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uefi-raw: Add explicit padding field for MemoryDescriptor + add test
By convention, structs having a C representation should be explicit with their in-between padding fields. This allows users memory-safe trivial serialization and deserialization by accessing the underlying memory as byte slice. Without this fields being explicit, Miri complains about uninitialized memory accesses, which is UB. uefi-raw: add test for MemoryDescriptor to catch Miri problems The underlying issue is that if someone wants to cast a slice of descriptors to a byte slice, the uninitialized **implicit** padding bytes are UB. Miri complains about that.
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‎uefi-raw/CHANGELOG.md‎

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@@ -1,5 +1,8 @@
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# uefi-raw - [Unreleased]
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## Changed
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- `MemoryDescriptor` now has an explicit padding field, allowing trivial
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memory-safe serialization and deserialization
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# uefi-raw - 0.8.0 (2024-09-09)
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‎uefi-raw/src/table/boot.rs‎

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@@ -350,7 +350,8 @@ bitflags! {
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pub struct MemoryDescriptor {
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/// Type of memory occupying this range.
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pub ty: MemoryType,
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// Implicit 32-bit padding.
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/// Padding field. Ignore.
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pub padding0: u32,
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/// Starting physical address.
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pub phys_start: PhysicalAddress,
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/// Starting virtual address.
@@ -370,6 +371,7 @@ impl Default for MemoryDescriptor {
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fn default() -> Self {
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Self {
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ty: MemoryType::RESERVED,
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padding0: 0,
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phys_start: 0,
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virt_start: 0,
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page_count: 0,
@@ -379,7 +381,7 @@ impl Default for MemoryDescriptor {
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}
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newtype_enum! {
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/// The type of a memory range.
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/// The type of memory range.
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///
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/// UEFI allows firmwares and operating systems to introduce new memory types
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/// in the `0x7000_0000..=0xFFFF_FFFF` range. Therefore, we don't know the full set
@@ -484,3 +486,35 @@ pub enum Tpl: usize => {
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/// Note that this is not necessarily the processor's page size. The UEFI page
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/// size is always 4 KiB.
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pub const PAGE_SIZE: usize = 4096;
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#[cfg(test)]
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mod tests {
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use core::{mem, slice};
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use super::*;
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// This tests succeeds if Miri doesn't complain about uninitialized memory.
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#[test]
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fn memory_descriptor_trivial_serialization() {
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let descs = [
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MemoryDescriptor {
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ty: MemoryType::CONVENTIONAL,
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padding0: 0,
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phys_start: 0x1000,
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virt_start: 0x1000,
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page_count: 1,
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att: Default::default(),
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},
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];
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let raw_bytes: &[u8] = {
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let ptr = descs.as_ptr().cast::<u8>();
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let len = mem::size_of_val(&descs);
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unsafe { slice::from_raw_parts(ptr.cast(), len) }
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};
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// Test succeeds if Miri doesn't complain about initialized memory
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// (implicit padding fields).
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dbg!(&raw_bytes);
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}
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}

‎uefi/src/mem/memory_map/impl_.rs‎

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@@ -432,20 +432,23 @@ mod tests {
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const BASE_MMAP_UNSORTED: [MemoryDescriptor; 3] = [
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MemoryDescriptor {
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ty: MemoryType::CONVENTIONAL,
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padding0: 0,
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phys_start: 0x3000,
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virt_start: 0x3000,
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page_count: 1,
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att: MemoryAttribute::WRITE_BACK,
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},
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MemoryDescriptor {
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ty: MemoryType::CONVENTIONAL,
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padding0: 0,
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phys_start: 0x2000,
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virt_start: 0x2000,
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page_count: 1,
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att: MemoryAttribute::WRITE_BACK,
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},
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MemoryDescriptor {
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ty: MemoryType::CONVENTIONAL,
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padding0: 0,
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phys_start: 0x1000,
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virt_start: 0x1000,
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page_count: 1,

‎uefi/src/mem/memory_map/mod.rs‎

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@@ -131,6 +131,7 @@ mod tests_mmap_artificial {
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const BASE: MemoryDescriptor = MemoryDescriptor {
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ty: TY,
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padding0: 0,
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phys_start: 0,
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virt_start: 0,
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page_count: 0,
@@ -169,6 +170,7 @@ mod tests_mmap_artificial {
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const BASE: MemoryDescriptor = MemoryDescriptor {
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ty: TY,
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padding0: 0,
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phys_start: 0,
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virt_start: 0,
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page_count: 0,
@@ -278,6 +280,7 @@ mod tests_mmap_real {
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| MemoryAttribute::WRITE_COMBINE
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| MemoryAttribute::WRITE_THROUGH
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| MemoryAttribute::WRITE_BACK,
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..Default::default()
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},
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MemoryDescriptor {
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ty: MemoryType::CONVENTIONAL,
@@ -288,6 +291,7 @@ mod tests_mmap_real {
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| MemoryAttribute::WRITE_COMBINE
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| MemoryAttribute::WRITE_THROUGH
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| MemoryAttribute::WRITE_BACK,
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..Default::default()
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},
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MemoryDescriptor {
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ty: MemoryType::BOOT_SERVICES_DATA,
@@ -298,6 +302,7 @@ mod tests_mmap_real {
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| MemoryAttribute::WRITE_COMBINE
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| MemoryAttribute::WRITE_THROUGH
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| MemoryAttribute::WRITE_BACK,
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..Default::default()
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},
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MemoryDescriptor {
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ty: MemoryType::CONVENTIONAL,
@@ -308,6 +313,7 @@ mod tests_mmap_real {
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| MemoryAttribute::WRITE_COMBINE
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| MemoryAttribute::WRITE_THROUGH
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| MemoryAttribute::WRITE_BACK,
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..Default::default()
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},
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MemoryDescriptor {
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ty: MemoryType::CONVENTIONAL,
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| MemoryAttribute::WRITE_COMBINE
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| MemoryAttribute::WRITE_THROUGH
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| MemoryAttribute::WRITE_BACK,
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..Default::default()
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},
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MemoryDescriptor {
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ty: MemoryType::ACPI_NON_VOLATILE,
@@ -328,6 +335,7 @@ mod tests_mmap_real {
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| MemoryAttribute::WRITE_COMBINE
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| MemoryAttribute::WRITE_THROUGH
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| MemoryAttribute::WRITE_BACK,
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..Default::default()
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},
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MemoryDescriptor {
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ty: MemoryType::CONVENTIONAL,
@@ -338,6 +346,7 @@ mod tests_mmap_real {
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| MemoryAttribute::WRITE_COMBINE
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| MemoryAttribute::WRITE_THROUGH
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| MemoryAttribute::WRITE_BACK,
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..Default::default()
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},
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MemoryDescriptor {
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ty: MemoryType::ACPI_NON_VOLATILE,
@@ -348,6 +357,7 @@ mod tests_mmap_real {
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| MemoryAttribute::WRITE_COMBINE
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| MemoryAttribute::WRITE_THROUGH
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| MemoryAttribute::WRITE_BACK,
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..Default::default()
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},
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MemoryDescriptor {
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ty: MemoryType::CONVENTIONAL,
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| MemoryAttribute::WRITE_COMBINE
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| MemoryAttribute::WRITE_THROUGH
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| MemoryAttribute::WRITE_BACK,
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..Default::default()
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},
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MemoryDescriptor {
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ty: MemoryType::ACPI_NON_VOLATILE,
@@ -368,6 +379,7 @@ mod tests_mmap_real {
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| MemoryAttribute::WRITE_COMBINE
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| MemoryAttribute::WRITE_THROUGH
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| MemoryAttribute::WRITE_BACK,
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..Default::default()
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},
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];
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assert_eq!(entries.as_slice(), &expected);

‎uefi/tests/memory_map.rs‎

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@@ -12,20 +12,23 @@ fn parse_boot_information_efi_mmap() {
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virt_start: 0x3000,
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page_count: 1,
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att: MemoryAttribute::WRITE_BACK,
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..Default::default()
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},
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MemoryDescriptor {
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ty: MemoryType::CONVENTIONAL,
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phys_start: 0x2000,
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virt_start: 0x2000,
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page_count: 1,
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att: MemoryAttribute::WRITE_BACK,
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..Default::default()
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},
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MemoryDescriptor {
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ty: MemoryType::CONVENTIONAL,
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phys_start: 0x1000,
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virt_start: 0x1000,
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page_count: 1,
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att: MemoryAttribute::WRITE_BACK,
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..Default::default()
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},
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];
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let map_size = mmap_source.len() * desc_size;

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