linux-kernel - [PATCH] rust: print: add safety comments for %pA formatting

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Message-ID: <20260205042132.40772-1-muchamadcoirulanwar@gmail.com>
Date: Thu,  5 Feb 2026 11:21:32 +0700
From: Muchamad Coirul Anwar <muchamadcoirulanwar@...il.com>
To: ojeda@...nel.org
Cc: boqun@...nel.org,
	gary@...yguo.net,
	bjorn3_gh@...tonmail.com,
	lossin@...nel.org,
	a.hindborg@...nel.org,
	aliceryhl@...gle.com,
	tmgross@...ch.edu,
	dakr@...nel.org,
	tamird@...nel.org,
	gregkh@...uxfoundation.org,
	fujita.tomonori@...il.com,
	andrewjballance@...il.com,
	rust-for-linux@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Muchamad Coirul Anwar <muchamadcoirulanwar@...il.com>
Subject: [PATCH] rust: print: add safety comments for %pA formatting

The safety comments in `rust_fmt_argument` and `call_printk` were
previously marked as TODO.

This patch adds the missing safety documentation explaining why
dereferencing the pointers and calling the C `_printk` function
is safe in these contexts. It clarifies the contract between
`lib/vsprintf.c` and the Rust implementation regarding the `%pA`
format specifier.

Signed-off-by: Muchamad Coirul Anwar <muchamadcoirulanwar@...il.com>
---
 rust/kernel/print.rs | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/rust/kernel/print.rs b/rust/kernel/print.rs
index 6fd84389a858..3e6ff8f42d95 100644
--- a/rust/kernel/print.rs
+++ b/rust/kernel/print.rs
@@ -29,7 +29,11 @@
     use fmt::Write;
     // SAFETY: The C contract guarantees that `buf` is valid if it's less than `end`.
     let mut w = unsafe { RawFormatter::from_ptrs(buf.cast(), end.cast()) };
-    // SAFETY: TODO.
+    // SAFETY: The C implementation of `vsprintf` (in `lib/vsprintf.c`) specifically
+    // calls this function ONLY when processing the `%pA` format specifier.
+    // On the Rust side (`call_printk`), we guarantee that `%pA` is always paired
+    // with a valid pointer to `fmt::Arguments`.
+    // Therefore, dereferencing `ptr` here is safe.
     let _ = w.write_fmt(unsafe { *ptr.cast::<fmt::Arguments<'_>>() });
     w.pos().cast()
 }
@@ -109,7 +113,9 @@ pub unsafe fn call_printk(
 ) {
     // `_printk` does not seem to fail in any path.
     #[cfg(CONFIG_PRINTK)]
-    // SAFETY: TODO.
+    // SAFETY: The format string is constructed to use `%pA`, which corresponds to the
+    // pointer to `fmt::Arguments` passed as the third argument.
+    // Since `args` is a valid reference, casting it to a pointer is safe.
     unsafe {
         bindings::_printk(
             format_string.as_ptr(),
-- 
2.50.0