I encountered this problem when implementing lazy allocation of file pages. When the kernel tries to access the contents of the user-unallocated page, it cannot properly identify it as coming from the user, resulting in a failure to trigger a page fault normally.
fn check_null_terminated<T: PartialEq + Default>(
start: VirtAddr,
access_flags: MappingFlags,
) -> LinuxResult<usize> {
let align = Layout::new::<T>().align();
if start.as_usize() & (align - 1) != 0 {
return Err(LinuxError::EFAULT);
}
let zero = T::default();
let mut page = start.align_down_4k();
let start = start.as_ptr_of::<T>();
let mut len = 0;
access_user_memory(|| {
loop {
if is_accessing_user_memory() {
// debug!("is_accessing_user_memory");
panic!("is_accessing_user_memory");
}
// SAFETY: This won't overflow the address space since we'll check
// it below.
let ptr = unsafe { start.add(len) };
while ptr as usize >= page.as_ptr() as usize {
// We cannot prepare `aspace` outside of the loop, since holding
// aspace requires a mutex which would be required on page
// fault, and page faults can trigger inside the loop.
// TODO: this is inefficient, but we have to do this instead of
// querying the page table since the page might has not been
// allocated yet.
let task = current();
let aspace = task.task_ext().process_data().aspace.lock();
if !aspace.check_region_access(
VirtAddrRange::from_start_size(page, PAGE_SIZE_4K),
access_flags,
) {
return Err(LinuxError::EFAULT);
}
page += PAGE_SIZE_4K;
}
// This might trigger a page fault
// SAFETY: The pointer is valid and points to a valid memory region.
if unsafe { ptr.read_volatile() } == zero {
break;
}
len += 1;
}
Ok(())
})?;
Ok(len)
}In the above situation, Starry cannot panic.
But the strange thing is that if I uncomment debug! or add a line of debug! it can panic normally again.
I encountered this problem when implementing lazy allocation of file pages. When the kernel tries to access the contents of the user-unallocated page, it cannot properly identify it as coming from the user, resulting in a failure to trigger a page fault normally.
In the above situation, Starry cannot panic.
But the strange thing is that if I uncomment debug! or add a line of debug! it can panic normally again.