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

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Date:   Sat, 11 Mar 2023 00:48:24 -0800
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 1/1] x86/vdso: use 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_.

Compile the 32-bit getcpu() on 64-bit Linux kernel with the 64-bit Linux kernel
GDT_ENTRY_CPUNODE.

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>
---
 arch/x86/include/asm/segment.h | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/include/asm/segment.h b/arch/x86/include/asm/segment.h
index 794f69625780..d3b4f8797054 100644
--- a/arch/x86/include/asm/segment.h
+++ b/arch/x86/include/asm/segment.h
@@ -119,7 +119,11 @@
 
 #define GDT_ENTRY_ESPFIX_SS		26
 #define GDT_ENTRY_PERCPU		27
+#ifndef BUILD_VDSO32_64
 #define GDT_ENTRY_CPUNODE		28
+#else
+#define GDT_ENTRY_CPUNODE		15
+#endif
 
 #define GDT_ENTRY_DOUBLEFAULT_TSS	31
 
-- 
2.34.1