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Is access to memory that spans the zero boundary in x86 allowed?

Is it allowed for one access to span the bounary between 0and 0xFFFFFF...in x86 1 ?

For example, provided that eax( raxin the 64-bit version) is zero, the following access is allowed:

mov ebx, DWORD [eax - 2]

I am interested in both x86 (32-bit) and x86-64 if the answers are different.

1 Of course, given that the area is displayed in your process, etc.

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I just tested this EFI program. (And it worked as expected.) If you want to reproduce this result, you will need an efi_printf implementation or some other way to view the result.

#include <stdint.h>
#include "efi.h"

uint8_t *p = (uint8_t *)0xfffffffffffffffcULL;

int main()
{
    uint64_t cr3;
    asm("mov %%cr3, %0" : "=r"(cr3));
    uint64_t *pml4 = (uint64_t *)(cr3 & ~0xfffULL);

    efi_printf("cr3 %lx\n", cr3);
    efi_printf("pml4[0] %lx\n", pml4[0]);
    uint64_t *pdpt = (uint64_t *)(pml4[0] & ~0xfffULL);
    efi_printf("pdpt[0] %lx\n", pdpt[0]);
    if (!(pdpt[0] & 1)) {
        pdpt[0] = (uint64_t)efi_alloc_pages(EFI_BOOT_SERVICES_DATA, 1) | 0x03;
        efi_printf("pdpt[0] %lx\n", pdpt[0]);
    }
    uint64_t *pd = (uint64_t *)(pdpt[0] & ~0xfffULL);
    efi_printf("pd[0] %lx\n", pd[0]);
    if (!(pd[0] & 1)) {
        pd[0] = (uint64_t)efi_alloc_pages(EFI_BOOT_SERVICES_DATA, 1) | 0x03;
        efi_printf("pd[0] %lx\n", pd[0]);
    }
    if (!(pd[0] & 0x80)) {
        uint64_t *pt = (uint64_t *)(pd[0] & ~0xfffULL);
        efi_printf("pt[0] %lx\n", pt[0]);
        if (!(pt[0] & 1)) {
            pt[0] = (uint64_t)efi_alloc_pages(EFI_BOOT_SERVICES_DATA, 1) | 0x03;
            efi_printf("pt[0] %lx\n", pt[0]);
        }
    }

    efi_printf("[0] = %08x\n", *(uint32_t *)(p+4));

    efi_printf("pml4[0x1ff] %lx\n", pml4[0x1ff]);
    if (pml4[0x1ff] == 0) {

        uint64_t *pt = (uint64_t *)efi_alloc_pages(EFI_BOOT_SERVICES_DATA, 4);
        uint64_t x = (uint64_t)pt;

        efi_printf("pt = %p\n", pt);

        pml4[0x1ff] = x | 0x3;
        pt[0x1ff] = x + 0x1000 | 0x3;
        pt[0x3ff] = x + 0x2000 | 0x3;
        pt[0x5ff] = x + 0x3000 | 0x3;

        *(uint32_t *)p = 0xabcdabcd;
        *(uint32_t *)(p + 4) = 0x12341234;

        efi_printf("[0] = %08x\n", *(uint32_t *)(p+4));
        efi_printf("[fffffffffffc] = %08x\n", *(uint32_t *)(x + 0x3ffc));

        *(uint32_t *)(p + 2) = 0x56785678;

        efi_printf("p[0] = %08x\n", ((uint32_t *)p)[0]);
        efi_printf("p[1] = %08x\n", ((uint32_t *)p)[1]);
    }

    return 0;
}

, , 4 :

[0] = 12341234
[fffffffffffc] = ABCDABCD
p[0] = 5678ABCD
p[1] = 12345678

0x56785678 16- 16- .

: p , GCC *(p+4) ud2

+4

, . @prl, , gnu-efi, Linux. wraptest.c:

#include <efi.h>
#include <efiapi.h>
#include <efilib.h>
#include <inttypes.h>
#include <stdint.h>

uint8_t *p = (uint8_t *)0xfffffffffffffffcULL;

EFI_STATUS
EFIAPI
efi_main (EFI_HANDLE ImageHandle, EFI_SYSTEM_TABLE *SystemTable)
{
    uint64_t cr3;

    InitializeLib(ImageHandle, SystemTable);
    asm("mov %%cr3, %0" : "=r"(cr3));
    uint64_t *pml4 = (uint64_t *)(cr3 & ~0xfffULL);

    Print(L"cr3 %lx\n", cr3);
    Print(L"pml4[0] %lx\n", pml4[0]);
    uint64_t *pdpt = (uint64_t *)(pml4[0] & ~0xfffULL);
    Print(L"pdpt[0] %lx\n", pdpt[0]);
    if (!(pdpt[0] & 1)) {
        uefi_call_wrapper(BS->AllocatePages, 4, AllocateAnyPages, \
                          EfiBootServicesData, 1, &pdpt[0]);
        pdpt[0] |= 0x03;
        Print(L"pdpt[0] %lx\n", pdpt[0]);
    }
    uint64_t *pd = (uint64_t *)(pdpt[0] & ~0xfffULL);
    Print(L"pd[0] %lx\n", pd[0]);
    if (!(pd[0] & 1)) {
        uefi_call_wrapper(BS->AllocatePages, 4, AllocateAnyPages, \
                          EfiBootServicesData, 1, &pd[0]);
        pd[0] |= 0x03;
        Print(L"pd[0] %lx\n", pd[0]);
    }
    if (!(pd[0] & 0x80)) {
        uint64_t *pt = (uint64_t *)(pd[0] & ~0xfffULL);
        Print(L"pt[0] %lx\n", pt[0]);
        if (!(pt[0] & 1)) {
            uefi_call_wrapper(BS->AllocatePages, 4, AllocateAnyPages, \
                              EfiBootServicesData, 1, &pt[0]);
            pt[0] |= 0x03;
            Print(L"pt[0] %lx\n", pt[0]);
        }
    }

    Print(L"[0] = %08x\n", *(uint32_t *)(p+4));

    Print(L"pml4[0x1ff] %lx\n", pml4[0x1ff]);
    if (pml4[0x1ff] == 0) {
        uint64_t *pt;
        uefi_call_wrapper(BS->AllocatePages, 4, AllocateAnyPages, \
                          EfiBootServicesData, 4, &pt);
        uint64_t x = (uint64_t)pt;

        Print(L"pt = %lx\n", pt);

        pml4[0x1ff] = x | 0x3;
        pt[0x1ff] = (x + 0x1000) | 0x3;
        pt[0x3ff] = (x + 0x2000) | 0x3;
        pt[0x5ff] = (x + 0x3000) | 0x3;

        *(uint32_t *)p = 0xabcdabcd;
        *(uint32_t *)(p + 4) = 0x12341234;

        Print(L"[0] = %08x\n", *(uint32_t *)(p+4));
        Print(L"[fffffffffffc] = %08x\n", *(uint32_t *)(x + 0x3ffc));

        /* This write should place 0x5678 in the last 16-bit word of memory
         * and 0x5678 at the first 16-bit word in memory. If the wrapping
         * works as expected p[0] should be 0x5678ABCD and
         * p[1] should be 0x12345678 when displayed. */
        *(uint32_t *)(p + 2) = 0x56785678;

        Print(L"p[0] = %08x\n", ((uint32_t *)p)[0]);
        Print(L"p[1] = %08x\n", ((uint32_t *)p)[1]);
    }

    return 0;
}

A Makefile, 64- Ubuntu, 64- Debian :

ARCH            ?= $(shell uname -m | sed s,i[3456789]86,ia32,)
ifneq ($(ARCH),x86_64)
LIBDIR          = /usr/lib32
else
LIBDIR          = /usr/lib
endif

OBJS            = wraptest.o
TARGET          = wraptest.efi

EFIINC          = /usr/include/efi
EFIINCS         = -I$(EFIINC) -I$(EFIINC)/$(ARCH) -I$(EFIINC)/protocol
LIB             = $(LIBDIR)
EFILIB          = $(LIBDIR)
EFI_CRT_OBJS    = $(EFILIB)/crt0-efi-$(ARCH).o
EFI_LDS         = $(EFILIB)/elf_$(ARCH)_efi.lds

CFLAGS          = $(EFIINCS) -fno-stack-protector -fpic \
                  -fshort-wchar -mno-red-zone -Wall -O3
ifeq ($(ARCH),x86_64)
  CFLAGS += -DEFI_FUNCTION_WRAPPER
endif

LDFLAGS         = -nostdlib -znocombreloc -T $(EFI_LDS) -shared \
                  -Bsymbolic -L $(EFILIB) -L $(LIB) $(EFI_CRT_OBJS)

all: $(TARGET)

wraptest.so: $(OBJS)
        ld $(LDFLAGS) $(OBJS) -o $@ -lefi -lgnuefi

%.efi: %.so
        objcopy -j .text -j .sdata -j .data -j .dynamic \
                -j .dynsym  -j .rel -j .rela -j .reloc \
                --target=efi-app-$(ARCH) $^ $@

, , x86-64. EFI :

make ARCH=x86_64

wraptest.efi, EFI. make

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