Fix exec_app_context corrupting stack

[hatoo]

A context switch will corrupt an app's stack.

I think this PR fix it.

I've found it by searching for an issue that

#![no_std]
#![no_main]

extern crate alloc;

use alloc::vec::Vec;
#[cfg_attr(target_os = "linux", no_main)]
use noli::prelude::*;
entry_point!(main);

fn main() -> u64 {
    let mut v: Vec<usize> = Vec::new();
    v.reserve(1); // This allocation is no problem but this increases segment size significantly because it requires noli's ALLOCATOR
    0
}

This app caused a crash when I ran it a few times in a row.

And I've concluded that a context switch accidentally overwrites ALLOCATOR(OS side)'s table's headers by 0 and header's linked list is cat because (None::<Option<Box<Header>>> is 0)

Then OS can't allocate a region for exec the app -> crash

[hikalium]

Thanks for the report! I created a repro branch and reproduced the issue with the following steps:

• Checkout this branch: https://github.com/hikalium/wasabi/tree/bug_24_stack_corrupt
• make run to launch the OS
• Run bug0 on wasabi to run the repro app 1000 times (thanks for providing the code!)

expected behavior: bug0 runs 1000 times without an error
actual behavior: it fails with the following log messages:

> bug0
[INFO]  os/src/cmd.rs:58 :  Executing cmd: ["bug0"]
[INFO]  os/src/cmd.rs:153:  try 0
[INFO]  os/src/cmd.rs:156:  Ok(0)
// snip
[INFO]  os/src/cmd.rs:153:  try 157
[PANIC] start
[PANIC] RIP = 0x000000003E20593D
[PANIC] RSP = 0x000000003E1DCD58
[PANIC] RBP = 0x000000003E1DCDD8
[PANIC] *RBP = 0x0000000000000000
[PANIC] dump_stack() = 0x000000003E205910
// snip
00000000: 02 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
00000010: 00 00 00 00 00 00 00 00 12 00 00 00 00 00 00 00 |................|
00000020: 01 00 00 00 00 00 00 00 20 00 00 00 0C 00 00 00 |........ .......|
00000030: 03 CE 1D 3E 00 00 00 00 88 CD 1D 3E 00 00 00 00 |...>.......>....|
[PANIC] PanicInfo { payload: Any { .. }, message: Some(assertion failed: self.canary == CANARY_SIGNATURE), location: Location { file: "os/src/allocator.rs", line: 112, col: 9 }, can_unwind: true, force_no_backtrace: false }

However, although the branch above contains the suggested fix, the error is still observed... let me look into it!

[hatoo]

I think OS error is because of memory shortage.
Everytime when exec an app, OS doesn't dealloc used app's region and stack. it leaks.

I got ALLOCATOR's dump after execution failed
b.txt
The headers looks good but it seems that there are no spaces to fit app'a region