Project 6

1
Project 6

• Requires working project 5
• Three main parts
• Message passing
• Input/Output redirection
• Users/Permissions

2
Message Passing

• Create with MessageQueueCreate(char name)
• If no message queue with name, create
• Otherwise, grant access to existing
• Use existing Read/Write/Close (new filesystem
  FS_TYPE_IPC)
• vfs.c Read example
• If (PFAT) do what it does now
• Else If (GOSFS) GOSFS_Read()
• Else if (IPC) Message_Queue_Read()
• You implement Message_Queue_Read
• Read from empty blocks a process
• Write to full blocks a process

3
Message Passing

• size 4

• Implement as circular queue

4
Input/Output Redirection

• Change Spawn_Program(char program) to
  Spawn_Program(char program, int stdin, int
  stdout)
• For every process file descriptor 0 is input
• For every process file descriptor 1 is output
• Make sure supplied file descriptors are valid

5
FS_TYPE_CONSOLE

• Console is a new file type in vsf
• vfs.c Read
• If (CONSOLE) Get_Key
• vsf.c Write
• If (CONSOLE) Print()
• Modify Print_String to call Write(1)
• Allows output redirection automatically

6
Redirection Example

• Parent process
• int fd Open(/d/output)
• Spawn_Program(/c/a.exe, 0, fd)
• Close(fd)
• Child process (a.exe)
• Write(1, string) // writes to file fd
• Printf(string) // writes to file fd

7
Permissions

• Each UserContext has a new field called uid
• Indicates user running this process
• Superuser is uid 0
• Used to test for GOSFS operations
• Open checks permissions requested versus existing
• Creating file requires write on parent directory
• CreateDirectory requires write on parent
  directory
• Delete requires write to access file/directory
• SetAcl requires write access to file/directory
• Read, Write, Seek, Stat, and Close do not check
  rights (checked in Open)
• Superuser (uid 0) always has all rights
  (independent of ACL)
• Used to pass permissions to spawned process
• More on this later

8
Modifying UID

• SetEffectiveUid(int uid)
• Sets the uid to supplied uid for current process
• Only sets if current user is superuser (uid 0)
• int GetUid()
• Returns current process uid

9
Spawn Modification

• New elf.c can spawn a process off GOSFS drive
• /d/a.exe will work
• How to set spawned process uid?
• Copy uid from parent process?
• Some processes need more rights than parent
• passwd command in UNIX
• Use setUid bit in GOSFSFileNode
• If setUid bit is 0
• Copy uid from parent process
• If setUid bit is 1
• Use acl0.uid as the process uid

10
GOSFSFileNode Enhancements

• aclEntry acl4
• Table of aclEntries
• Each entry contains a uid and permissions
• acl0 is the owner
• Set when the file/directory is created set to
  have full rights
• setUid1
• Indicates whether program should run with
  permissions of parent process or of file owner

11
SetAcl

• SYS_SET_ACL
• SetAcl(char name, int uid, int permissions)
• Permissions can be
• O_READ, O_WRITE, O_READ O_WRITE, 0
• Find file by name
• Store permissions for supplied uid
• if uid already in acl list update
• if permissions 0 erase rights for uid