CS330 Assignment-2: System calls and exception handlers Solved

Recap: gemOS is in 64-bit mode executing itself as the first context (say the boot context). The boot context sets up the page table, stack, segment registers for itself. Further, it implements basic input output to a serial console, and puts itself into a basic shell.

At this stage, GemOS implements a command called init which creates the first user process (named as the init process with PID = 1). Source code for init process can be found in user/init.c file. The current init process invokes two system calls—getpid() and exit(). The objective of the assignment is to implement new system calls and exception handlers and enable support for lazy memory allocation. To handle the system calls, GemOS installs an IDT entry at offset 0x80. The system call handlers in in GemOS are defined in entry.c.

Part A
In this part of the assignment, you are required to implement three system calls as described below,

int write(char *buf, int length)

Outputs the characters in buffer into the console and returns the number of bytes written. The length parameter is limited to 1024 bytes. You must check the validity of buf address by accessing the init process page table. Note that, for convenience, CR3 is not switched on syscall or exception entry points. write system call should return -1 for invalid memory addresses and incorrect values of length.

void *expand(u32 size, int flags)

Expands the memory area by number of pages specified in the size parameter where maximum value of size can be 512. The memory segment is selected based on values of flags—MAP RD expands the MM SEG RODATA segment while MAP WR expands the MM SEG DATA segment. These segments are part of struct exec ctx and can be accessed by accessing the member named mms, which is an array of struct mm segment.

struct mm_segment{ unsigned long start; unsigned long end; unsigned long next_free;

u32 access_flags;    /*R=1, W=2, X=4, S=8*/ };

An element next free is added in this assignment to denote the next unused virtual address (at 4K boundary). So at any point of time, for MM SEG RODATA and MM SEG DATA, the allowed virtual address range is start to next free - 1. expand system call should expand the allowed virtual address range by ‘size’ number of pages and return the start of newly added virtual address region (in other words, return value of next free before expansion). Note that, expand does not allocate any PFN to map the expanded virtual address region. void *shrink (u32 size, flags)

Shrinks the memory area by number of pages specified in the size parameter. The shrink system call should bring down the next free marker of the memory segment by ‘size’ number of pages and return the current end address of the virtual address segment (updated next free). The memory segment is selected depending on the flags argument, similar to the expand system call. Further, the shrink system call must release all the PFNs mapped to by virtual pages being released and update the page table entries appropriately. Both expand and shrink system calls should return NULL (0) if the size argument grows the segment beyond mm segment.end or shrinks the segment below mm segment.start, respectively.

Part B
In this part of the assignment, you are required to implement two exception handlers in gemOS as described below.

Divide-by-zero

If the user process (in this case init) performs a division by zero, this exception handler is invoked. In the IDT, the handler (handle div by zero defined in entry.c) is already registered. You are required to implement the handler in which you should call do exit() after printing an error message as, “Div-by-zero detected at [RIP]”, where RIP is the instruction pointer address of the faulting instruction. Note that, the hardware does not push any error code into the OS stack before invoking the handler. The OS stack points to user RIP when the exception occurs (see slide 20 of Userland.pdf).

Page fault

If the user process (in this case init) accesses any virtual address for which valid mapping is not present or permissions (R/W or U/S) are not sufficient in the page tables, the page fault exception handler will be invoked. In the IDT, the handler (handle page fault) defined in entry.c is already registered. When the exception occurs, the hardware will push an error code along with user RIP and other information into the OS stack as shown in slide 20 of Userland.pdf. The CR2 register contains the faulting virtual address. Relevant fields of the hardware generated error code are described below.

bit-0: value 0 implies present bit in page table is not set in one of the paging levels, value 1 indicates

protection violation.

bit-1: value 0 implies the access causing the fault is a read, value 1 indicates the access is a write.

bit-2: value 0 implies the access causing the fault is from CPL=0, value 1 indicates the access is from CPL=3

You can ignore all other bits.

Your page fault handler should handle the fault for different virtual memory segments differently. If the virtual address does not belong to any of the following memory segments, you should print an error message with RIP, accessed virtual address and error code before terminating the process by invoking do exit. MM SEGMENT DATA: A page fault in this region can occur only if the page table mapping for a virtual address is not present. Before creating a new mapping in the page tables, you must check the next free value of the memory segment. If the address is not legitimate (not between start and next free) you should terminate the process by printing an error message with accessed virtual address, RIP and error code.

MM SEGMENT RODATA: A page fault to this region can occur if the page table mapping for a virtual address is not present or the access is a WRITE access. Before creating a new mapping in the page tables, you must check the next free value of the memory segment. If the address is not legitimate (not between start and next free) or the access is a WRITE access, you should terminate the process by printing an error message with accessed virtual address, RIP and error code.