In an assembly program, the .text section is loaded at 0x08048000; the .data and the .bss section comes after that.
What would happen if I don't put an exit syscall in the .text section? Would it lead to the .data and the .bss section being interpreted as code causing "unpredictable" behavior? When will the program terminate -- probably after every "instruction" is executed?
I can easily write a program without the exit syscall, but testing if .data and .bss gets executed is something I don't know because I guess I would have to know the real machine code that is generated under-the-hoods to understand that.
I think this question is more about "How would OS and CPU handle such a scenario?" than assembly language, but it is still interesting to know for assembly programmers etc.
The processor does not know where your code ends. It faithfully executes one instruction after another until execution is redirected elsewhere (e.g. by a jump, call, interrupt, system call, or similar).
If your code ends without jumping elsewhere, the processor continues executing whatever is in memory after your code. It is fairly unpredictable what exactly happens, but eventually, your code typically crashes because it tries to execute an invalid instruction or tries to access memory that it is not allowed to access.
If neither happens and no jump occurs, eventually the processor tries to execute unmapped memory or memory that is marked as “not executable” as code, causing a segmentation violation. On Linux, this raises a SIGSEGV or SIGBUS. When unhandled, these terminate your process and optionally produce core dumps.
If you're curious, run under a debugger and look at disassembly of the faulting instruction.