michaelkuc6/blog_os
1
0
Fork 0
mirror of https://github.com/phil-opp/blog_os.git synced 2025-12-16 22:37:49 +00:00
Code Issues Projects Releases Wiki Activity

Merge pull request #311 from phil-opp/page-table-post-updates

Browse Source 
Bring “Page Tables” post up to date again
This commit is contained in:
Philipp Oppermann
2017-04-13 19:52:45 +02:00
committed by GitHub
parent e48aa7e68e 493dede919
commit 1ad30d488c
2 changed files with 11 additions and 68 deletions
Download Patch File Download Diff File Expand all files Collapse all files

2
Cargo.toml
View File

@@ -11,7 +11,7 @@ once = "0.3.2"
rlibc = "1.0"
spin = "0.4.5"
volatile = "0.1.0"
x86_64 = "0.1.0"
x86_64 = "0.1.2"
[dependencies.hole_list_allocator]
path = "libs/hole_list_allocator"

77
blog/content/post/06-page-tables.md
View File

@@ -776,7 +776,7 @@ To test it, we add a `test_paging` function in `memory/paging/mod.rs`:
pub fn test_paging<A>(allocator: &mut A)
where A: FrameAllocator
{
let page_table = unsafe { ActivePageTable::new() };
let mut page_table = unsafe { ActivePageTable::new() };
// test it
}
@@ -792,63 +792,6 @@ let mut frame_allocator = ...;
memory::test_paging(&mut frame_allocator);
```
### translate
First, we translate some addresses:
```rust
// address 0 is mapped
println!("Some = {:?}", page_table.translate(0));
// second P1 entry
println!("Some = {:?}", page_table.translate(4096));
// second P2 entry
println!("Some = {:?}", page_table.translate(512 * 4096));
// 300th P2 entry
println!("Some = {:?}", page_table.translate(300 * 512 * 4096));
// second P3 entry
println!("None = {:?}", page_table.translate(512 * 512 * 4096));
// last mapped byte
println!("Some = {:?}", page_table.translate(512 * 512 * 4096 - 1));
```
Currently, the first GiB of the address space is identity-mapped. Thus all addresses in this area should translate to `Some(x)`, where `x` is the virtual address. Only the second last address, `512 * 512 * 4096`, is not in that area and should resolve to `None`.
But the output shows two `None` lines:
```
Some = Some(0)
Some = Some(4096)
Some = Some(2097152)
Some = Some(629145600)
None = None
Some = None
```
The last line is wrong. But why?
In fact, all addresses above `344 * 512 * 4096` seem to get translated to `None`. But even worse, there are some wrong translations, too. For example, on my machine `357 * 512 * 4096` translates to roughly `255TiB`:
```
Some(280735973961728)
```
Something is terribly wrong here. But it's not our code.
The reason for this bug is a silent stack overflow. Remember, our `.bss` section in the `boot.asm` file looks like this:
```nasm
section .bss
align 4096
p4_table:
resb 4096
p3_table:
resb 4096
p2_table:
resb 4096
stack_bottom:
resb 4096
stack_top:
```
So a stack overflow overwrites the P2 table, starting at the last entry. But the CPU still uses the memory as page table entries. And if the stack bytes contain the present byte, it seems to point to a frame and `translate` returns a (wrong) `Some`.
To fix it, we double the stack size to `4096 * 2`. Now the last byte gets translated to `Some(1073741823)` correctly. To avoid this kind of bug in the future, we need to add a guard page to the stack, which causes an exception on stack overflow. We will do that in the next post when we remap the kernel.
### map_to
Let's test the `map_to` function:
@@ -905,21 +848,21 @@ So to fix our `unmap` function, we need to remove the cached translation from th
```toml
[dependencies]
...
x86_64 = "0.1.0"
x86_64 = "0.1.2"
```
Now we can use it to fix `unmap`:
```rust
...
p1[page.p1_index()].set_unused();
unsafe {
use x86_64::instructions::tlb;
use x86_64::VirtualAddress;
tlb::flush(VirtualAddress(page.start_address()));
}
// TODO free p(1,2,3) table if empty
//allocator.deallocate_frame(frame);
p1[page.p1_index()].set_unused();
use x86_64::instructions::tlb;
use x86_64::VirtualAddress;
tlb::flush(VirtualAddress(page.start_address()));
// TODO free p(1,2,3) table if empty
//allocator.deallocate_frame(frame);
}
```
Now the desired page fault occurs even when we access the page before.