linux-kernel - [kconfig][RFC] getting things like CONFIG_ARCH_HAS_SYSCALL

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Date:   Tue, 9 Aug 2022 23:57:02 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     linux-kbuild@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [kconfig][RFC] getting things like CONFIG_ARCH_HAS_SYSCALL_WRAPPER
 without bothering with selects

	Sometimes we have situations when we want linux/foo.h
include asm/bar.h if the latter exists; one example is
includes of asm/syscall_wrapper.h.  Right now there are two
ways to do that:
	1) ARCH_HAS_BAR_H, explicitly selected by architectures
that have asm/bar.h and include guarded by #ifdef CONFIG_ARCH_HAS_BAR_H
	2) include/asm-default/bar.h, either empty or with
something like #define __ARCH_HAS_NO_BAR_H, with mandatory-y += bar.h
in include/asm-generic/Kbuild and unconditional includes of asm/bar.h,
possibly followed by #ifdef __ARCH_HAS_NO_BAR_H ... #endif

However, kconfig could do (1) without selects - something like
bool ARCH_HAS_BAR_H
	def_bool $(header-exists,bar.h)

Does anybody see problems with the patch below?

Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
---
diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 1652a9800ebe..277437f329ce 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -42,7 +42,6 @@ config ARM64
 	select ARCH_HAS_STRICT_MODULE_RWX
 	select ARCH_HAS_SYNC_DMA_FOR_DEVICE
 	select ARCH_HAS_SYNC_DMA_FOR_CPU
-	select ARCH_HAS_SYSCALL_WRAPPER
 	select ARCH_HAS_TEARDOWN_DMA_OPS if IOMMU_SUPPORT
 	select ARCH_HAS_TICK_BROADCAST if GENERIC_CLOCKEVENTS_BROADCAST
 	select ARCH_HAS_VM_GET_PAGE_PROT
diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
index 91c0b80a8bf0..86e905b8462c 100644
--- a/arch/s390/Kconfig
+++ b/arch/s390/Kconfig
@@ -78,7 +78,6 @@ config S390
 	select ARCH_HAS_SET_MEMORY
 	select ARCH_HAS_STRICT_KERNEL_RWX
 	select ARCH_HAS_STRICT_MODULE_RWX
-	select ARCH_HAS_SYSCALL_WRAPPER
 	select ARCH_HAS_UBSAN_SANITIZE_ALL
 	select ARCH_HAS_VDSO_DATA
 	select ARCH_HAVE_NMI_SAFE_CMPXCHG
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index be0b95e51df6..64592d027a0d 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -92,7 +92,6 @@ config X86
 	select ARCH_HAS_STRICT_KERNEL_RWX
 	select ARCH_HAS_STRICT_MODULE_RWX
 	select ARCH_HAS_SYNC_CORE_BEFORE_USERMODE
-	select ARCH_HAS_SYSCALL_WRAPPER
 	select ARCH_HAS_UBSAN_SANITIZE_ALL
 	select ARCH_HAS_VM_GET_PAGE_PROT
 	select ARCH_HAS_DEBUG_WX
diff --git a/init/Kconfig b/init/Kconfig
index c7900e8975f1..ce88397c4e46 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -2257,4 +2257,4 @@ config ARCH_HAS_SYNC_CORE_BEFORE_USERMODE
 # kernel/time/posix-stubs.c. All these overrides need to be available in
 # <asm/syscall_wrapper.h>.
 config ARCH_HAS_SYSCALL_WRAPPER
-	def_bool n
+	def_bool $(header-exists,syscall_wrapper.h)
diff --git a/scripts/Kconfig.include b/scripts/Kconfig.include
index 0496efd6e117..465bb836f82a 100644
--- a/scripts/Kconfig.include
+++ b/scripts/Kconfig.include
@@ -23,6 +23,10 @@ success = $(if-success,$(1),y,n)
 # Return n if <command> exits with 0, y otherwise
 failure = $(if-success,$(1),n,y)
 
+# $(header-exists,<header>)
+# Return y if arch/$(ARCH)/include/asm/<header> exists, n otherwise
+header-exists = $(success,test -e $(srctree)/arch/$(ARCH)/include/asm/$(1))
+
 # $(cc-option,<flag>)
 # Return y if the compiler supports <flag>, n otherwise
 cc-option = $(success,mkdir .tmp_$$$$; trap "rm -rf .tmp_$$$$" EXIT; $(CC) -Werror $(CLANG_FLAGS) $(1) -c -x c /dev/null -o .tmp_$$$$/tmp.o)