boost modify_cmdline speed by starting replacement at its offset on disk

2 files changed, 14 insertions, 3 deletions

diff --git a/src/error.rs b/src/error.rs
index 6f8e217..97fa882 100644
--- a/src/error.rs
+++ b/src/error.rs
@@ -4,6 +4,8 @@ use thiserror::Error;
 
 #[derive(Debug, Error)]
 pub enum Error {
+    #[error("can't find cmdline (/cmdline.txt) on boot partition")]
+    NoCmdline,
     #[error("can't find disk device")]
     NoDiskDev,
     #[error("no private keys found in file")]
diff --git a/src/main.rs b/src/main.rs
index 89db44c..f72da42 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -279,12 +279,21 @@ where
 
 fn modify_cmdline(old: &str, new: &str) -> Result<()> {
     let boot = boot_dev()?;
+    let mut buf = fs::read(boot)?;
 
-    let mut cmdline = fs::read(boot)?;
-    replace_slice(&mut cmdline, old.as_bytes(), new.as_bytes());
-    fs::write(boot, cmdline)?;
+    let cmdline_buf = fs::read("/boot/cmdline.txt")?;
+    let cmdline_offset = buf
+        .windows(cmdline_buf.len())
+        .position(|window| window == cmdline_buf)
+        .ok_or(Error::NoCmdline)?
+        / 512
+        + 1;
 
+    replace_slice(&mut buf[cmdline_offset..], old.as_bytes(), new.as_bytes());
+
+    fs::write(boot, buf)?;
     nix::unistd::sync();
+
     Ok(())
 }