text: // SPDX-License-Identifier: GPL-2.0
  /*
   * kaslr.c
   *
   * This contains the routines needed to generate a reasonable level of
   * entropy to choose a randomized kernel base address offset in support
   * of Kernel Address Space Layout Randomization (KASLR). Additionally
   * handles walking the physical memory maps (and tracking memory regions
   * to avoid) in order to select a physical memory location that can
   * contain the entire properly aligned running kernel image.
   *
   */
  
  /*
   * isspace() in linux/ctype.h is expected by next_args() to filter
   * out "space/lf/tab". While boot/ctype.h conflicts with linux/ctype.h,
   * since isdigit() is implemented in both of them. Hence disable it
   * here.
   */
  #define BOOT_CTYPE_H
  
  /*
   * _ctype[] in lib/ctype.c is needed by isspace() of linux/ctype.h.
   * While both lib/ctype.c and lib/cmdline.c will bring EXPORT_SYMBOL
   * which is meaningless and will cause compiling error in some cases.
   */
  #define __DISABLE_EXPORTS
  
  #include "misc.h"
  #include "error.h"
  #include "../string.h"
  
  #include <generated/compile.h>
  #include <linux/module.h>
  #include <linux/uts.h>
  #include <linux/utsname.h>
  #include <linux/ctype.h>
  #include <linux/efi.h>
  #include <generated/utsrelease.h>
  #include <asm/efi.h>
  
  /* Macros used by the included decompressor code below. */
  #define STATIC
  #include <linux/decompress/mm.h>
  
  #ifdef CONFIG_X86_5LEVEL
  unsigned int __pgtable_l5_enabled;
  unsigned int pgdir_shift __ro_after_init = 39;
  unsigned int ptrs_per_p4d __ro_after_init = 1;
  #endif
  
  extern unsigned long get_cmd_line_ptr(void);
  
  /* Used by PAGE_KERN* macros: */
  pteval_t __default_kernel_pte_mask __read_mostly = ~0;
  
  /* Simplified build-specific string for starting entropy. */
  static const char build_str[] = UTS_RELEASE " (" LINUX_COMPILE_BY "@"
  		LINUX_COMPILE_HOST ") (" LINUX_COMPILER ") " UTS_VERSION;
  
  static unsigned long rotate_xor(unsigned long hash, const void *area,
  				size_t size)
  {
  	size_t i;
  	unsigned long *ptr = (unsigned long *)area;
  
  	for (i = 0; i < size / sizeof(hash); i++) {
  		/* Rotate by odd number of bits and XOR. */
  		hash = (hash << ((sizeof(hash) * 8) - 7)) | (hash >> 7);
  		hash ^= ptr[i];
  	}
  
  	return hash;
  }
  
  /* Attempt to create a simple but unpredictable starting entropy. */
  static unsigned long get_boot_seed(void)
  {
  	unsigned long hash = 0;
  
  	hash = rotate_xor(hash, build_str, sizeof(build_str));
  	hash = rotate_xor(hash, boot_params, sizeof(*boot_params));
  
  	return hash;
  }
  
  #define KASLR_COMPRESSED_BOOT
  #include "../../lib/kaslr.c"
  
  struct mem_vector {
  	unsigned long long start;
  	unsigned long long size;
  };
  
  /* Only supporting at most 4 unusable memmap regions with kaslr */
  #define MAX_MEMMAP_REGIONS	4
  
  static bool memmap_too_large;
  
  
  /* Store memory limit specified by "mem=nn[KMG]" or "memmap=nn[KMG]" */
  static unsigned long long mem_limit = ULLONG_MAX;
  
  
  enum mem_avoid_index {
  	MEM_AVOID_ZO_RANGE = 0,
  	MEM_AVOID_INITRD,
  	MEM_AVOID_CMDLINE,
  	MEM_AVOID_BOOTPARAMS,
  	MEM_AVOID_MEMMAP_BEGIN,
  	MEM_AVOID_MEMMAP_END = MEM_AVOID_MEMMAP_BEGIN + MAX_MEMMAP_REGIONS - 1,
  	MEM_AVOID_MAX,
  };
  
  static struct mem_vector mem_avoid[MEM_AVOID_MAX];
  
  static bool mem_overlaps(struct mem_vector *one, struct mem_vector *two)
  {
  	/* Item one is entirely before item two. */
  	if (one->start + one->size <= two->start)
  		return false;
  	/* Item one is entirely after item two. */
  	if (one->start >= two->start + two->size)
  		return false;
  	return true;
  }
  
  char *skip_spaces(const char *str)
  {
  	while (isspace(*str))
  		++str;
  	return (char *)str;
  }
  #include "../../../../lib/ctype.c"
  #include "../../../../lib/cmdline.c"
  
  static int
  parse_memmap(char *p, unsigned long long *start, unsigned long long *size)
  {
  	char *oldp;
  
  	if (!p)
  		return -EINVAL;
  
  	/* We don't care about this option here */
  	if (!strncmp(p, "exactmap", 8))
  		return -EINVAL;
  
  	oldp = p;
  	*size = memparse(p, &p);
  	if (p == oldp)
  		return -EINVAL;
  
  	switch (*p) {
  	case '#':
  	case '$':
  	case '!':
  		*start = memparse(p + 1, &p);
  		return 0;
  	case '@':
  		/* memmap=nn@ss specifies usable region, should be skipped */
  		*size = 0;
  		/* Fall through */
  	default:
  		/*
  		 * If w/o offset, only size specified, memmap=nn[KMG] has the
  		 * same behaviour as mem=nn[KMG]. It limits the max address
  		 * system can use. Region above the limit should be avoided.
  		 */
  		*start = 0;
  		return 0;
  	}
  
  	return -EINVAL;
  }
  
  static void mem_avoid_memmap(char *str)
  {
  	static int i;
  
  	if (i >= MAX_MEMMAP_REGIONS)
  		return;
  
  	while (str && (i < MAX_MEMMAP_REGIONS)) {
  		int rc;
  		unsigned long long start, size;
  		char *k = strchr(str, ',');
  
  		if (k)
  			*k++ = 0;
  
  		rc = parse_memmap(str, &start, &size);
  		if (rc < 0)
  			break;
  		str = k;
  
  		if (start == 0) {
  			/* Store the specified memory limit if size > 0 */
  			if (size > 0)
  				mem_limit = size;
  
  			continue;
  		}
  
  		mem_avoid[MEM_AVOID_MEMMAP_BEGIN + i].start = start;
  		mem_avoid[MEM_AVOID_MEMMAP_BEGIN + i].size = size;
  		i++;
  	}
  
  	/* More than 4 memmaps, fail kaslr */
  	if ((i >= MAX_MEMMAP_REGIONS) && str)
  		memmap_too_large = true;
  }
  
  /* Store the number of 1GB huge pages which users specified: */
  static unsigned long max_gb_huge_pages;
  
  static void parse_gb_huge_pages(char *param, char *val)
  {
  	static bool gbpage_sz;
  	char *p;
  
  	if (!strcmp(param, "hugepagesz")) {
  		p = val;
  		if (memparse(p, &p) != PUD_SIZE) {
  			gbpage_sz = false;
  			return;
  		}
  
  		if (gbpage_sz)
  			warn("Repeatedly set hugeTLB page size of 1G!\n");
  		gbpage_sz = true;
  		return;
  	}
  
  	if (!strcmp(param, "hugepages") && gbpage_sz) {
  		p = val;
  		max_gb_huge_pages = simple_strtoull(p, &p, 0);
  		return;
  	}
  }
  
  
  static int handle_mem_options(void)
  {
  	char *args = (char *)get_cmd_line_ptr();
  	size_t len = strlen((char *)args);
  	char *tmp_cmdline;
  	char *param, *val;
  	u64 mem_size;
  
  	if (!strstr(args, "memmap=") && !strstr(args, "mem=") &&
  		!strstr(args, "hugepages"))
  		return 0;
  
  	tmp_cmdline = malloc(len + 1);
  	if (!tmp_cmdline)
  		error("Failed to allocate space for tmp_cmdline");
  
  	memcpy(tmp_cmdline, args, len);
  	tmp_cmdline[len] = 0;
  	args = tmp_cmdline;
  
  	/* Chew leading spaces */
  	args = skip_spaces(args);
  
  	while (*args) {
  		args = next_arg(args, &param, &val);
  		/* Stop at -- */
  		if (!val && strcmp(param, "--") == 0) {
  			warn("Only '--' specified in cmdline");
  			free(tmp_cmdline);
  			return -1;
  		}
  
  		if (!strcmp(param, "memmap")) {
  			mem_avoid_memmap(val);
  		} else if (strstr(param, "hugepages")) {
  			parse_gb_huge_pages(param, val);
  		} else if (!strcmp(param, "mem")) {
  			char *p = val;
  
  			if (!strcmp(p, "nopentium"))
  				continue;
  			mem_size = memparse(p, &p);
  			if (mem_size == 0) {
  				free(tmp_cmdline);
  				return -EINVAL;
  			}
  			mem_limit = mem_size;
  		}
  	}
  
  	free(tmp_cmdline);
  	return 0;
  }
  
  /*
   * In theory, KASLR can put the kernel anywhere in the range of [16M, 64T).
   * The mem_avoid array is used to store the ranges that need to be avoided
   * when KASLR searches for an appropriate random address. We must avoid any
   * regions that are unsafe to overlap with during decompression, and other
   * things like the initrd, cmdline and boot_params. This comment seeks to
   * explain mem_avoid as clearly as possible since incorrect mem_avoid
   * memory ranges lead to really hard to debug boot failures.
   *
   * The initrd, cmdline, and boot_params are trivial to identify for
   * avoiding. They are MEM_AVOID_INITRD, MEM_AVOID_CMDLINE, and
   * MEM_AVOID_BOOTPARAMS respectively below.
   *
   * What is not obvious how to avoid is the range of memory that is used
   * during decompression (MEM_AVOID_ZO_RANGE below). This range must cover
   * the compressed kernel (ZO) and its run space, which is used to extract
   * the uncompressed kernel (VO) and relocs.
   *
   * ZO's full run size sits against the end of the decompression buffer, so
   * we can calculate where text, data, bss, etc of ZO are positioned more
   * easily.
   *
   * For additional background, the decompression calculations can be found
   * in header.S, and the memory diagram is based on the one found in misc.c.
   *
   * The following conditions are already enforced by the image layouts and
   * associated code:
   *  - input + input_size >= output + output_size
   *  - kernel_total_size <= init_size
   *  - kernel_total_size <= output_size (see Note below)
   *  - output + init_size >= output + output_size
   *
   * (Note that kernel_total_size and output_size have no fundamental
   * relationship, but output_size is passed to choose_random_location
   * as a maximum of the two. The diagram is showing a case where
   * kernel_total_size is larger than output_size, but this case is
   * handled by bumping output_size.)
   *
   * The above conditions can be illustrated by a diagram:
   *
   * 0   output            input            input+input_size    output+init_size
   * |     |                 |                             |             |
   * |     |                 |                             |             |
   * |-----|--------|--------|--------------|-----------|--|-------------|
   *                |                       |           |
   *                |                       |           |
   * output+init_size-ZO_INIT_SIZE  output+output_size  output+kernel_total_size
   *
   * [output, output+init_size) is the entire memory range used for
   * extracting the compressed image.
   *
   * [output, output+kernel_total_size) is the range needed for the
   * uncompressed kernel (VO) and its run size (bss, brk, etc).
   *
   * [output, output+output_size) is VO plus relocs (i.e. the entire
   * uncompressed payload contained by ZO). This is the area of the buffer
   * written to during decompression.
   *
   * [output+init_size-ZO_INIT_SIZE, output+init_size) is the worst-case
   * range of the copied ZO and decompression code. (i.e. the range
   * covered backwards of size ZO_INIT_SIZE, starting from output+init_size.)
   *
   * [input, input+input_size) is the original copied compressed image (ZO)
   * (i.e. it does not include its run size). This range must be avoided
   * because it contains the data used for decompression.
   *
   * [input+input_size, output+init_size) is [_text, _end) for ZO. This
   * range includes ZO's heap and stack, and must be avoided since it
   * performs the decompression.
   *
   * Since the above two ranges need to be avoided and they are adjacent,
   * they can be merged, resulting in: [input, output+init_size) which
   * becomes the MEM_AVOID_ZO_RANGE below.
   */
  static void mem_avoid_init(unsigned long input, unsigned long input_size,
  			   unsigned long output)
  {
  	unsigned long init_size = boot_params->hdr.init_size;
  	u64 initrd_start, initrd_size;
  	u64 cmd_line, cmd_line_size;
  	char *ptr;
  
  	/*
  	 * Avoid the region that is unsafe to overlap during
  	 * decompression.
  	 */
  	mem_avoid[MEM_AVOID_ZO_RANGE].start = input;
  	mem_avoid[MEM_AVOID_ZO_RANGE].size = (output + init_size) - input;
  	add_identity_map(mem_avoid[MEM_AVOID_ZO_RANGE].start,
  			 mem_avoid[MEM_AVOID_ZO_RANGE].size);
  
  	/* Avoid initrd. */
  	initrd_start  = (u64)boot_params->ext_ramdisk_image << 32;
  	initrd_start |= boot_params->hdr.ramdisk_image;
  	initrd_size  = (u64)boot_params->ext_ramdisk_size << 32;
  	initrd_size |= boot_params->hdr.ramdisk_size;
  	mem_avoid[MEM_AVOID_INITRD].start = initrd_start;
  	mem_avoid[MEM_AVOID_INITRD].size = initrd_size;
  	/* No need to set mapping for initrd, it will be handled in VO. */
  
  	/* Avoid kernel command line. */
  	cmd_line  = (u64)boot_params->ext_cmd_line_ptr << 32;
  	cmd_line |= boot_params->hdr.cmd_line_ptr;
  	/* Calculate size of cmd_line. */
  	ptr = (char *)(unsigned long)cmd_line;
  	for (cmd_line_size = 0; ptr[cmd_line_size++];)
  		;
  	mem_avoid[MEM_AVOID_CMDLINE].start = cmd_line;
  	mem_avoid[MEM_AVOID_CMDLINE].size = cmd_line_size;
  	add_identity_map(mem_avoid[MEM_AVOID_CMDLINE].start,
  			 mem_avoid[MEM_AVOID_CMDLINE].size);
  
  	/* Avoid boot parameters. */
  	mem_avoid[MEM_AVOID_BOOTPARAMS].start = (unsigned long)boot_params;
  	mem_avoid[MEM_AVOID_BOOTPARAMS].size = sizeof(*boot_params);
  	add_identity_map(mem_avoid[MEM_AVOID_BOOTPARAMS].start,
  			 mem_avoid[MEM_AVOID_BOOTPARAMS].size);
  
  	/* We don't need to set a mapping for setup_data. */
  
  	/* Mark the memmap regions we need to avoid */
  	handle_mem_options();
  
  #ifdef CONFIG_X86_VERBOSE_BOOTUP
  	/* Make sure video RAM can be used. */
  	add_identity_map(0, PMD_SIZE);
  #endif
  }
  
  /*
   * Does this memory vector overlap a known avoided area? If so, record the
   * overlap region with the lowest address.
   */
  static bool mem_avoid_overlap(struct mem_vector *img,
  			      struct mem_vector *overlap)
  {
  	int i;
  	struct setup_data *ptr;
  	unsigned long earliest = img->start + img->size;
  	bool is_overlapping = false;
  
  	for (i = 0; i < MEM_AVOID_MAX; i++) {
  		if (mem_overlaps(img, &mem_avoid[i]) &&
  		    mem_avoid[i].start < earliest) {
  			*overlap = mem_avoid[i];
  			earliest = overlap->start;
  			is_overlapping = true;
  		}
  	}
  
  	/* Avoid all entries in the setup_data linked list. */
  	ptr = (struct setup_data *)(unsigned long)boot_params->hdr.setup_data;
  	while (ptr) {
  		struct mem_vector avoid;
  
  		avoid.start = (unsigned long)ptr;
  		avoid.size = sizeof(*ptr) + ptr->len;
  
  		if (mem_overlaps(img, &avoid) && (avoid.start < earliest)) {
  			*overlap = avoid;
  			earliest = overlap->start;
  			is_overlapping = true;
  		}
  
  		ptr = (struct setup_data *)(unsigned long)ptr->next;
  	}
  
  	return is_overlapping;
  }
  
  struct slot_area {
  	unsigned long addr;
  	int num;
  };
  
  #define MAX_SLOT_AREA 100
  
  static struct slot_area slot_areas[MAX_SLOT_AREA];
  
  static unsigned long slot_max;
  
  static unsigned long slot_area_index;
  
  static void store_slot_info(struct mem_vector *region, unsigned long image_size)
  {
  	struct slot_area slot_area;
  
  	if (slot_area_index == MAX_SLOT_AREA)
  		return;
  
  	slot_area.addr = region->start;
  	slot_area.num = (region->size - image_size) /
  			CONFIG_PHYSICAL_ALIGN + 1;
  
  	if (slot_area.num > 0) {
  		slot_areas[slot_area_index++] = slot_area;
  		slot_max += slot_area.num;
  	}
  }
  
  /*
   * Skip as many 1GB huge pages as possible in the passed region
   * according to the number which users specified:
   */
  static void
  process_gb_huge_pages(struct mem_vector *region, unsigned long image_size)
  {
  	unsigned long addr, size = 0;
  	struct mem_vector tmp;
  	int i = 0;
  
  	if (!max_gb_huge_pages) {
  		store_slot_info(region, image_size);
  		return;
  	}
  
  	addr = ALIGN(region->start, PUD_SIZE);
  	/* Did we raise the address above the passed in memory entry? */
  	if (addr < region->start + region->size)
  		size = region->size - (addr - region->start);
  
  	/* Check how many 1GB huge pages can be filtered out: */
  	while (size > PUD_SIZE && max_gb_huge_pages) {
  		size -= PUD_SIZE;
  		max_gb_huge_pages--;
  		i++;
  	}
  
  	/* No good 1GB huge pages found: */
  	if (!i) {
  		store_slot_info(region, image_size);
  		return;
  	}
  
  	/*
  	 * Skip those 'i'*1GB good huge pages, and continue checking and
  	 * processing the remaining head or tail part of the passed region
  	 * if available.
  	 */
  
  	if (addr >= region->start + image_size) {
  		tmp.start = region->start;
  		tmp.size = addr - region->start;
  		store_slot_info(&tmp, image_size);
  	}
  
  	size  = region->size - (addr - region->start) - i * PUD_SIZE;
  	if (size >= image_size) {
  		tmp.start = addr + i * PUD_SIZE;
  		tmp.size = size;
  		store_slot_info(&tmp, image_size);
  	}
  }
  
  static unsigned long slots_fetch_random(void)
  {
  	unsigned long slot;
  	int i;
  
  	/* Handle case of no slots stored. */
  	if (slot_max == 0)
  		return 0;
  
  	slot = kaslr_get_random_long("Physical") % slot_max;
  
  	for (i = 0; i < slot_area_index; i++) {
  		if (slot >= slot_areas[i].num) {
  			slot -= slot_areas[i].num;
  			continue;
  		}
  		return slot_areas[i].addr + slot * CONFIG_PHYSICAL_ALIGN;
  	}
  
  	if (i == slot_area_index)
  		debug_putstr("slots_fetch_random() failed!?\n");
  	return 0;
  }
  
  static void process_mem_region(struct mem_vector *entry,
  			       unsigned long minimum,
  			       unsigned long image_size)
  {
  	struct mem_vector region, overlap;
  	struct slot_area slot_area;
  	unsigned long start_orig, end;
  	struct mem_vector cur_entry;
  
  	/* On 32-bit, ignore entries entirely above our maximum. */
  	if (IS_ENABLED(CONFIG_X86_32) && entry->start >= KERNEL_IMAGE_SIZE)
  		return;
  
  	/* Ignore entries entirely below our minimum. */
  	if (entry->start + entry->size < minimum)
  		return;
  
  	/* Ignore entries above memory limit */
  	end = min(entry->size + entry->start, mem_limit);
  	if (entry->start >= end)
  		return;
  	cur_entry.start = entry->start;
  	cur_entry.size = end - entry->start;
  
  	region.start = cur_entry.start;
  	region.size = cur_entry.size;
  
  	/* Give up if slot area array is full. */
  	while (slot_area_index < MAX_SLOT_AREA) {
  		start_orig = region.start;
  
  		/* Potentially raise address to minimum location. */
  		if (region.start < minimum)
  			region.start = minimum;
  
  		/* Potentially raise address to meet alignment needs. */
  		region.start = ALIGN(region.start, CONFIG_PHYSICAL_ALIGN);
  
  		/* Did we raise the address above the passed in memory entry? */
  		if (region.start > cur_entry.start + cur_entry.size)
  			return;
  
  		/* Reduce size by any delta from the original address. */
  		region.size -= region.start - start_orig;
  
  		/* On 32-bit, reduce region size to fit within max size. */
  		if (IS_ENABLED(CONFIG_X86_32) &&
  		    region.start + region.size > KERNEL_IMAGE_SIZE)
  			region.size = KERNEL_IMAGE_SIZE - region.start;
  
  		/* Return if region can't contain decompressed kernel */
  		if (region.size < image_size)
  			return;
  
  		/* If nothing overlaps, store the region and return. */
  		if (!mem_avoid_overlap(&region, &overlap)) {
  			process_gb_huge_pages(&region, image_size);
  			return;
  		}
  
  		/* Store beginning of region if holds at least image_size. */
  		if (overlap.start > region.start + image_size) {
  			struct mem_vector beginning;
  
  			beginning.start = region.start;
  			beginning.size = overlap.start - region.start;
  			process_gb_huge_pages(&beginning, image_size);
  		}
  
  		/* Return if overlap extends to or past end of region. */
  		if (overlap.start + overlap.size >= region.start + region.size)
  			return;
  
  		/* Clip off the overlapping region and start over. */
  		region.size -= overlap.start - region.start + overlap.size;
  		region.start = overlap.start + overlap.size;
  	}
  }
  
  #ifdef CONFIG_EFI
  /*
   * Returns true if mirror region found (and must have been processed
   * for slots adding)
   */
  static bool
  process_efi_entries(unsigned long minimum, unsigned long image_size)
  {
  	struct efi_info *e = &boot_params->efi_info;
  	bool efi_mirror_found = false;
  	struct mem_vector region;
  	efi_memory_desc_t *md;
  	unsigned long pmap;
  	char *signature;
  	u32 nr_desc;
  	int i;
  
  	signature = (char *)&e->efi_loader_signature;
  	if (strncmp(signature, EFI32_LOADER_SIGNATURE, 4) &&
  	    strncmp(signature, EFI64_LOADER_SIGNATURE, 4))
  		return false;
  
  #ifdef CONFIG_X86_32
  	/* Can't handle data above 4GB at this time */
  	if (e->efi_memmap_hi) {
  		warn("EFI memmap is above 4GB, can't be handled now on x86_32. EFI should be disabled.\n");
  		return false;
  	}
  	pmap =  e->efi_memmap;
  #else
  	pmap = (e->efi_memmap | ((__u64)e->efi_memmap_hi << 32));
  #endif
  
  	nr_desc = e->efi_memmap_size / e->efi_memdesc_size;
  	for (i = 0; i < nr_desc; i++) {
  		md = efi_early_memdesc_ptr(pmap, e->efi_memdesc_size, i);
  		if (md->attribute & EFI_MEMORY_MORE_RELIABLE) {
  			efi_mirror_found = true;
  			break;
  		}
  	}
  
  	for (i = 0; i < nr_desc; i++) {
  		md = efi_early_memdesc_ptr(pmap, e->efi_memdesc_size, i);
  
  		/*
  		 * Here we are more conservative in picking free memory than
  		 * the EFI spec allows:
  		 *
  		 * According to the spec, EFI_BOOT_SERVICES_{CODE|DATA} are also
  		 * free memory and thus available to place the kernel image into,
  		 * but in practice there's firmware where using that memory leads
  		 * to crashes.
  		 *
  		 * Only EFI_CONVENTIONAL_MEMORY is guaranteed to be free.
  		 */
  		if (md->type != EFI_CONVENTIONAL_MEMORY)
  			continue;
  
  		if (efi_mirror_found &&
  		    !(md->attribute & EFI_MEMORY_MORE_RELIABLE))
  			continue;
  
  		region.start = md->phys_addr;
  		region.size = md->num_pages << EFI_PAGE_SHIFT;
  		process_mem_region(&region, minimum, image_size);
  		if (slot_area_index == MAX_SLOT_AREA) {
  			debug_putstr("Aborted EFI scan (slot_areas full)!\n");
  			break;
  		}
  	}
  	return true;
  }
  #else
  static inline bool
  process_efi_entries(unsigned long minimum, unsigned long image_size)
  {
  	return false;
  }
  #endif
  
  static void process_e820_entries(unsigned long minimum,
  				 unsigned long image_size)
  {
  	int i;
  	struct mem_vector region;
  	struct boot_e820_entry *entry;
  
  	/* Verify potential e820 positions, appending to slots list. */
  	for (i = 0; i < boot_params->e820_entries; i++) {
  		entry = &boot_params->e820_table[i];
  		/* Skip non-RAM entries. */
  		if (entry->type != E820_TYPE_RAM)
  			continue;
  		region.start = entry->addr;
  		region.size = entry->size;
  		process_mem_region(&region, minimum, image_size);
  		if (slot_area_index == MAX_SLOT_AREA) {
  			debug_putstr("Aborted e820 scan (slot_areas full)!\n");
  			break;
  		}
  	}
  }
  
  static unsigned long find_random_phys_addr(unsigned long minimum,
  					   unsigned long image_size)
  {
  	/* Check if we had too many memmaps. */
  	if (memmap_too_large) {
  		debug_putstr("Aborted memory entries scan (more than 4 memmap= args)!\n");
  		return 0;
  	}
  
  	/* Make sure minimum is aligned. */
  	minimum = ALIGN(minimum, CONFIG_PHYSICAL_ALIGN);
  
  	if (process_efi_entries(minimum, image_size))
  		return slots_fetch_random();
  
  	process_e820_entries(minimum, image_size);
  	return slots_fetch_random();
  }
  
  static unsigned long find_random_virt_addr(unsigned long minimum,
  					   unsigned long image_size)
  {
  	unsigned long slots, random_addr;
  
  	/* Make sure minimum is aligned. */
  	minimum = ALIGN(minimum, CONFIG_PHYSICAL_ALIGN);
  	/* Align image_size for easy slot calculations. */
  	image_size = ALIGN(image_size, CONFIG_PHYSICAL_ALIGN);
  
  	/*
  	 * There are how many CONFIG_PHYSICAL_ALIGN-sized slots
  	 * that can hold image_size within the range of minimum to
  	 * KERNEL_IMAGE_SIZE?
  	 */
  	slots = (KERNEL_IMAGE_SIZE - minimum - image_size) /
  		 CONFIG_PHYSICAL_ALIGN + 1;
  
  	random_addr = kaslr_get_random_long("Virtual") % slots;
  
  	return random_addr * CONFIG_PHYSICAL_ALIGN + minimum;
  }
  
  /*
   * Since this function examines addresses much more numerically,
   * it takes the input and output pointers as 'unsigned long'.
   */
  void choose_random_location(unsigned long input,
  			    unsigned long input_size,
  			    unsigned long *output,
  			    unsigned long output_size,
  			    unsigned long *virt_addr)
  {
  	unsigned long random_addr, min_addr;
  
  	if (cmdline_find_option_bool("nokaslr")) {
  		warn("KASLR disabled: 'nokaslr' on cmdline.");
  		return;
  	}
  
  #ifdef CONFIG_X86_5LEVEL
  	if (__read_cr4() & X86_CR4_LA57) {
  		__pgtable_l5_enabled = 1;
  		pgdir_shift = 48;
  		ptrs_per_p4d = 512;
  	}
  #endif
  
  	boot_params->hdr.loadflags |= KASLR_FLAG;
  
  	/* Prepare to add new identity pagetables on demand. */
  	initialize_identity_maps();
  
  	/* Record the various known unsafe memory ranges. */
  	mem_avoid_init(input, input_size, *output);
  
  	/*
  	 * Low end of the randomization range should be the
  	 * smaller of 512M or the initial kernel image
  	 * location:
  	 */
  	min_addr = min(*output, 512UL << 20);
  
  	/* Walk available memory entries to find a random address. */
  	random_addr = find_random_phys_addr(min_addr, output_size);
  	if (!random_addr) {
  		warn("Physical KASLR disabled: no suitable memory region!");
  	} else {
  		/* Update the new physical address location. */
  		if (*output != random_addr) {
  			add_identity_map(random_addr, output_size);
  			*output = random_addr;
  		}
  
  		/*
  		 * This loads the identity mapping page table.
  		 * This should only be done if a new physical address
  		 * is found for the kernel, otherwise we should keep
  		 * the old page table to make it be like the "nokaslr"
  		 * case.
  		 */
  		finalize_identity_maps();
  	}
  
  
  	/* Pick random virtual address starting from LOAD_PHYSICAL_ADDR. */
  	if (IS_ENABLED(CONFIG_X86_64))
  		random_addr = find_random_virt_addr(LOAD_PHYSICAL_ADDR, output_size);
  	*virt_addr = random_addr;
  }
  