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error: invalid output size for constraint '=a' for ARCH=i386 after e2ffa15b9baa #2121
Description
After commit e2ffa15b9baa ("kbuild: Disable CC_HAS_ASM_GOTO_OUTPUT on clang < 17"), which exposes a separate code path that was not used before, there is an error when building mm/maccess.c for ARCH=i386 with LLVM 15 or 16 (the only supported versions impacted):
$ make -skj"$(nproc)" ARCH=i386 LLVM=1 clean defconfig mm/maccess.o
mm/maccess.c:41:3: error: invalid output size for constraint '=a'
copy_from_kernel_nofault_loop(dst, src, size, u64, Efault);
^
mm/maccess.c:22:3: note: expanded from macro 'copy_from_kernel_nofault_loop'
__get_kernel_nofault(dst, src, type, err_label); \
^
arch/x86/include/asm/uaccess.h:630:18: note: expanded from macro '__get_kernel_nofault'
__get_user_size(*((type *)(dst)), (__force type __user *)(src), \
^
mm/maccess.c:41:3: error: invalid output size for constraint '=a'
mm/maccess.c:22:3: note: expanded from macro 'copy_from_kernel_nofault_loop'
__get_kernel_nofault(dst, src, type, err_label); \
^
arch/x86/include/asm/uaccess.h:630:18: note: expanded from macro '__get_kernel_nofault'
__get_user_size(*((type *)(dst)), (__force type __user *)(src), \
^
Activity
nickdesaulniers commentedon Mar 30, 2026
So this only affects older releases of clang?
nathanchance commentedon Mar 30, 2026
I think the
error: invalid output size for constraint '=a'could still be seen even with tip of tree clang but that code path is not taken when asm goto with outputs is used (soclang-17and newer is “fixed”).nickdesaulniers commentedon Mar 30, 2026
Can you help reduce a test case to report upstream? (Low priority)