Gsoc-2024-NetBSD

Setroot:

flowchart TD  
    A[setroot Start] --> B{md_is_root?}  
    B -->|Yes| C[setroot_md]  
    B -->|No| D{TFTPROOT defined?}  
      
    D -->|Yes| E[tftproot_dhcpboot]  
    E -->|Success| C[setroot_md]
    C --> G  
    D -->|No| G[setroot_nfs]  
    E -->|Failure| G  
      
    G --> H{rootspec == NULL & bootdv == NULL?}  
    H -->|Yes| I[Set RB_ASKNAME]  
    H -->|No| J  
      
    I --> J{Root Device Loop}  
    J -->|RB_ASKNAME set| K[setroot_ask]
    K --> |root_device==NULL| J  
    J -->|RB_ASKNAME not set| L[setroot_root]  
    L --> N[Sleep 1 sec]  
    L -->|root_device == NULL| M{Wait timeout?}  
    %%M -->|No| N[Sleep 1 sec]
    N --> M
    M -->|No| J  
    M -->|Yes| I
    %%N --> L  
    %%setroot_root -> sleep 1 sec -> if no timeout -> root device loop else -> set rb_askname
%%[Tuesday, November 5, 2024] [11:37:43 PM IST] <good>	and setroot_ask if root_device == NULL -> root device loop
      
    K --> P[Configure root_device]  
    L -->|root_device found| P  
      
    P --> Q[setroot_dump]  
    Q --> R[End]

During a system boot up, based on kernel Config file, system mounts root filesystem. This is handled by setroot function in kern_subr.c. device and partition are defined with syntax

config netbsd root on XXX_device type XXX

device name with partition is loaded onto rootspec variable which is used by setroot and associated functions to select a device. device can also be a network device in that case a valid interface name should be specified. if instead of a device ‘ ? ‘ is specified, setroot uses boot device(from bootspec) as root device. Boot device is set by findroot function. Findroot is machine dependent. x86/x86_autoconf.c contains function which interact with BIOS entries to configure boot spec variable. it also has different cases for different device type Christoph badura’s documentation.

  1. Primary Functions:
    • setroot(): Main entry point that orchestrates root device selection
    • setroot_nfs(): Handles NFS root configuration
    • setroot_md(): Sets up memory disk as root device
    • setroot_ask(): Interactive root device configuration
    • setroot_root(): core root device configuration
    • setroot_dump(): Configures dump device
  2. Key Control Flow:
    a) Initial Setup:
    • Checks if config has specified rootdev and partition (through rootspec), otherwise uses dev and partition system booted from ( bootspec)
    • Verifies if memory disk should be root (md_is_root)
    • Handles TFTPROOT configuration if enabled

    b) Device Selection:

    • If no boot device and no root specified, enters interactive mode
    • Waits up to ROOT_WAITTIME (default 20 seconds) for device availability
    • Falls back to interactive mode i.e. asks user for root device and dump device if device doesn’t appear

    c) Interactive Configuration (setroot_ask):

    • Prompts for root device
    • Prompts for dump device
    • Prompts for filesystem type
    • Supports special commands (halt, reboot, ddb) debugger is only supported if src was built with debugger option
  3. Supported features types:
    a) Device Support:
    • Handles disk devices with partitions
    • Supports network interfaces
    • Supports memory disk devices
    • Supports disk wedges

    b) Filesystem Support:

    • Generic filesystem support
    • NFS support
    • Allows custom filesystem type selection

    c) Dump Device Configuration:

    • Automatic selection based on root device
    • Manual configuration support
    • Support for “none” as dump device
  4. Important Variables:
    • rootdev: Device number for root device
    • dumpdev: Device number for dump device
    • dumpcdev: Character device for dumps
    • root_device: Device structure for root device
    • booted_device: Device through which system booted. Obtained through BIOS entries
    • rootspec: Specification for root device from config file
    • bootspec: Boot device specification
  5. Special Handling:
    a) NFS Root:
    • Automatically selects the network interface if needed.
    • Skips partition handling for network devices.

    b) Memory Disk:

    • Special handling when md_is_root is set.
    • Creates md0 device if needed.

    c) Dump Device Selection:

    • Uses partition ‘b’ by default on the root disk.
    • Allows “none” as a valid option.
    • Special handling for network root devices.
  6. Error Handling:
    • Timeout mechanism for device availability
    • Fallback to interactive mode on failures
    • Validation of device and filesystem selections
    • Clear error messages for invalid selections

Setroot_md and md is disk

sets parameter device to md device of name “md0” , after reading the block device.

Tftproot_dhcp

The tftproot_dhcpboot is to set up a root filesystem for the device using TFTP (Trivial File Transfer Protocol) over NFS (Network File System) based on DHCP (Dynamic Host Configuration Protocol) information.

  1. Function Signature and Initializations:
    • tftproot_dhcpboot takes a device_t type argument bootdv (a device pointer to the boot device).
    • Initializes necessary local variables such as nd (NFS diskless information), ifp (network interface), trh (a TFTP root handle), and error (initially set to -1, which indicates failure if nothing is done).
  2. Check if Root Specification Exists:
    • If rootspec is non-NULL (indicating a specific network interface was given for booting), the function iterates through all available network interfaces (IFNET_READER_FOREACH) to find one matching rootspec.
    • If found, it assigns the matching interface to ifp and exits the loop.
  3. Fallback to Boot Device:
    • If no matching interface is found (ifp is still NULL) and bootdv is non-NULL, the function checks if bootdv is a network interface (using device_class).
    • It then tries to match bootdv’s name (device_xname(bootdv)) with the names of available network interfaces, setting ifp if a match is found.
  4. Validation of Network Interface:
    • If ifp is still NULL, the function outputs a debug message indicating failure to locate the interface, then jumps to the cleanup and exit section (out label).
  5. Find and Validate Root Device:
    • Calls device_find_by_xname to locate the device object for ifp using its if_xname.
    • If the device (dv) is not found or is not of the network interface type (DV_IFNET), it logs a debug message and exits.
  6. Assign Root Device and Allocate NFS Structure:
    • Sets root_device to dv (the found network device).
    • Assigns curlwp (current lightweight process, l) and allocates memory for nd (an nfs_diskless structure), zero-initializing it. This structure will contain essential NFS boot information.
    • Sets nd->nd_ifp to ifp and marks nd->nd_nomount to 1, indicating no mounting is performed during this phase.
  7. NFS Boot Initialization:
    • Calls nfs_boot_init, which initializes network-based boot information for NFS using nd and l. If this function fails, it logs a debug message and exits.
  8. Remove TFTP Prefix from Boot File:
    • Strips the “tftp:” prefix from nd->nd_bootfile, which represents the boot file path. If the boot file starts with “tftp:”, it removes this prefix for easier handling in subsequent steps.
  9. TFTP Root Handle Setup and File Retrieval:
    • Initializes trh to zero and assigns nd to trh.trh_nd, setting the trh_block to 1.
    • Calls tftproot_getfile, which retrieves the boot file via TFTP. If this function fails, it logs a debug message and exits.
  10. Final Error Handling and Cleanup:
    • Sets error to 0, indicating success.
    • In the out label section, it frees the allocated nd memory if it was allocated.
    • Returns error, which will be 0 on success or an error code if any part of the function failed.

Function Workflow

The tftproot_dhcpboot function:

  1. Determines the appropriate network interface (ifp) using either rootspec or bootdv.
  2. Ensures the found device is a network interface.
  3. Allocates and initializes an NFS diskless structure.
  4. Sets up the root device and initializes boot information for NFS.
  5. Prepares the TFTP boot file by stripping unnecessary prefixes.
  6. Retrieves the boot file via TFTP and returns success or failure.

Setroot_ask

flowchart TD
    A[Start]
    A --> B{Input loop} 
    B --> C{bootdv!= NULL}
    C -->|Yes| D[Show default bootdv]
    C -->|No| E[Prompt user for root device]
    D --> E	
    E --> F{User input for root device}
    F -->|No input| G[Set to bootdv]
    F -->|Wildcard input| H[Search matching devices]
    F -->|Valid input| I[Set rootdv and exit loop]
    H --> I

    I --> ZA{Breaks input loop}
    G --> ZA
    ZA --> |No| B			
    ZA -->|Yes| J[Set rootdev to nrootdev]

    J --> K[Default Dump Device Setup]
    K --> L{Root device supports partitions?}
    L -->|Yes| M[Set defdumpdv to rootdv]
    L -->|No| N[Set defdumpdv to NULL]

    M --> O[Dump Device Selection]
    N --> O
    O --> P{User input for dump device}
    P -->|empty input| Q{Is defdumpdv available?}
    Q -->|Yes| R[Set dumpdv to second partition of rootdv]
    R --> S[Set dumpdev to ndumpdev identifier]
    Q -->|No| T[Skip setting dump device]
    P -->|Input none| T
    P -->|Valid input| S
    T --> S

    S --> U[Filesystem Type Selection]
    U --> V{User input for filesystem}
    V -->|empty input| W[Set rootfstype to default deffsname]
    V -->|Command input halt reboot ddb| X[Execute command]
    V -->|Filesystem not valid| Y[List available filesystems]
    Y --> V
    W --> Z[Set rootfstype based on input or default]
    X --> Z

    Z --> AA[Root Device Validation and Final Setup]
    AA --> AB{Is rootdv class network or disk?}
    AB -->|Yes| AC[Print root device details]
    AB -->|No| AD[Display error message]

    AC --> AE[Set root_device and call setroot_dump]
    AD --> AE
    AE --> AF[End]

Setroot_root

Purpose

The setroot_root function is responsible for configuring the root device of the system during the boot process if root device was specified and boot device was found. It selects the appropriate root device based on a specified configuration (rootspec) or falls back to the boot device if rootspec is not defined. This function also determines the major device number and partition of the root device, ensuring that the system can mount the root filesystem correctly.

Parameters

Internal Variables

Function Workflow

  1. Checking for Root Device Specification (rootspec):
    • If rootspec is NULL, the function defaults to using bootdv as the root device.
    • If bootdv is defined, majdev is assigned the major number of bootdv.
    • For disk devices, it calculates the root device ID (rootdev) using MAKEDISKDEV if the device uses partitions, or makedev otherwise.
  2. Parsing the rootspec:
    • If rootspec is defined, parsedisk attempts to parse it and assign the resulting device to rootdv.
    • If parsing fails, rootdev and rootdv are computed from the major device name (rootdevname) and device unit.
    • The function then calls finddevice to locate rootdv based on the constructed device name in buf.
  3. Error Handling:
    • If majdev is invalid or rootdv is NULL, error messages are printed, and the function exits early.
  4. Final Root Device Determination:
    • After successfully identifying rootdv, the function checks the device class of rootdv to ensure it is either a network interface (DV_IFNET) or a disk (DV_DISK).
    • It then displays the name of the root device and appends the partition letter if the device supports partitions.
  5. Setting Global Root Device:
    • The global root_device is assigned to rootdv, and setroot_dump is called to complete the root device setup.

Notes

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