linux-kernel - [PATCH v2 1/1] x86/vdso: choose the right GDT_ENTRY_CPUNODE for 32-bit getcpu() on 64-bit kernel

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Date:   Tue, 21 Mar 2023 23:17:58 -0700
From:   Xin Li <xin3.li@...el.com>
To:     linux-kernel@...r.kernel.org, x86@...nel.org
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com, bigeasy@...utronix.de,
        yujie.liu@...el.com, shan.kang@...el.com
Subject: [PATCH v2 1/1] x86/vdso: choose the right GDT_ENTRY_CPUNODE for 32-bit getcpu() on 64-bit kernel

The vDSO getcpu() reads CPU ID from the GDT_ENTRY_CPUNODE entry when the RDPID
instruction is not available. And GDT_ENTRY_CPUNODE is defined as 28 on 32-bit
Linux kernel and 15 on 64-bit. But the 32-bit getcpu() on 64-bit Linux kernel
is compiled with 32-bit Linux kernel GDT_ENTRY_CPUNODE, i.e., 28, beyond the
64-bit Linux kernel GDT limit. Thus, it just fails _silently_.

When BUILD_VDSO32_64 is defined, choose the 64-bit Linux kernel GDT definitions
to compile the 32-bit getcpu().

Fixes: 877cff5296faa6e ("x86/vdso: Fake 32bit VDSO build on 64bit compile for vgetcpu")
Reported-by: kernel test robot <yujie.liu@...el.com>
https://lore.kernel.org/oe-lkp/202303020903.b01fd1de-yujie.liu@intel.com/
Reported-by: Shan Kang <shan.kang@...el.com>
Signed-off-by: Xin Li <xin3.li@...el.com>
---

Change since v1:
* Define all of the GDT_ENTRY_* macros in *one* place, then make that *one*
  place depend on BUILD_VDSO32_64 (Dave Hansen).
---
 arch/x86/include/asm/segment.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/segment.h b/arch/x86/include/asm/segment.h
index 794f69625780..9d6411c65920 100644
--- a/arch/x86/include/asm/segment.h
+++ b/arch/x86/include/asm/segment.h
@@ -56,7 +56,7 @@
 
 #define GDT_ENTRY_INVALID_SEG	0
 
-#ifdef CONFIG_X86_32
+#if defined(CONFIG_X86_32) && !defined(BUILD_VDSO32_64)
 /*
  * The layout of the per-CPU GDT under Linux:
  *
-- 
2.34.1