mprotect fails after MacOS 11.2 on arm mac

I got the following error when I ran qemu on arm mac(MacOS 11.2).

```
$ ./qemu-system-x86_64
qemu-system-x86_64: qemu_mprotect__osdep: mprotect failed: Permission denied
**
ERROR:../tcg/tcg.c:844:tcg_region_init: assertion failed: (!rc)
Bail out! ERROR:../tcg/tcg.c:844:tcg_region_init: assertion failed: (!rc)
[1]    34898 abort      ./qemu-system-x86_64
```

I tested the same version of qemu on intel mac(MacOS 11.2), but it works fine.

And my friend told me that they did not have this error with MacOS 11.1.

So, I think it is CPU architecture or an OS version dependent error.


Environment:

Qemu commit id: d0dddab40e472ba62b5f43f11cc7dba085dabe71
OS: MacOS 11.2(20D64)
Hardware: MacBook Air (M1, 2020)


How to build:

```
mkdir build/
cd build/
../configure --target-list=aarch64-softmmu,x86_64-softmmu
make
```


How to reproduce:

```
./qemu-system-x86_64
```


Error message:

```
$ ./qemu-system-x86_64
qemu-system-x86_64: qemu_mprotect__osdep: mprotect failed: Permission denied
**
ERROR:../tcg/tcg.c:844:tcg_region_init: assertion failed: (!rc)
Bail out! ERROR:../tcg/tcg.c:844:tcg_region_init: assertion failed: (!rc)
[1]    34898 abort      ./qemu-system-x86_64
```

Thanks for submitting the ticket.
I've just stumbled upon it after updating to 11.2.

The question was already asked on apple developer forums: https://developer.apple.com/forums/thread/672804

And there's a thread going on with regard to broken nodejs on 11.2:
https://github.com/nodejs/node/issues/37061#issuecomment-774175983

I hit the same problem and did some initial investigation with Toshifumi.

Here is a more exhaustive test program I wrote based on the post on the Apple Developer Forums and the result shows that very interesting behavior of mmap and mprotect since macOS 11.2. 

https://gist.github.com/hikalium/75ae822466ee4da13cbbe486498a191f

I and my friend confirmed that all mmap & following mprotect calls with any protection bit combinations are succeeded up to 11.1 on M1 Mac but starting from 11.2 mprotect starts failing if we call mmap with PROT_WRITE + PROT_EXEC. (Surprisingly, mmap itself is not failing even on those patterns.)

It looks like the allocation of code gen buffer in QEMU uses this combination at mmap call:
https://github.com/qemu/qemu/blob/master/accel/tcg/translate-all.c#L1294

So maybe we need to specify PROT_NONE instead on the initial mmap and change it appropriately afterwards to make it working on M1 Mac after 11.2.

(We tried to fix it but we have no sufficient knowledge about tcg... Could you take a look into it?)

The patch can be used as a workaround for now:
diff --git a/util/osdep.c b/util/osdep.c
index 66d01b9160..76be8c295b 100644
--- a/util/osdep.c
+++ b/util/osdep.c
@@ -110,6 +110,9 @@ int qemu_mprotect_none(void *addr, size_t size)
 {
 #ifdef _WIN32
     return qemu_mprotect__osdep(addr, size, PAGE_NOACCESS);
+#elif defined(__APPLE__) && defined(__arm64__)
+    /* Workaround mprotect (RWX->NONE) issue on Big Sur 11.2 */
+    return 0;
 #else
     return qemu_mprotect__osdep(addr, size, PROT_NONE);
 #endif

It works for me when I use "./configure --enable-debug-tcg --extra-cflags=-I/opt/homebrew/include".

Fixed here:
https://gitlab.com/qemu-project/qemu/-/commit/c118881ee607dcac