[PATCH v2] rust: print: add safety documentation for %pA handling

[Shivendra Sharma <shivendra02467@gmail.com>]

Add missing safety documentation to `rust_fmt_argument` and `call_printk`.
These comments clarify the safety requirements for dereferencing
pointers and calling the C `_printk` function, specifically
addressing the contract between `lib/vsprintf.c` and Rust regarding
the `%pA` format specifier.

The safety guarantee is made generic to cover all users of `%pA`,
including `seq_file.rs`.

Suggested-by: Alice Ryhl <aliceryhl@google.com>
Co-developed-by: Muchamad Coirul Anwar <muchamadcoirulanwar@gmail.com>
Signed-off-by: Muchamad Coirul Anwar <muchamadcoirulanwar@gmail.com>
Signed-off-by: Shivendra Sharma <shivendra02467@gmail.com>
---
v2:
  - Combined work with Muchamad Coirul Anwar.
  - Refined safety comments to be generic per Alice Ryhl's feedback.
  - Added formal "# Safety" section to rust_fmt_argument per Miguel Ojeda's
    previous suggestion.
  - Added documentation to include/linux/sprintf.h.
  - Links to previous versions:
    v1 (Shivendra): https://lore.kernel.org/rust-for-linux/20260128202130.196419-1-shivendra02467@gmail.com/
    v1 (Muchamad): https://lore.kernel.org/rust-for-linux/20260205042132.40772-1-muchamadcoirulanwar@gmail.com/

 include/linux/sprintf.h |  7 ++++++-
 rust/kernel/print.rs    | 19 +++++++++++++++----
 2 files changed, 21 insertions(+), 5 deletions(-)

diff --git a/include/linux/sprintf.h b/include/linux/sprintf.h
index f06f7b785091..f915829cbba5 100644
--- a/include/linux/sprintf.h
+++ b/include/linux/sprintf.h
@@ -25,7 +25,12 @@ __scanf(2, 0) int vsscanf(const char *, const char *, va_list);
 extern bool no_hash_pointers;
 void hash_pointers_finalize(bool slub_debug);
 
-/* Used for Rust formatting ('%pA') */
+/**
+ * Used for Rust formatting ('%pA').
+ *
+ * Safety preconditions are documented in the Rust implementation
+ * (rust/kernel/print.rs).
+ */
 char *rust_fmt_argument(char *buf, char *end, const void *ptr);
 
 #endif	/* _LINUX_KERNEL_SPRINTF_H */
diff --git a/rust/kernel/print.rs b/rust/kernel/print.rs
index 6fd84389a858..15d4039a7646 100644
--- a/rust/kernel/print.rs
+++ b/rust/kernel/print.rs
@@ -19,7 +19,12 @@
 };
 
 // Called from `vsprintf` with format specifier `%pA`.
-#[expect(clippy::missing_safety_doc)]
+///
+/// # Safety
+///
+/// - `buf` must be valid for writes until `end`.
+/// - `ptr` must point to a valid `fmt::Arguments` instance.
+/// - The `fmt::Arguments` must remain valid for the duration of the call.
 #[export]
 unsafe extern "C" fn rust_fmt_argument(
     buf: *mut c_char,
@@ -27,9 +32,13 @@
     ptr: *const c_void,
 ) -> *mut c_char {
     use fmt::Write;
-    // SAFETY: The C contract guarantees that `buf` is valid if it's less than `end`.
+    // SAFETY: The safety requirements of this function (see above)
+    // guarantee that `buf` and `end` define a valid range.
     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, we always pair `%pA` with a valid pointer to
+    // `fmt::Arguments`, satisfying the requirements of this function (see above).
     let _ = w.write_fmt(unsafe { *ptr.cast::<fmt::Arguments<'_>>() });
     w.pos().cast()
 }
@@ -109,7 +118,9 @@ 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(),
-- 
2.51.2