Fix shmat() to work correctly

shmat() maps the memory region without executable permission by default, while Noah tries to map the region onto the VM with executable permission, and it fails. It also incorrectly returns the host address instead of the guest address.

Author: utshina

src/mm/shm.c

@@ -48,10 +48,10 @@ DEFINE_SYSCALL(shmat, int, shmid, gaddr_t, addr, int, shmflg)
   pthread_rwlock_wrlock(&proc.mm->alloc_lock);
   addr = alloc_region(len);
   do_munmap(addr, len);
-  record_region(proc.mm, ptr, addr, len, LINUX_PROT_READ | LINUX_PROT_WRITE | LINUX_PROT_EXEC, LINUX_MAP_PRIVATE | LINUX_MAP_FIXED, -1, 0);
-  vmm_mmap(addr, len, HV_MEMORY_READ | HV_MEMORY_WRITE | HV_MEMORY_EXEC, ptr);
+  record_region(proc.mm, ptr, addr, len, LINUX_PROT_READ | LINUX_PROT_WRITE, LINUX_MAP_PRIVATE | LINUX_MAP_FIXED, -1, 0);
+  vmm_mmap(addr, len, HV_MEMORY_READ | HV_MEMORY_WRITE, ptr);
   pthread_rwlock_unlock(&proc.mm->alloc_lock);
-  return (uint64_t) ptr;
+  return (uint64_t) addr;
 }
 
 DEFINE_SYSCALL(shmctl, int, shmid, int, cmd, gaddr_t, buf_ptr)