[PATCH v1 25/28] rust: add `build_error` crate

Miguel Ojeda
2022-11-10T16:45:21Z

From: Gary Guo <gary@garyguo.net>

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 <gary@garyguo.net>
[Reworded, adapted for upstream and applied latest changes]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
---
lib/Kconfig.debug | 16 ++++++++++++++++
rust/Makefile | 22 +++++++++++++++++-----
rust/build_error.rs | 24 ++++++++++++++++++++++++
rust/exports.c | 5 +++++
scripts/generate_rust_analyzer.py | 8 +++++++-
5 files changed, 69 insertions(+), 6 deletions(-)
create mode 100644 rust/build_error.rs

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 29280072dc0e..452c9f06c2bc 100644
--- 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 7700d3853404..ff70c4c916f8 100644
--- 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
index 000000000000..0ff6b33059aa
--- /dev/null
+++ b/rust/build_error.rs
@@ -0,0 +1,24 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Build-time error.
+//!
+//! This crate provides a function `build_error`, which will panic in
+//! compile-time if executed in 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, but those
+//! functions could still be called in the runtime).
+
+#![no_std]
+
+/// Panics if executed in const context, or triggers a build error if not.
+#[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 bb7cc64cecd0..83e2a7070cae 100644
--- 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 75bb611bd751..ecc7ea9a4dcf 100755
--- 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"] = {
--
2.38.1