linux-kernel - [RFT PATCH v2 13/23] x86/linkage: Add SYM_PIC_ALIAS() macro helper to emit symbol aliases

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Message-ID: <20250504095230.2932860-38-ardb+git@google.com>
Date: Sun,  4 May 2025 11:52:43 +0200
From: Ard Biesheuvel <ardb+git@...gle.com>
To: linux-kernel@...r.kernel.org
Cc: linux-efi@...r.kernel.org, x86@...nel.org, 
	Ard Biesheuvel <ardb@...nel.org>, Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...nel.org>, 
	Dionna Amalie Glaze <dionnaglaze@...gle.com>, Kevin Loughlin <kevinloughlin@...gle.com>, 
	Tom Lendacky <thomas.lendacky@....com>
Subject: [RFT PATCH v2 13/23] x86/linkage: Add SYM_PIC_ALIAS() macro helper to
 emit symbol aliases

From: Ard Biesheuvel <ardb@...nel.org>

Startup code that may execute from the early 1:1 mapping of memory will
be confined into its own address space, and only be permitted to access
ordinary kernel symbols if this is known to be safe.

Introduce a macro helper SYM_PIC_ALIAS() that emits a __pi_ prefixed
alias for a symbol, which allows startup code to access it.

Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
---
 arch/x86/include/asm/linkage.h | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/arch/x86/include/asm/linkage.h b/arch/x86/include/asm/linkage.h
index b51d8a4673f5..9d38ae744a2e 100644
--- a/arch/x86/include/asm/linkage.h
+++ b/arch/x86/include/asm/linkage.h
@@ -141,5 +141,15 @@
 #define SYM_FUNC_START_WEAK_NOALIGN(name)		\
 	SYM_START(name, SYM_L_WEAK, SYM_A_NONE)
 
+/*
+ * Expose 'sym' to the startup code in arch/x86/boot/startup/, by emitting an
+ * alias prefixed with __pi_
+ */
+#ifdef __ASSEMBLER__
+#define SYM_PIC_ALIAS(sym)	SYM_ALIAS(__pi_ ## sym, sym, SYM_L_GLOBAL)
+#else
+#define SYM_PIC_ALIAS(sym)	extern typeof(sym) __PASTE(__pi_, sym) __alias(sym)
+#endif
+
 #endif /* _ASM_X86_LINKAGE_H */
 
-- 
2.49.0.906.g1f30a19c02-goog