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DPager.c
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483 lines (394 loc) · 14.9 KB
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#include <assert.h>
#include <elf.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mman.h>
#include <unistd.h>
#include <signal.h>
#include <ucontext.h>
// ELF magic numbers
#define EI_MAG0 0
#define EI_MAG1 1
#define EI_MAG2 2
#define EI_MAG3 3
#define ELFMAG0 0x7f
#define ELFMAG1 'E'
#define ELFMAG2 'L'
#define ELFMAG3 'F'
#define PAGE_SIZE 4096
#define STACK_ALIGNMENT 16
extern char **environ;
// Size of the ELF magic number
#define SELFMAG 4
Elf64_Addr e_entry;
int global_fd;
Elf64_Ehdr elf_header;
Elf64_Phdr *ph;
// Had to add GNU property (elf.h did not have it on lab machine)
#define PT_GNU_PROPERTY 0x6474e553
int load_elf_binary(int argc, char *argv[], Elf64_Ehdr *header) {
// for command line argument!
if (argc < 2) {
printf("Usage: %s <executable>\n", argv[0]);
return 1;
}
// Open the ELF file
int fd = open(argv[1], O_RDONLY);
if (fd < 0) {
perror("Failed to open file");
return 1;
} else {
printf("Successfully opened file. \n");
}
// Read the ELF header
if (read(fd, &elf_header, sizeof(Elf64_Ehdr)) != sizeof(Elf64_Ehdr)) {
perror("Failed to read ELF header");
close(fd);
return 1;
} else {
printf("Successfully read ELF header. \n");
}
e_entry = elf_header.e_entry;
// check to see if we are dealing with an elf file!
unsigned char elf_magic[SELFMAG] = {ELFMAG0, ELFMAG1, ELFMAG2, ELFMAG3};
if (memcmp(elf_header.e_ident, elf_magic, SELFMAG) == 0) {
printf("We are dealing with an ELF file. \n");
} else {
return 0;
}
// Allocate memory for program headers
ph = (Elf64_Phdr *)malloc(elf_header.e_phnum * sizeof(Elf64_Phdr));
if (ph == NULL) {
perror("Failed to allocate memory for program headers");
close(fd);
return 1;
}
// seeks to the start of the program headers
if (lseek(fd, elf_header.e_phoff, SEEK_SET) == (off_t)-1) {
perror("Failed to seek to program headers.\n");
} else {
printf("Successfully seeked program headers. \n");
}
// Read all program headers into memory
if (read(fd, ph, elf_header.e_phnum * sizeof(Elf64_Phdr)) != elf_header.e_phnum * sizeof(Elf64_Phdr)) {
perror("Failed to read program headers");
free(ph);
close(fd);
return 1;
}
printf("Successfully read program headers.\n");
header = &elf_header;
global_fd = fd;
printf("addr of elf_header %p\n", header);
printf("Elf loading complete.\n");
return 0;
}
/**
* Routine for checking stack made for child program.
* top_of_stack: stack pointer that will given to child program as %rsp
* argc: Expected number of arguments
* argv: Expected argument strings
*/
void stack_check(void* top_of_stack, uint64_t argc, char** argv) {
printf("----- stack check -----\n");
assert(((uint64_t)top_of_stack) % 8 == 0);
printf("top of stack is 8-byte aligned\n");
uint64_t* stack = top_of_stack;
uint64_t actual_argc = *(stack++);
printf("argc: %lu\n", actual_argc);
assert(actual_argc == argc);
for (int i = 0; i < argc; i++) {
char* argp = (char*)*(stack++);
assert(strcmp(argp, argv[i]) == 0);
printf("arg %d: %s\n", i, argp);
}
// Argument list ends with null pointer
assert(*(stack++) == 0);
int envp_count = 0;
while (*(stack++) != 0)
envp_count++;
printf("env count: %d\n", envp_count);
Elf64_auxv_t* auxv_start = (Elf64_auxv_t*)stack;
Elf64_auxv_t* auxv_null = auxv_start;
while (auxv_null->a_type != AT_NULL) {
auxv_null++;
}
printf("aux count: %lu\n", auxv_null - auxv_start);
printf("----- end stack check -----\n");
}
#define DEFAULT_STACK_SIZE_KB 80
typedef struct {
void *base;
size_t size;
} stack_info_t;
int allocate_stack(stack_info_t *stack, size_t size_kb, void *desired_addr) {
long page_size = sysconf(_SC_PAGESIZE);
if (page_size == -1) {
perror("Failed to get page size");
return -1;
}
size_t stack_size = size_kb * 1024;
if (stack_size % page_size != 0) {
stack_size = ((stack_size / page_size) + 1) * page_size;
}
void *stack_base = mmap(desired_addr, stack_size, PROT_READ | PROT_WRITE,
MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
if (stack_base == MAP_FAILED) {
perror("Failed to allocate stack");
return -1;
}
stack->base = stack_base;
stack->size = stack_size;
return 0;
}
void free_stack(stack_info_t *stack) {
if (stack->base != NULL) {
if (munmap(stack->base, stack->size) == -1) {
perror("Failed to free stack");
}
stack->base = NULL;
stack->size = 0;
}
}
int setup_the_stack(int argc, char *argv[], char *envp[],
Elf64_Ehdr *elf_header) {
stack_info_t stack;
void *desired_addr = (void *)0x7000000;
size_t stack_size_kb = DEFAULT_STACK_SIZE_KB;
// allocate the stack
if (allocate_stack(&stack, stack_size_kb, desired_addr) == -1) {
fprintf(stderr, "Failed to allocate stack\n");
return 1;
}
printf("Stack allocated successfully:\n");
printf(" Base address: %p\n", stack.base);
printf(" Size: %zu bytes\n", stack.size);
void *stack_top = (void *)(stack.base + stack.size);
int num_env_vars = count_env_vars();
printf("Number of environment variables: %d\n", num_env_vars);
// Allocate memory for env_vars strings
size_t env_vars_total_len = 0;
for (int i = 0; envp[i] != NULL; i++) {
env_vars_total_len += strlen(envp[i]) + 1;
}
char *env_vars_buffer = (char *)malloc(env_vars_total_len);
char *env_vars_ptr = env_vars_buffer;
for (int i = 0; envp[i] != NULL; i++) {
size_t len = strlen(envp[i]);
memcpy(env_vars_ptr, envp[i], len + 1);
envp[i] = env_vars_ptr;
env_vars_ptr += len + 1;
}
// Allocate memory for cmd_args strings
size_t cmd_args_total_len = 0;
for (int i = 0; i < argc; i++) {
cmd_args_total_len += strlen(argv[i]) + 1;
}
char *cmd_args_buffer = (char *)malloc(cmd_args_total_len);
char *cmd_args_ptr = cmd_args_buffer;
for (int i = 0; i < argc; i++) {
size_t len = strlen(argv[i]);
memcpy(cmd_args_ptr, argv[i], len + 1);
argv[i] = cmd_args_ptr;
cmd_args_ptr += len + 1;
}
// calculate the address of auxv using pointer arithmetic
Elf64_auxv_t *auxv = (Elf64_auxv_t *)(envp + num_env_vars + 1);
// count the number of auxiliary vector entries
int aux_entries = count_auxv_entries(auxv);
printf("Number of auxiliary vector entries: %d\n", aux_entries);
Elf64_auxv_t *vectors =
(Elf64_auxv_t *)malloc(aux_entries * sizeof(Elf64_auxv_t));
if (vectors == NULL) {
perror("Failed to allocate auxiliary vector");
return -1;
}
memset(vectors, 0, aux_entries * sizeof(Elf64_auxv_t));
Elf64_auxv_t *auxv_ptr = (Elf64_auxv_t *)auxv;
memcpy(vectors, auxv_ptr, aux_entries * sizeof(Elf64_auxv_t));
size_t stack_ptr = (size_t)stack_top;
stack_ptr -= aux_entries * sizeof(Elf64_auxv_t);
stack_ptr -= (argc + num_env_vars + 2) * sizeof(char *);
stack_top =
(char **)((stack_ptr & ~(STACK_ALIGNMENT - 1)) & ~(STACK_ALIGNMENT - 1));
Elf64_Addr stack_top_addr = (Elf64_Addr)stack_top;
*(long *)stack_top = (long)argc;
stack_top += sizeof(long);
char **argv_ptr = (char **)stack_top;
stack_top += sizeof(char *) * argc;
argv_ptr[0] = cmd_args_buffer;
cmd_args_buffer +=
strlen(cmd_args_buffer) + 1;
*(char **)stack_top = NULL;
stack_top += sizeof(char *);
char **envp_ptr = (char **)stack_top;
stack_top += sizeof(char *) * num_env_vars;
// Direct manipulation of env_vars_buffer within the loop
for (int i = 0; i < num_env_vars - 1; ++i) {
envp_ptr[i] = env_vars_buffer;
env_vars_buffer +=
strlen(env_vars_buffer) + 1;
}
envp_ptr[num_env_vars - 1] = NULL;
memcpy(stack_top, vectors, sizeof(Elf64_auxv_t) * aux_entries);
stack_top += sizeof(Elf64_auxv_t) * aux_entries;
stack_check((void *)stack_top_addr, argc, (char **)argv_ptr);
printf("Stack setup completed\n");
printf("Beginning to execute assembly code\n");
// // using the neat trick described in handout:
// // will use the ret instruction to perform a return, which will pop the
// // address from the top of the stack and jump to that address.
asm("xor %rax, %rax");
asm("xor %rbx, %rbx");
asm("xor %rcx, %rcx");
asm("xor %rdx, %rdx");
asm("mov %0, %%rsp" : : "r"(stack_top_addr));
asm("push %0" : : "r"(e_entry));
asm("ret");
free_stack(&stack);
return 0;
}
int count_auxv_entries(Elf64_auxv_t *auxv) {
int count = 0;
while (auxv->a_type != AT_NULL) {
count++;
auxv++;
}
return count;
}
int count_env_vars_recursive(char **env) {
if (*env == NULL) {
return 0;
}
return 1 + count_env_vars_recursive(env + 1);
}
int count_env_vars() { return count_env_vars_recursive(environ); }
// void segv_handler(int sig, siginfo_t *info, void *ucontext) {
// void *fault_addr = info->si_addr;
// printf("Handling SIGSEGV at address: %p\n", fault_addr);
// printf("Global fd: %d\n", global_fd);
// // Determine the system's page size
// size_t page_size = sysconf(_SC_PAGE_SIZE);
// // Iterate over program headers to find the relevant segment
// for (int i = 0; i < elf_header.e_phnum; ++i) {
// Elf64_Phdr phdr = ph[i];
// if (phdr.p_type == PT_LOAD) {
// uintptr_t start_addr = phdr.p_vaddr;
// uintptr_t end_addr = start_addr + phdr.p_memsz;
// if (fault_addr >= (void *)start_addr && fault_addr < (void *)end_addr) {
// // Fault address is within the current loadable segment
// int prot = PROT_READ | PROT_WRITE | PROT_EXEC;
// int flags = MAP_PRIVATE | MAP_ANON;
// // Calculate the page-aligned start address and adjusted map size
// uintptr_t page_aligned_start_addr = start_addr & ~(page_size - 1);
// size_t offset = start_addr - page_aligned_start_addr;
// size_t adjusted_map_size = phdr.p_memsz + offset;
// // Use page_aligned_start_addr for mmap
// void *segment = mmap((void *)page_aligned_start_addr, adjusted_map_size, prot, flags, -1, 0);
// if (segment == MAP_FAILED) {
// perror("Failed to mmap segment");
// exit(1);
// }
// // Copy segment data from the ELF file into the mapped memory
// if (pread(global_fd, segment + offset, phdr.p_filesz, phdr.p_offset) != phdr.p_filesz) {
// perror("Failed to read segment data");
// exit(1);
// }
// printf("mmap result: %p, page-aligned start addr: %p, map size: %zu, protection flags: %d\n",
// segment, (void *)page_aligned_start_addr, adjusted_map_size, prot);
// return; // Successfully handled the segmentation fault
// }
// }
// }
// // If we reach here, the fault address was not within a loadable segment
// fprintf(stderr, "Invalid memory access at address: %p\n", fault_addr);
// exit(1);
// }
void segv_handler(int sig, siginfo_t *info, void *ucontext) {
// Print basic information about the signal received
printf("Received signal: %d\n", sig);
// Access the faulting address from the siginfo_t structure
void *fault_addr = info->si_addr;
// if (fault_addr == NULL) {
// force_seg_fault();
// }
printf("Handling SIGSEGV at address: %p\n", fault_addr);
printf("Global fd: %d\n", global_fd);
// Determine the system's page size for memory mapping
size_t page_size = sysconf(_SC_PAGE_SIZE);
printf("System page size: %zu bytes\n", page_size);
// Iterate over program headers to find if the fault address falls within a segment
int segment_found = 0;
for (int i = 0; i < elf_header.e_phnum; i++) {
Elf64_Phdr phdr = ph[i];
// Check if the program header is for a loadable segment
if (phdr.p_type == PT_LOAD) {
uintptr_t start_addr = phdr.p_vaddr;
uintptr_t end_addr = start_addr + phdr.p_memsz;
// Check if the fault address is within the segment
if (fault_addr >= (void *)start_addr && fault_addr < (void *)end_addr) {
segment_found = 1;
printf("Fault address is within segment [%d]: %p - %p\n", i, (void *)start_addr, (void *)end_addr);
int prot = PROT_READ | PROT_WRITE | PROT_EXEC;
int flags = MAP_PRIVATE | MAP_ANON;
// Calculate the page-aligned address of the faulting page
uintptr_t page_aligned_fault_addr = (uintptr_t)fault_addr & ~(page_size - 1);
// Calculate the file offset for mapping, ensuring it's page-aligned
off_t file_offset = phdr.p_offset + (page_aligned_fault_addr - start_addr);
// Map only the page that caused the fault
void *segment = mmap((void *)page_aligned_fault_addr, page_size, prot, flags, -1, 0);
if (segment == MAP_FAILED) {
perror("Failed to mmap segment");
exit(1);
}
// Check if the segment data is large enough to read
if (phdr.p_filesz + phdr.p_vaddr >= page_aligned_fault_addr) {
// Calculate the size of data to read based on file and memory sizes
size_t read_size = phdr.p_filesz - (page_aligned_fault_addr - start_addr);
if (read_size > page_size) {
read_size = page_size;
}
// Read the segment data from the file into the mapped area
if (pread(global_fd, page_aligned_fault_addr, read_size, file_offset) != read_size) {
perror("Failed to read segment data");
exit(1);
}
printf("Mapped and read segment successfully. Address: %p, Size: %zu bytes\n", segment, read_size);
printf("Offset: %ld\n", file_offset);
return;
} else {
printf("Mapped segment successfully. Address: %p, Size: %zu bytes\n", segment, 0);
return;
}
}
}
}
if (!segment_found) {
fprintf(stderr, "Invalid memory access at address: %p\n", fault_addr);
fprintf(stderr, "Fault address does not fall within any loadable segment\n");
exit(1);
}
}
// void force_seg_fault() {
// int *zero = NULL;
// return *zero;
// }
void setup_signal_handler() {
struct sigaction sa;
memset(&sa, 0, sizeof(struct sigaction));
sa.sa_sigaction = segv_handler;
sa.sa_flags = SA_SIGINFO;
if (sigaction(SIGSEGV, &sa, NULL) == -1) {
perror("Failed to set up signal handler");
exit(1);
}
}
int main(int argc, char *argv[], char *envp[]) {
Elf64_Ehdr header;
load_elf_binary(argc, argv, &header);
setup_signal_handler();
setup_the_stack(argc - 1, &argv[1], envp, &header);
return 0;
}