[PATCH v3 2/3] rust: time: make Delta division and remainder fail consistently

[chenhan <chenhan0017.work@gmail.com>]

Delta's division and rem_nanos() use Rust operators on 64-bit and C
helpers on 32-bit. The report shows that rem_nanos(0) can return 10 for
a 10 ns dividend on ARMv7, while the same call panics on x86-64. The
i64::MIN / -1 case also differs, and neither API documents these inputs.

Check for zero divisors and i64::MIN with -1 before the architecture-
specific code, and document the panic conditions on each method. This
matches Rust's signed division and remainder operators, including when
overflow checks are disabled. Valid-input results and API signatures
are unchanged; invalid inputs now panic on 32-bit too.

No in-tree calls to either API were found at the base commit. The checks
stay in the existing methods so the fix does not depend on introducing
a math module.

Fixes: 4521438fb076 ("rust: time: Implement basic arithmetic operations for Delta")
Reported-by: Georgios Androutsopoulos <georgeandrout13@gmail.com>
Closes: https://github.com/Rust-for-Linux/linux/issues/1254
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: chenhan <chenhan0017.work@gmail.com>
---
 rust/kernel/time.rs | 30 ++++++++++++++++++++++++++++--
 1 file changed, 28 insertions(+), 2 deletions(-)

diff --git a/rust/kernel/time.rs b/rust/kernel/time.rs
index 16b44e82d2ac..4415e91a5227 100644
--- a/rust/kernel/time.rs
+++ b/rust/kernel/time.rs
@@ -412,8 +412,20 @@ fn mul_assign(&mut self, rhs: i64) {
 impl ops::Div for Delta {
     type Output = i64;
 
+    /// # Panics
+    ///
+    /// Panics if `rhs` is zero, or if `self` represents [`i64::MIN`] nanoseconds
+    /// and `rhs` represents `-1` nanosecond.
     #[inline]
     fn div(self, rhs: Self) -> Self::Output {
+        if rhs.value == 0 {
+            panic!("attempt to divide by zero");
+        }
+
+        if self.value == i64::MIN && rhs.value == -1 {
+            panic!("attempt to divide with overflow");
+        }
+
         #[cfg(CONFIG_64BIT)]
         {
             self.value / rhs.value
@@ -421,7 +433,7 @@ fn div(self, rhs: Self) -> Self::Output {
 
         #[cfg(not(CONFIG_64BIT))]
         {
-            // SAFETY: This function is always safe to call regardless of the input values
+            // SAFETY: The checks above exclude a zero divisor and `i64::MIN / -1`.
             unsafe { bindings::div64_s64(self.value, rhs.value) }
         }
     }
@@ -615,8 +627,21 @@ pub fn to_jiffies_timeout(self) -> Delta<Jiffy> {
     }
 
     /// Return `self % divisor` where `divisor` is in nanoseconds.
+    ///
+    /// # Panics
+    ///
+    /// Panics if `divisor` is zero, or if `self` represents [`i64::MIN`]
+    /// nanoseconds and `divisor` is `-1`.
     #[inline]
     pub fn rem_nanos(self, divisor: i32) -> Self {
+        if divisor == 0 {
+            panic!("attempt to calculate the remainder with a divisor of zero");
+        }
+
+        if self.value == i64::MIN && divisor == -1 {
+            panic!("attempt to calculate the remainder with overflow");
+        }
+
         #[cfg(CONFIG_64BIT)]
         {
             Self {
@@ -628,7 +653,8 @@ pub fn rem_nanos(self, divisor: i32) -> Self {
         {
             let mut rem = 0;
 
-            // SAFETY: `rem` is in the stack, so we can always provide a valid pointer to it.
+            // SAFETY: The checks above exclude a zero divisor and `i64::MIN / -1`.
+            // `&mut rem` provides a valid, aligned, writable pointer for this call.
             unsafe { bindings::div_s64_rem(self.as_nanos(), divisor, &mut rem) };
 
             Self {
-- 
2.34.1