Assignment-2: System calls and exception handlers

Assignment-2: System calls and exception handlers
Description
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.
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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.
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MM SEGMENT STACK: A page fault to this region can occur 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 start and end value of
this memory segment. If the address is not legitimate (not between start and end), you should terminate
the process by printing an error message with accessed virtual address, RIP and error code.
For all of the above cases, if the virtual address is legitimate, you must install the page table for the accessed
virtual address (like assignment-1) before returning to the user process. Note that, after fixing up the page
fault, you must execute iretq instruction by placing the stack pointer register (RSP) to the user process
RIP as shown in slide 20 of Userland.pdf. Note that, the page table structure is same as assignment-1.
Submission guidelines
– You are required to submit entry.c file with implementations for system call and page fault handlers.
– Remove/comment all print statements used for debugging.
– Your implementation will be tested with several test cases. So, your implementation should be generic.
– Maximum size of allocated physical memory will not exceed 16MB at any point of time. This will be
ensured in all the test cases.
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