问题
A prior related question was answered. Thank you! However this creates a new question for me. Why does nasm put data bytes at two different memory locations? I include program information and other data dump below.
---------- code snippet compiled with nasm, ld -----------------
section .text
...
zero: jmp short two
one: pop ebx
xor eax, eax
mov [ebx+12], eax
mov [ebx+8], ebx
mov [ebx+7], al
lea ecx, [ebx+8]
lea edx, [ebx+12]
mov al, 11
int 0x80
two: call one
section .data align=1
msg: db '/bin/sh0argvenvp'
-------- readelf output to show load locations --------
readelf -Wl myshdb
Elf file type is EXEC (Executable file)
Entry point 0x8048080
There are 2 program headers, starting at offset 52
Program Headers:
Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
LOAD 0x000000 0x08048000 0x08048000 0x0009d 0x0009d R E 0x1000
LOAD 0x00009d 0x0804909d 0x0804909d 0x00010 0x00010 RW 0x1000
Section to Segment mapping:
Segment Sections...
00 .text
01 .data
-------------- run with gdb and debug step to mov instructions ----------
---------------registers--------------
EAX: 0x0
EBX: 0x804809d ("/bin/sh0argvenvp")
----------- memory address checks ------------
gdb-peda$ p zero
$15 = {<text variable, no debug info>} 0x8048080 <zero>
gdb-peda$ p one
$16 = {<text variable, no debug info>} 0x8048082 <one>
gdb-peda$ p two
$17 = {<text variable, no debug info>} 0x8048098 <two>
gdb-peda$ p $ebx
$18 = 0x804809d
gdb-peda$ p msg
$19 = 0x6e69622f
gdb-peda$ x 0x804809d
0x804809d: "/bin/sh0argvenvp"
gdb-peda$ x msg
0x6e69622f: <error: Cannot access memory at address 0x6e69622f>
In other words, the string message is available from a memory location directly after code (0x804809d). Yet msg label maps to 0x6e69622f, which is the label to my data. How can use gdb to see data at the second address? Is nasm putting the data at two different locations? Why?
回答1:
Let's look at the LOAD
segments:
Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
LOAD 0x000000 0x08048000 0x08048000 0x0009d 0x0009d R E 0x1000
LOAD 0x00009d 0x0804909d 0x0804909d 0x00010 0x00010 RW 0x1000
The first one instructs the loader to mmap
0x9d
bytes from file offset 0
into virtual memory at address 0x08048000
.
The loader can't do exactly that, because memory mapping only works at one page (4096 bytes) granularity. So it mmap
s the .text
, and everything that follows it in the file, up to one page, at address 0x08048000
.
This means that whatever .data
followed .text
in the file after offset 0x9d
will appear at address 0x0804809d
and later, but with wrong permissions (R
ead and E
xecute).
The second LOAD
segment instructs the loader to mmap
file contents, starting at offset 0x9d
at virtual address 0x0804909d
.
The loader can't do exactly that either for the same "page granularity" reason.
Instead, it will round down the offset and the address, and mmap
file contents starting from offset 0
at address 0x08049000
.
That that means that whatever .text
preceded .data
in the file will appear at address before 0x0804909d
, again with the wrong permissions (R
ead and W
rite this time).
You can confirm that that's what's happening by using GDB x/10i 0x8049080
-- you will see exactly the same instructions as with x/10i 0x8048080
.
You can also observe the actual mmap
system calls the loader performed with strace
.
来源:https://stackoverflow.com/questions/62885381/why-does-my-data-section-appear-twice-in-the-compiled-binary-ubuntu-x86-nasm