rust: add `build_error` crate

The `build_error` crate provides a function `build_error` which
will panic at compile-time if executed in const context and,
by default, will cause a build error if not executed at compile
time and the optimizer does not optimise away the call.

The `CONFIG_RUST_BUILD_ASSERT_ALLOW` kernel option allows to
relax the default build failure and convert it to a runtime
check. If the runtime check fails, `panic!` will be called.

Its functionality will be exposed to users as a couple macros in
the `kernel` crate in the following patch, thus some documentation
here refers to them for simplicity.

Signed-off-by: Gary Guo <[email protected]>
Reviewed-by: Wei Liu <[email protected]>
[Reworded, adapted for upstream and applied latest changes]
Signed-off-by: Miguel Ojeda <[email protected]>

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 29280072dc0e41a4dc8f893cdafadd5c9a3e36ca..452c9f06c2bc08468bfb32ec19982ee9ce722c27 100644 (file)
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -2801,6 +2801,22 @@ config RUST_OVERFLOW_CHECKS
 
          If unsure, say Y.
 
+config RUST_BUILD_ASSERT_ALLOW
+       bool "Allow unoptimized build-time assertions"
+       depends on RUST
+       help
+         Controls how are `build_error!` and `build_assert!` handled during build.
+
+         If calls to them exist in the binary, it may indicate a violated invariant
+         or that the optimizer failed to verify the invariant during compilation.
+
+         This should not happen, thus by default the build is aborted. However,
+         as an escape hatch, you can choose Y here to ignore them during build
+         and let the check be carried at runtime (with `panic!` being called if
+         the check fails).
+
+         If unsure, say N.
+
 endmenu # "Rust"
 
 source "Documentation/Kconfig"

diff --git a/rust/Makefile b/rust/Makefile
index 7700d3853404e5553c8f06b862608de7a89fae80..ff70c4c916f802fa8e1c894e81bd9bd84387f1e3 100644 (file)
--- a/rust/Makefile
+++ b/rust/Makefile
@@ -19,6 +19,12 @@ obj-$(CONFIG_RUST) += alloc.o bindings.o kernel.o
 always-$(CONFIG_RUST) += exports_alloc_generated.h exports_bindings_generated.h \
     exports_kernel_generated.h
 
+ifdef CONFIG_RUST_BUILD_ASSERT_ALLOW
+obj-$(CONFIG_RUST) += build_error.o
+else
+always-$(CONFIG_RUST) += build_error.o
+endif
+
 obj-$(CONFIG_RUST) += exports.o
 
 # Avoids running `$(RUSTC)` for the sysroot when it may not be available.
@@ -108,7 +114,7 @@ rustdoc-alloc: $(src)/alloc/lib.rs rustdoc-core rustdoc-compiler_builtins FORCE
        $(call if_changed,rustdoc)
 
 rustdoc-kernel: private rustc_target_flags = --extern alloc \
-    --extern macros=$(objtree)/$(obj)/libmacros.so \
+    --extern build_error --extern macros=$(objtree)/$(obj)/libmacros.so \
     --extern bindings
 rustdoc-kernel: $(src)/kernel/lib.rs rustdoc-core rustdoc-macros \
     rustdoc-compiler_builtins rustdoc-alloc $(obj)/libmacros.so \
@@ -126,6 +132,9 @@ quiet_cmd_rustc_test_library = RUSTC TL $<
                -L$(objtree)/$(obj)/test \
                --crate-name $(subst rusttest-,,$(subst rusttestlib-,,$@)) $<
 
+rusttestlib-build_error: $(src)/build_error.rs rusttest-prepare FORCE
+       $(call if_changed,rustc_test_library)
+
 rusttestlib-macros: private rustc_target_flags = --extern proc_macro
 rusttestlib-macros: private rustc_test_library_proc = yes
 rusttestlib-macros: $(src)/macros/lib.rs rusttest-prepare FORCE
@@ -216,9 +225,9 @@ rusttest-macros: $(src)/macros/lib.rs rusttest-prepare FORCE
        $(call if_changed,rustdoc_test)
 
 rusttest-kernel: private rustc_target_flags = --extern alloc \
-    --extern macros --extern bindings
+    --extern build_error --extern macros --extern bindings
 rusttest-kernel: $(src)/kernel/lib.rs rusttest-prepare \
-    rusttestlib-macros rusttestlib-bindings FORCE
+    rusttestlib-build_error rusttestlib-macros rusttestlib-bindings FORCE
        $(call if_changed,rustc_test)
        $(call if_changed,rustc_test_library)
 
@@ -366,6 +375,9 @@ $(obj)/alloc.o: private rustc_target_flags = $(alloc-cfgs)
 $(obj)/alloc.o: $(src)/alloc/lib.rs $(obj)/compiler_builtins.o FORCE
        $(call if_changed_dep,rustc_library)
 
+$(obj)/build_error.o: $(src)/build_error.rs $(obj)/compiler_builtins.o FORCE
+       $(call if_changed_dep,rustc_library)
+
 $(obj)/bindings.o: $(src)/bindings/lib.rs \
     $(obj)/compiler_builtins.o \
     $(obj)/bindings/bindings_generated.rs \
@@ -373,8 +385,8 @@ $(obj)/bindings.o: $(src)/bindings/lib.rs \
        $(call if_changed_dep,rustc_library)
 
 $(obj)/kernel.o: private rustc_target_flags = --extern alloc \
-    --extern macros --extern bindings
-$(obj)/kernel.o: $(src)/kernel/lib.rs $(obj)/alloc.o \
+    --extern build_error --extern macros --extern bindings
+$(obj)/kernel.o: $(src)/kernel/lib.rs $(obj)/alloc.o $(obj)/build_error.o \
     $(obj)/libmacros.so $(obj)/bindings.o FORCE
        $(call if_changed_dep,rustc_library)
 

diff --git a/rust/build_error.rs b/rust/build_error.rs
new file mode 100644 (file)
index 0000000..fa24eee
--- /dev/null
+++ b/rust/build_error.rs
@@ -0,0 +1,31 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Build-time error.
+//!
+//! This crate provides a [const function][const-functions] `build_error`, which will panic in
+//! compile-time if executed in [const context][const-context], and will cause a build error
+//! if not executed at compile time and the optimizer does not optimise away the call.
+//!
+//! It is used by `build_assert!` in the kernel crate, allowing checking of
+//! conditions that could be checked statically, but could not be enforced in
+//! Rust yet (e.g. perform some checks in [const functions][const-functions], but those
+//! functions could still be called in the runtime).
+//!
+//! For details on constant evaluation in Rust, please see the [Reference][const-eval].
+//!
+//! [const-eval]: https://doc.rust-lang.org/reference/const_eval.html
+//! [const-functions]: https://doc.rust-lang.org/reference/const_eval.html#const-functions
+//! [const-context]: https://doc.rust-lang.org/reference/const_eval.html#const-context
+
+#![no_std]
+
+/// Panics if executed in [const context][const-context], or triggers a build error if not.
+///
+/// [const-context]: https://doc.rust-lang.org/reference/const_eval.html#const-context
+#[inline(never)]
+#[cold]
+#[export_name = "rust_build_error"]
+#[track_caller]
+pub const fn build_error(msg: &'static str) -> ! {
+    panic!("{}", msg);
+}

diff --git a/rust/exports.c b/rust/exports.c
index bb7cc64cecd0ddce7e4b727581d1f842b5e79a84..83e2a7070cae88bb3ff06626b124e94565568bb8 100644 (file)
--- a/rust/exports.c
+++ b/rust/exports.c
@@ -19,3 +19,8 @@
 #include "exports_alloc_generated.h"
 #include "exports_bindings_generated.h"
 #include "exports_kernel_generated.h"
+
+// For modules using `rust/build_error.rs`.
+#ifdef CONFIG_RUST_BUILD_ASSERT_ALLOW
+EXPORT_SYMBOL_RUST_GPL(rust_build_error);
+#endif

diff --git a/scripts/generate_rust_analyzer.py b/scripts/generate_rust_analyzer.py
index 75bb611bd7516cd12f5948a2194420c18d2e27cb..ecc7ea9a4dcfc6f1c89f289f8866bad9ed1e430b 100755 (executable)
--- a/scripts/generate_rust_analyzer.py
+++ b/scripts/generate_rust_analyzer.py
@@ -67,6 +67,12 @@ def generate_crates(srctree, objtree, sysroot_src):
     )
     crates[-1]["proc_macro_dylib_path"] = "rust/libmacros.so"
 
+    append_crate(
+        "build_error",
+        srctree / "rust" / "build_error.rs",
+        ["core", "compiler_builtins"],
+    )
+
     append_crate(
         "bindings",
         srctree / "rust"/ "bindings" / "lib.rs",
@@ -78,7 +84,7 @@ def generate_crates(srctree, objtree, sysroot_src):
     append_crate(
         "kernel",
         srctree / "rust" / "kernel" / "lib.rs",
-        ["core", "alloc", "macros", "bindings"],
+        ["core", "alloc", "macros", "build_error", "bindings"],
         cfg=cfg,
     )
     crates[-1]["source"] = {