Abstraction Layer (HAL) Purpose: Standard interface for hardware. Structure: Multiple modules for specific hardware types (e.g., WiFi, Bluetooth). Function: Loads appropriate module when hardware access is requested. Android Runtime(ART) & System Libraries ART: Each app runs in its own VM (via DEX files). Features: Ahead-of-Time (AOT) and Just-in-Time (JIT) compilation, optimized garbage collection (GC), and debugging-related support. Native System Libraries: User space daemons, HAL, boot animation. Init Process (pid=1): Parent of all user processes. Starts daemons (ueventd, logd, healthd, installd, adbd, and lmkd) and services (servicemanager, bootanim). Starts the Zygote process (parent of all Java processes). Framework Layer Zygote Process: Forked by init (in init.rc), loads VM, preloads classes/resources, starts Zygote socket. System Server: Forked from Zygote, manages Java framework including ActivityManager, WindowManager, PackageManager, PowerManager and other services. Media Server: Forked from init, manages C++ services (AudioFlinger, CameraService, etc.). App Layer Launcher: First app process from Zygote (user’s desktop). Other Apps: Browser, Phone, Email, etc., each in its own process, all forked from Zygote. Syscall & JNI Syscall: Interface between Native (user space) and Kernel. JNI: Bridge between Java and Native (C/C++) code. Appendix JNI - Java Native Interface It defines a way for the bytecode that Android compiles from managed code (written in the Java or Kotlin programming languages) to interact with native code (written in C/C++). JNI is vendor-neutral, has support for loading code from dynamic shared libraries, and while cumbersome at times is reasonably efficient. ...

Overview of DNS The Domain Name System (DNS) is the cornerstone of the modern internet, acting as a distributed and hierarchical naming system that translates human-readable domain names (e.g., example.com) into IP addresses (e.g., 192.0.2.1) that networking equipment uses to route traffic. DNS Resolution Flow User Request: The user enters a domain in their browser. Recursive Resolver: The request first hits a recursive resolver (typically provided by the user’s ISP or a public resolver like Google or Cloudflare). Root Servers: If not cached, the resolver queries one of the root DNS servers (13 root server clusters). TLD Servers: The root server responds with the TLD (e.g., .com) nameservers. Authoritative Server: The resolver queries the TLD server, which responds with the authoritative nameserver for the domain. Final Resolution: The resolver queries the authoritative nameserver, retrieves the required record (e.g., A record), and returns it to the user. DNS Hierarchy Root Zone: Managed by IANA, root servers handle the top level of DNS. TLD Zone: Top-Level Domains like .com, .net, .org, governed by registries. Authoritative Zones: Domains and subdomains managed by authoritative nameservers, controlled by domain owners. Major DNS Record Types A (Address Record) Purpose: Maps a domain to an IPv4 address. Example: api.example.com. IN A 192.0.2.1 Use Cases: Web servers, API endpoints. AAAA (IPv6 Address Record) Purpose: Maps a domain to an IPv6 address. Example: api.example.com. IN AAAA 2001:db8::1 Use Cases: IPv6-compliant services. CNAME (Canonical Name Record) Purpose: Points a subdomain to another domain (alias). Constraints: Cannot be used at the root of a domain. Example: www.example.com. IN CNAME example.com. Use Cases: Load-balanced endpoints, third-party services. ALIAS (Non-standard, provider-specific) Purpose: Like CNAME but usable at the root domain. Example (Cloudflare or Route 53): example.com. IN ALIAS example.net. Use Cases: Root domain pointing to load balancer or CDN. MX (Mail Exchange Record) Purpose: Defines mail servers for a domain. Priority Field: Lower numbers have higher priority. Example: example.com. IN MX 10 mail1.example.com. example.com. IN MX 20 mail2.example.com. Use Cases: Email routing and delivery. NS (Name Server Record) Purpose: Specifies authoritative DNS servers for a domain. Example: example.com. IN NS ns1.exampledns.com. example.com. IN NS ns2.exampledns.com. Use Cases: Delegating zones. SOA (Start of Authority) Purpose: Metadata for the DNS zone. Fields: Primary NS Admin email Serial number (zone version) Refresh, retry, expire, minimum TTL Example: example.com. IN SOA ns1.exampledns.com. admin.example.com. ( 2025052101 ; serial 3600 ; refresh 600 ; retry 604800 ; expire 86400 ; minimum ) TXT (Text Record) Purpose: Stores arbitrary text data. Use Cases: SPF (Sender Policy Framework): example.com. IN TXT "v=spf1 include:_spf.google.com ~all" DKIM (DomainKeys Identified Mail): default._domainkey.example.com. IN TXT "v=DKIM1; k=rsa; p=..." DMARC: _dmarc.example.com. IN TXT "v=DMARC1; p=none; rua=mailto:dmarc@example.com" PTR (Pointer Record) Purpose: Reverse DNS lookup. Example: 1.2.0.192.in-addr.arpa. IN PTR api.example.com. Use Cases: Email server validation, diagnostics. SRV (Service Locator Record) Purpose: Defines the location of services by name. Fields: Priority, weight, port, target. Example: _sip._tcp.example.com. IN SRV 10 60 5060 sipserver.example.com. Use Cases: SIP, XMPP, LDAP. CAA (Certificate Authority Authorization) Purpose: Specifies which CAs are allowed to issue certificates. Example: example.com. IN CAA 0 issue "letsencrypt.org" Use Cases: TLS certificate issuance control. Real-World Examples and Configurations Subdomain Examples api.example.com. IN A 192.0.2.10 mail.example.com. IN MX 10 mailhost.example.com. ftp.example.com. IN CNAME files.example.net. Multiple Records example.com. IN MX 10 mx1.example.com. example.com. IN MX 20 mx2.example.com. example.com. IN A 192.0.2.1 example.com. IN A 192.0.2.2 TTL and Priority Tuning www.example.com. 3600 IN CNAME webhost.example.net. example.com. 300 IN MX 10 mail1.example.com. Common Pitfalls and Misconfigurations CNAME at Root Domain Using CNAME at the zone apex (example.com.) is invalid and breaks RFC compliance. Use ALIAS or ANAME records if supported by your DNS provider. ...

Generating a Patch Using diff 1. Extract the Original Source Ensure you have a pristine copy of the original source for comparison. You can extract it from the tarball in the dl directory: tar -xf dl/<package>-<version>.tar.gz -C /tmp/ This will create a directory like /tmp/<package>-<version>/. Using buildroot’s mechanism Clean the build directory and apply current patches of buildroot make <pkg>-dirclean # Remove <pkg> build directory make <pkg>-extract # Extract <pkg> sources make <pkg>-patch # Apply patches to <pkg> (Optional) cp -r output/build/<package> /tmp/ 2. Generate the Patch Use the diff command to create a unified diff between the original and modified sources: ...

Overview ssh-audit is a tool for ssh server & client configuration auditing(banner, key exchange, encryption, mac, compression, compatibility, security, etc). Installation On Arch pacman -S ssh-audit pip pip3 install ssh-audit Generic - Download and extract tarball from release Usage ./ssh-audit.py 192.168.101.201 References https://github.com/jtesta/ssh-audit https://www.ssh-audit.com/hardening_guides.html

This File contains all the markdown formatting which include generic markdown as well as obsidian specific markdown. Markdown Conversion Test Headings H1 H2 H3 H4 H5 H6 Text Formatting Italic Italic Bold Bold Bold Italic Strikethrough Lists Unordered list Nested item Deeper item Ordered list Second item Sub-item Sub-item Links Standard Markdown Link Obsidian internal link: [0-Inbox/Obsidian]({< ref “/posts/0-inbox/obsidian/” >}}) Obsidian link with alias: [0-Inbox/Obsidian|Custom Text]({< ref “/posts/0-inbox/obsidian|custom-text/” >}}) Obsidian section link: [0-Inbox/Obsidian#Themes]({< ref “/posts/0-inbox/obsidian#themes/” >}}) Obsidian section link with alias: [0-Inbox/Obsidian#Themes|Alias Text]({< ref “/posts/0-inbox/obsidian#themes|alias-text/” >}}) Mixed content: Regular and [../0-Inbox/Obsidian Export to HUGO]({< ref “/posts/../0-inbox/obsidian-export-to-hugo/” >}}) Obsidian section link display content: Images Markdown image: Obsidian image: Obsidian image relative path: Code Inline code example. ...

Install nfs-kernel-server mkdir ~/Public Export the rootfs folder Add the following entry to /etc/exports: /home/rishav/Public *(rw,async,nohide,insecure,no_root_squash,no_subtree_check) Update the NFS server’s exports sudo exportfs -arv Make the NFS server serve over UDP Add the following to /etc/nfs.conf [nfsd] udp=y vers2=y Restart the NFS server systemctl restart nfs-server.service

Process Mount recovery partition into temporary location mkdir -p /mnt/recovery mount /dev/mmcblk0p6 /mnt/recovery Prepare old_root directory mkdir -p /mnt/recovery/mnt/old_root Switch root using pivot_root cd /mnt/recovery pivot_root . mnt/old_root Now: New root is /mnt/recovery (i.e., /) The previous root (e.g., initramfs or mainfs) is now mounted at /mnt/old_root Note: The chroot must be available under the old root and under the new root(recovery) Remount /proc, /sys, /dev, etc mount -t proc proc /proc mount -t sysfs sysfs /sys mount -o bind /mnt/old_root/dev /dev mount -o bind /mnt/old_root/tmp /tmp mount -o bind /mnt/old_root/run /run Start a shell inside of new root exec /bin/sh Mount root filesystem from NFS Setup nfs-service on host machine (refer [3-Resource/Linux/NFS|this]({< ref “/posts/3-resource/linux/nfs|this/” >}})) Mount NFS filesystem into the board mount -t nfs -o nolock 192.168.1.27:/home/rishav/Public /mnt/nfsroot Mount ext image as a loopback device mkdir -p /mnt/local cp /mnt/nfs/rootfs.ext4 /tmp/rootfs.ext4 mount -o loop /tmp/rootfs.ext4 /mnt/local References Manual page of pivot_root Also see difference between pivot_root and chroot

1. Overview Process Context The kernel executes code on behalf of a user-space process (e.g., handling a system call like read() or write()). Key Properties: Associated with a struct task_struct (process descriptor). Can sleep (use blocking functions like mutex_lock()). Can access user-space memory (via copy_from_user()). Interrupt Context “Atomic context” or “Interrupt context”, The kernel executes code to handle a hardware interrupt or softirq (e.g., a network packet arriving) Key Properties: No associated process (current macro points to an idle task). Cannot sleep (blocking functions like kmalloc(GFP_KERNEL) are forbidden). Runs with interrupts disabled (on the current CPU). 2. CPU Execution States in ARM ARM architectures (e.g., ARMv8-A) define exception levels (ELs) that correspond to CPU execution states: ...