Project 6
• Requires working project 5
• Three main parts
– Message passing
– Input/Output redirection
– Users/Permissions
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
Message Passing
•size = 4
•Implement as circular queue
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
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
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
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
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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
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
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 acl[0].uid as the process’ uid
GOSFSFileNode Enhancements
• aclEntry acl[4]
– Table of aclEntries
– Each entry contains a uid and permissions
– acl[0] is the owner
• Set when the file/directory is created set to have full
rights
• setUid:1;
– Indicates whether program should run with
permissions of parent process or of file owner
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