Re: [PATCH v1 5/5] rust: Add warn_on and warn_on_once

On Tue, 10 Dec 2024 10:05:07 +0100
Alice Ryhl [off-list ref] wrote:

On Tue, Dec 10, 2024 at 1:19 AM FUJITA Tomonori
[off-list ref] wrote:

quoted

Add warn_on and warn_on_once macros. Wrapping the C's WARN_* and BUG_*
macros doesn't work so this uses the assembly code exported by the C
side via ARCH_WARN_ASM macro. Like the static branch code, this
generates the assembly code for rust dynamically by using the C
preprocessor.

file()! macro doesn't work for the Rust inline assembly in the same
way as __FILE__ for the C inline assembly. So the code to handle a
file name is different from the C assembly code (similar to the
arm64/loongarch assembly).

ASM_REACHABLE definition works in the same way to get objtool's
reachable asm code. The architectures which use objtool (x86 and
loongarch) needs it.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>

quoted

+#[macro_export]
+#[doc(hidden)]
+#[cfg(all(CONFIG_BUG, not(CONFIG_UML)))]
+#[cfg(any(target_arch = "x86_64", target_arch = "riscv64"))]

quoted

+#[macro_export]
+#[doc(hidden)]
+#[cfg(all(CONFIG_BUG, not(CONFIG_UML)))]
+#[cfg(any(target_arch = "aarch64", target_arch = "loongarch64"))]

What's the reason for this arch-specific code? The file!()/line!()
invocations? Could they be passed as an argument to the asm instead so
that we don't need target_arch cfgs? I understand that they don't work
exactly the same way, but maybe it could still work?

Because of "error: named argument never used" in Rust inline assembly:

All the archs define ARCH_WARN_ASM macro in the same way:

#define ARCH_WARN_ASM(file, line, flags, size)

However, only x86 and risc-v asm code use the size argument. Without
the cfgs, I'll get the following on arm64/loongarch:

error: named argument never used
  --> /home/fujita/git/linux-rust/drivers/block/rnull.rs:54:9
   |
54 |         warn_on!(true);
   |         ^^^^^^^^^^^^^^ named argument never used
   |
   = help: if this argument is intentionally unused, consider using it in an asm comment: `"/* {size} */"`
   = note: this error originates in the macro `$crate::warn_flags` which comes from the expansion of the macro `warn_on` (in Nightly builds, run with -Z macro-backtrace for more info)


Any way to make the compiler to ignore this?

quoted

+#[macro_export]
+#[doc(hidden)]
+#[cfg(all(CONFIG_BUG, CONFIG_UML))]
+macro_rules! warn_flags {
+    ($flags:expr) => {
+        // SAFETY: Just an FFI call.
+        unsafe {
+            $crate::bindings::warn_slowpath_fmt(
+                $crate::c_str!(::core::file!()).as_ptr() as *const ::core::ffi::c_char,
+                line!() as i32,
+                $flags as u32,
+                ::core::ptr::null() as *const ::core::ffi::c_char,

I wonder if this could be written to utilize Location::caller()
instead so that `#[track_caller]` works?

You meant that we could make warn_flags() function instead of macro
with Location::caller()?

If so, we need to add cfgs to warn_on and warn_on_once because both macro
and function of warn_flags are necessary?