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Why do we need MAP_PRIVATE flag when mapping memory?（1个答案）
7分钟前关门了。
我试着做一个非常简单的内存分配器，但是它在输出时给我错误，另外print_sys_error不能正常工作。如果我用下面的代码调用它，它就能工作：

_start:
    sub $16, %rsp # preallocate stack

    mov $20, %rdx
    mov $STDOUT, %rdi
    movq $_start.0, (%rsp)
    jmp print_sys_error

但在实际使用中不起作用。代码如下：

.global _start

.text

_start:
    sub $16, %rsp # preallocate stack

    mov $0x1000, %rsi # allocate 4 KIB
    call alloc
    mov %rax, 8(%rsp) # save page address for later

    movq $_start.0, (%rsp) # prepearing for potential call of print_sys_error
    mov $STDOUT, %rdi

    cmpq $0, %rdx
    jne print_sys_error

    mov 8(%rsp), %rax # get back page address
    movb $0xff, (%rax)
    movb $0xff, 0xfff(%rax)

    mov $SYS_EXIT, %rax # exit successfully
    xor %rdi, %rdi
    syscall

_start.0:
    sub $8, %rsp # stack got smaller after return

    mov $SYS_EXIT, %rax # fail successfully
    mov $1, %rdi
    syscall

### put file descriptor to %rdi and error code to %rdx (can't be over 34)
print_sys_error:
    call sys_error_to_str
    mov $SYS_WRITE, %rax
    syscall
    ret

### put error code to %rdx (can't be over 34)
### get string address in %rsi and string length in %rdx
sys_error_to_str:
    shl $4, %rdx
    mov error_table(%rdx), %rsi
    mov (error_table + 8)(%rdx), %rdx
    ret

### put new page size is rsi
### get error code in %rdx (0 if no error) and address in %rax (same as %rdx if error)
alloc:
    mov $SYS_MMAP, %rax
    xor %rdi, %rdi # page address, zero so set by kernel
    mov $(PROT_READ | PROT_WRITE | PROT_EXEC), %rdx # page access flags
    mov $(MAP_ANONYMOUS), %r10 # page/fd settings
    mov $-1, %r8 # fd (not used)
    xor %r9, %r9 # file offset (not used)
    syscall # page address in %rax

    cmpq $35, %rax # compare to error number (+1 as I see in glibc)
    jl alloc.0 # jump if error

    xor %rdx, %rdx # zero output error
    ret

alloc.0:

    mov %rax, %rdx # set output error
    ret

.data

.equ SYS_READ, 0 # for sys read `xor %rax %rax` can be used 
.equ SYS_WRITE, 1
.equ SYS_MMAP, 9
.equ SYS_EXIT, 60

.equ STDIN, 0
.equ STDOUT, 1
.equ STDERR, 2

.equ MAP_ANONYMOUS,    0x10
# some other map constants ...

.equ PROT_READ,             0x1
# some other prot constants ...

.section .data.sys_errors

.equ ERR_PERM, 1 # no permissions
.equ ERR_NOFILE, 2 # so such file or directory
# some other ERR constants

### each table element has address to the string and it's length, both values are qwords so each element has 16 bytes
error_table:
    .quad ERR_WRONG_STR
    .quad ERR_WRONG_STR_LEN

    .quad ERR_PERM_STR
    .quad ERR_PERM_STR_LEN
# ...

ERR_WRONG_STR:
    .asciz "wrong error"
    .equ ERR_WRONG_STR_LEN, . - ERR_WRONG_STR - 1

ERR_PERM_STR:
    .asciz "no permissions"
    .equ ERR_PERM_STR_LEN, . - ERR_PERM_STR - 1
# ...

堆栈溢出问题的代码很长，但我不认为它很复杂。
我试着在网上寻找更多关于mmap系统调用和系统调用的信息，我不记得我是怎么处理代码来修复它的，但是我花了很长时间。