From d48d7f10b56e74296eb065de995a23f1c6173547 Mon Sep 17 00:00:00 2001
From: Zach Reizner <zachr@google.com>
Date: Tue, 9 Jan 2018 15:19:23 -0800
Subject: [PATCH] kvm: add dirty_log_bitmap_size helper

For the callers of get_dirty_log, they must pass in bitmap large enough
to hold the log. Instead of duplicating that calculation inside the
method and the caller, this change provides a public helper usable by
both to calculate the bitmap's minimum size.

TEST=./build_test
BUG=chromium:800626

Change-Id: I349aa0dd7e78e7f9ea3d35ba0acba8088fd77a4e
Reviewed-on: https://chromium-review.googlesource.com/857909
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Zach Reizner <zachr@chromium.org>
Reviewed-by: Dylan Reid <dgreid@chromium.org>
---
 kvm/src/lib.rs | 26 +++++++++++++++++++++++---
 1 file changed, 23 insertions(+), 3 deletions(-)

diff --git a/kvm/src/lib.rs b/kvm/src/lib.rs
index 69ad3a7832..3dee94cf08 100644
--- a/kvm/src/lib.rs
+++ b/kvm/src/lib.rs
@@ -56,6 +56,17 @@ unsafe fn set_user_memory_region<F: AsRawFd>(fd: &F,
     if ret == 0 { Ok(()) } else { errno_result() }
 }
 
+/// Helper function to determine the size in bytes of a dirty log bitmap for the given memory region
+/// size.
+///
+/// # Arguments
+///
+/// * `size` - Number of bytes in the memory region being queried.
+pub fn dirty_log_bitmap_size(size: usize) -> usize {
+    let page_size = pagesize();
+    (((size + page_size - 1) / page_size) + 7) / 8
+}
+
 /// A wrapper around opening and using `/dev/kvm`.
 ///
 /// Useful for querying extensions and basic values from the KVM backend. A `Kvm` is required to
@@ -266,11 +277,10 @@ impl Vm {
     /// region `slot` represents. For example, if the size of `slot` is 16 pages, `dirty_log` must
     /// be 2 bytes or greater.
     pub fn get_dirty_log(&self, slot: u32, dirty_log: &mut [u8]) -> Result<()> {
-        let page_size = pagesize();
         match self.device_memory.get(&slot) {
             Some(mmap) => {
-                // Ensures that there are as many bits in dirty_log as there are pages in the mmap.
-                if (mmap.size() / page_size) > (dirty_log.len() << 3) {
+                // Ensures that there are as many bytes in dirty_log as there are pages in the mmap.
+                if dirty_log_bitmap_size(mmap.size()) > dirty_log.len() {
                     return Err(Error::new(-EINVAL));
                 }
                 let mut dirty_log_kvm = kvm_dirty_log {
@@ -828,6 +838,16 @@ impl CpuId {
 mod tests {
     use super::*;
 
+    #[test]
+    fn dirty_log_size() {
+        let page_size = pagesize();
+        assert_eq!(dirty_log_bitmap_size(0), 0);
+        assert_eq!(dirty_log_bitmap_size(page_size), 1);
+        assert_eq!(dirty_log_bitmap_size(page_size * 8), 1);
+        assert_eq!(dirty_log_bitmap_size(page_size * 8 + 1), 2);
+        assert_eq!(dirty_log_bitmap_size(page_size * 100), 13);
+    }
+
     #[test]
     fn new() {
         Kvm::new().unwrap();