Help: syscall read has weird behaviours

[realhackcraft]

This is not related to your repo or the book, just thought you might be the best person to ask:

I created an assembly program to read 1 byte from user input and store it in the text section (not best practice, but just to test things out).

Here is my code:

.global _start
.align 4

_start:
   ; Read user input into buffer
    mov x0, 0              ; File descriptor: stdin
    adr x1, input_buffer   ; Buffer to store the input
    mov x2, 1              ; Number of bytes to read
    mov x16, 3             ; System call number for read (sys_read)
    svc 0x80               ; Make the system call

   ; Write the input to stdout
    mov x0, 1              ; File descriptor: stdout
    adr x1, input_buffer   ; Address of the buffer
    mov x2, 1              ; Number of bytes to write
    mov x16, 4             ; System call number for write (sys_write)
    svc 0x80               ; Make the system call

input_buffer:
    .space 1

When I type a single byte in my terminal and press enter, it doesn't print anything and returns with code 0.

I decompiled the binary and found that input_buffer was at 0x0000100003f99. I used lldb to set a breakpoint on this instruction: mov x0, 1 // File descriptor: stdout and found the value at that address were:

0x100003f99: 0x00 0x00 0x00 0x01 0x00 0x00 0x00 0x1c
0x100003fa1: 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x1c

Which is not the byte I typed. I typed r (0x72)

I think I didn't use the read syscall properly, but have no idea how. Could you give some guidance?

[realhackcraft]

I solved it. I had to make the label in the .data section to put it in writable memory.

[below]

Excellent that you found the solution! Feel free to add it as an example and send me a PR!