Improve BUILD_BUG_ON error message with clang #1173

pcc · 2020-10-09T19:40:17Z

With clang BUILD_BUG_ON failures only appear at link time due to a lack of support for __attribute__((error)). The error messages look like this:

ld.lld: error: undefined symbol: __compiletime_assert_296
>>> referenced by signal_compat.c
>>>               kernel/signal_compat.o:(sigaction_compat_abi) in archive arch/x86/built-in.a

and it can be hard to trace them back to the caller.

I had the idea of putting the error message in the symbol name so that it survives until link time. Here's a quick proof of concept:

diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
index 4e035aca6f7e..f553ad004e5b 100644
--- a/include/linux/bitfield.h
+++ b/include/linux/bitfield.h
@@ -109,10 +109,9 @@
                (typeof(_mask))(((_reg) & (_mask)) >> __bf_shf(_mask)); \
        })
 
-extern void __compiletime_error("value doesn't fit into mask")
-__field_overflow(void);
-extern void __compiletime_error("bad bitfield mask")
-__bad_mask(void);
+extern void __field_overflow(void)
+       __compiletime_error("value doesn't fit into mask");
+extern void __bad_mask(void) __compiletime_error("bad bitfield mask");
 static __always_inline u64 field_multiplier(u64 field)
 {
        if ((field | (field - 1)) & ((field | (field - 1)) + 1))
diff --git a/include/linux/compiler-clang.h b/include/linux/compiler-clang.h
index cee0c728d39a..f4ad81f6f75e 100644
--- a/include/linux/compiler-clang.h
+++ b/include/linux/compiler-clang.h
@@ -61,3 +61,5 @@
 #if __has_feature(shadow_call_stack)
 # define __noscs       __attribute__((__no_sanitize__("shadow-call-stack")))
 #endif
+
+#define __compiletime_error(msg) __asm__(msg)
diff --git a/include/linux/thread_info.h b/include/linux/thread_info.h
index e93e249a4e9b..e6476b1be8cc 100644
--- a/include/linux/thread_info.h
+++ b/include/linux/thread_info.h
@@ -124,10 +124,10 @@ static inline void check_object_size(const void *ptr, unsigned long n,
 { }
 #endif /* CONFIG_HARDENED_USERCOPY */
 
-extern void __compiletime_error("copy source size is too small")
-__bad_copy_from(void);
-extern void __compiletime_error("copy destination size is too small")
-__bad_copy_to(void);
+extern void __bad_copy_from(void)
+       __compiletime_error("copy source size is too small");
+extern void __bad_copy_to(void)
+       __compiletime_error("copy destination size is too small");
 
 static inline void copy_overflow(int size, unsigned long count)
 {

With that and an artificial BUILD_BUG_ON failure injected into the kernel:

diff --git a/arch/x86/kernel/signal_compat.c b/arch/x86/kernel/signal_compat.c
index 862e69f6054f..5dfba690cf6b 100644
--- a/arch/x86/kernel/signal_compat.c
+++ b/arch/x86/kernel/signal_compat.c
@@ -164,7 +164,7 @@ static inline void signal_compat_build_tests(void)
        BUILD_BUG_ON(offsetof(siginfo_t, si_arch)      != 0x1C);
        BUILD_BUG_ON(offsetof(compat_siginfo_t, si_call_addr) != 0x0C);
        BUILD_BUG_ON(offsetof(compat_siginfo_t, si_syscall)   != 0x10);
-       BUILD_BUG_ON(offsetof(compat_siginfo_t, si_arch)      != 0x14);
+       BUILD_BUG_ON(offsetof(compat_siginfo_t, si_arch)      != 0x18);
 
        /* any new si_fields should be added here */
 }

the error message looks like this:

ld.lld: error: undefined symbol: BUILD_BUG_ON failed: offsetof(compat_siginfo_t, si_arch) != 0x18
>>> referenced by signal_compat.c
>>>               kernel/signal_compat.o:(sigaction_compat_abi) in archive arch/x86/built-in.a

Not perfect but at least you can see the error message. It does require LLVM_IAS=1 because GNU as doesn't like these weird symbol names. I don't know if the kernel folks will go for this though.

The text was updated successfully, but these errors were encountered:

nickdesaulniers · 2020-10-21T21:38:02Z

cc @arndb

nickdesaulniers · 2021-01-21T19:36:48Z

See also: https://llvm.org/pr16428

nickdesaulniers · 2021-01-21T19:39:53Z

Additional context: https://lore.kernel.org/lkml/CAKwvOdko=t=wBsiwwMaWOLuir09jLi-r_oyr=1q-HRsi=-DjoA@mail.gmail.com/

nickdesaulniers · 2021-07-15T02:01:29Z

https://reviews.llvm.org/D106030

nickdesaulniers · 2021-07-15T02:02:31Z

I also have this diff to send to LKML:

diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index cb9217fc60af..4ee87cbe7208 100644
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -44,7 +44,6 @@
 #define __compiletime_object_size(obj) __builtin_object_size(obj, 0)
 
 #define __compiletime_warning(message) __attribute__((__warning__(message)))
-#define __compiletime_error(message) __attribute__((__error__(message)))
 
 #if defined(LATENT_ENTROPY_PLUGIN) && !defined(__CHECKER__)
 #define __latent_entropy __attribute__((latent_entropy))
diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h
index 2487be0e7199..fcdcb711d715 100644
--- a/include/linux/compiler_attributes.h
+++ b/include/linux/compiler_attributes.h
@@ -137,6 +137,12 @@
 # define __designated_init
 #endif
 
+#if __has_attribute(__error__)
+# define __compiletime_error(msg)       __attribute__((__error__(msg)))
+#else
+# define __compiletime_error(msg)
+#endif
+
 /*
  * Optional: not supported by clang
  *
diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index e4ea86fc584d..cee45de0f61b 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -297,9 +297,6 @@ struct ftrace_likely_data {
 #ifndef __compiletime_warning
 # define __compiletime_warning(message)
 #endif
-#ifndef __compiletime_error
-# define __compiletime_error(message)
-#endif
 
 #ifdef __OPTIMIZE__
 # define __compiletime_assert(condition, msg, prefix, suffix)          \

nickdesaulniers · 2021-07-20T18:08:41Z

kernel patch: https://lore.kernel.org/lkml/20210720180441.2893919-1-ndesaulniers@google.com/T/#u

I'm working on adding support for __attribute__((__error__(""))) and __attribute__((__warning__(""))) to Clang. To make use of these in __compiletime_error and __compiletime_warning (as used by BUILD_BUG and friends) for newer clang and detect/fallback for older versions of clang, move these to compiler_attributes.h and guard them with __has_attribute preprocessor guards. Link: https://reviews.llvm.org/D106030 Link: https://bugs.llvm.org/show_bug.cgi?id=16428 Link: ClangBuiltLinux#1173 Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>

nickdesaulniers · 2021-08-04T18:52:11Z

v2 kernel patch: https://lore.kernel.org/lkml/20210802202326.1817503-1-ndesaulniers@google.com/

Add support for the GNU C style __attribute__((error(""))) and __attribute__((warning(""))). These attributes are meant to be put on declarations of functions whom should not be called. They are frequently used to provide compile time diagnostics similar to _Static_assert, but which may rely on non-ICE conditions (ie. relying on compiler optimizations). This is also similar to diagnose_if function attribute, but can diagnose after optimizations have been run. While users may instead simply call undefined functions in such cases to get a linkage failure from the linker, these provide a much more ergonomic and actionable diagnostic to users and do so at compile time rather than at link time. Users instead may be able use inline asm .err directives. These are used throughout the Linux kernel in its implementation of BUILD_BUG and BUILD_BUG_ON macros. These macros generally cannot be converted to use _Static_assert because many of the parameters are not ICEs. The Linux kernel still needs to be modified to make use of these when building with Clang; I have a patch that does so I will send once this feature is landed. To do so, we create a new IR level Function attribute, "dontcall" (both error and warning boil down to one IR Fn Attr). Then, similar to calls to inline asm, we attach a !srcloc Metadata node to call sites of such attributed callees. The backend diagnoses these during instruction selection, while we still know that a call is a call (vs say a JMP that's a tail call) in an arch agnostic manner. The frontend then reconstructs the SourceLocation from that Metadata, and determines whether to emit an error or warning based on the callee's attribute. Link: https://bugs.llvm.org/show_bug.cgi?id=16428 Link: ClangBuiltLinux/linux#1173 Reviewed By: aaron.ballman Differential Revision: https://reviews.llvm.org/D106030

nickdesaulniers · 2021-08-25T22:49:09Z

fixed in llvm-14:

Let's wait to close this out though until @ojeda picks up the kernel patch (with suggestion to s/clang-13/clang-14/) then that hits mainline.

Clang 14 will add support for __attribute__((__error__(""))) and __attribute__((__warning__(""))). To make use of these in __compiletime_error and __compiletime_warning (as used by BUILD_BUG and friends) for newer clang and detect/fallback for older versions of clang, move these to compiler_attributes.h and guard them with __has_attribute preprocessor guards. Link: https://reviews.llvm.org/D106030 Link: https://bugs.llvm.org/show_bug.cgi?id=16428 Link: ClangBuiltLinux#1173 Signed-off-by: Nick Desaulniers <ndesaulniers@google.com> Reviewed-by: Nathan Chancellor <nathan@kernel.org> Reviewed-by: Kees Cook <keescook@chromium.org> [Reworded, landed in Clang 14] Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

nickdesaulniers · 2021-09-14T16:32:25Z

b83a908
v5.15-rc1

kees · 2021-09-15T07:38:45Z

Awesome! I can confirm this works as expected for the FORTIFY_SOURCE warnings:
https://godbolt.org/z/qGrPqceWf

nickdesaulniers added the feature-request Not a bug per-se label Oct 14, 2020

nickdesaulniers added the Reported upstream This bug was filed on LLVM’s issue tracker, Phabricator, or the kernel mailing list. label Jan 21, 2021

nickdesaulniers self-assigned this Jul 15, 2021

nickdesaulniers added the [PATCH] Submitted A patch has been submitted for review label Jul 15, 2021

nickdesaulniers added the [FIXED][LLVM] 14 This bug was fixed in LLVM 14.x label Aug 25, 2021

nickdesaulniers closed this as completed Sep 14, 2021

nickdesaulniers added [FIXED][LINUX] 5.15 This bug was fixed in Linux 5.15 and removed [PATCH] Submitted A patch has been submitted for review labels Sep 14, 2021

caoman mentioned this issue Jul 23, 2022

8290900: Build failure with Clang 14+ due to function warning attribute openjdk/jdk#9621

Closed

3 tasks

Improve BUILD_BUG_ON error message with clang #1173

Improve BUILD_BUG_ON error message with clang #1173

pcc commented Oct 9, 2020

nickdesaulniers commented Oct 21, 2020

nickdesaulniers commented Jan 21, 2021 •

edited by nathanchance

nickdesaulniers commented Jan 21, 2021

nickdesaulniers commented Jul 15, 2021

nickdesaulniers commented Jul 15, 2021

nickdesaulniers commented Jul 20, 2021

nickdesaulniers commented Aug 4, 2021

nickdesaulniers commented Aug 25, 2021

nickdesaulniers commented Sep 14, 2021

kees commented Sep 15, 2021

Improve BUILD_BUG_ON error message with clang #1173

Improve BUILD_BUG_ON error message with clang #1173

Comments

pcc commented Oct 9, 2020

nickdesaulniers commented Oct 21, 2020

nickdesaulniers commented Jan 21, 2021 • edited by nathanchance

nickdesaulniers commented Jan 21, 2021

nickdesaulniers commented Jul 15, 2021

nickdesaulniers commented Jul 15, 2021

nickdesaulniers commented Jul 20, 2021

nickdesaulniers commented Aug 4, 2021

nickdesaulniers commented Aug 25, 2021

nickdesaulniers commented Sep 14, 2021

kees commented Sep 15, 2021

nickdesaulniers commented Jan 21, 2021 •

edited by nathanchance