linux-kernel - [PATCH v2 9/9] x86/entry/vdso32: when using int $0x80, use it directly

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Message-ID: <20251112043730.992152-10-hpa@zytor.com>
Date: Tue, 11 Nov 2025 20:37:27 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: "H. Peter Anvin" <hpa@...or.com>, "Jason A. Donenfeld" <Jason@...c4.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        "Theodore Ts'o" <tytso@....edu>,
        Thomas Weißschuh <thomas.weissschuh@...utronix.de>,
        Xin Li <xin@...or.com>, Andrew Cooper <andrew.cooper3@...rix.com>,
        Andy Lutomirski <luto@...nel.org>, Ard Biesheuvel <ardb@...nel.org>,
        Borislav Petkov <bp@...en8.de>, Brian Gerst <brgerst@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>, James Morse <james.morse@....com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Josh Poimboeuf <jpoimboe@...nel.org>, Kees Cook <kees@...nel.org>,
        Nam Cao <namcao@...utronix.de>, Oleg Nesterov <oleg@...hat.com>,
        Perry Yuan <perry.yuan@....com>, Thomas Gleixner <tglx@...utronix.de>,
        Thomas Huth <thuth@...hat.com>, Uros Bizjak <ubizjak@...il.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-sgx@...r.kernel.org, x86@...nel.org
Subject: [PATCH v2 9/9] x86/entry/vdso32: when using int $0x80, use it directly

When neither sysenter32 nor syscall32 is available (on either
FRED-capable 64-bit hardware or old 32-bit hardware), there is no
reason to do a bunch of stack shuffling in __kernel_vsyscall.
Unfortunately, just overwriting the initial "push" instructions will
mess up the CFI annotations, so suffer the 3-byte NOP if not
applicable.

Similarly, inline the int $0x80 when doing inline system calls in the
vdso instead of calling __kernel_vsyscall.

Signed-off-by: H. Peter Anvin (Intel) <hpa@...or.com>
---
 arch/x86/entry/vdso/vdso32/system_call.S | 18 ++++++++++++++----
 arch/x86/include/asm/vdso/sys_call.h     |  4 +++-
 2 files changed, 17 insertions(+), 5 deletions(-)

diff --git a/arch/x86/entry/vdso/vdso32/system_call.S b/arch/x86/entry/vdso/vdso32/system_call.S
index 7b1c0f16e511..9157cf9c5749 100644
--- a/arch/x86/entry/vdso/vdso32/system_call.S
+++ b/arch/x86/entry/vdso/vdso32/system_call.S
@@ -14,6 +14,18 @@
 	ALIGN
 __kernel_vsyscall:
 	CFI_STARTPROC
+
+	/*
+	 * If using int $0x80, there is no reason to muck about with the
+	 * stack here. Unfortunately just overwriting the push instructions
+	 * would mess up the CFI annotations, but it is only a 3-byte
+	 * NOP in that case. This could be avoided by patching the
+	 * vdso symbol table (not the code) and entry point, but that
+	 * would a fair bit of tooling work or by simply compiling
+	 * two different vDSO images, but that doesn't seem worth it.
+	 */
+	ALTERNATIVE "int $0x80; ret", "", X86_FEATURE_SYSFAST32
+
 	/*
 	 * Reshuffle regs so that all of any of the entry instructions
 	 * will preserve enough state.
@@ -52,11 +64,9 @@ __kernel_vsyscall:
 	#define SYSENTER_SEQUENCE	"movl %esp, %ebp; sysenter"
 	#define SYSCALL_SEQUENCE	"movl %ecx, %ebp; syscall"
 
-	/* If SYSENTER (Intel) or SYSCALL32 (AMD) is available, use it. */
-	ALTERNATIVE_2 "", SYSENTER_SEQUENCE, X86_FEATURE_SYSFAST32, \
-			  SYSCALL_SEQUENCE,  X86_FEATURE_SYSCALL32
+	ALTERNATIVE SYSENTER_SEQUENCE, SYSCALL_SEQUENCE, X86_FEATURE_SYSCALL32
 
-	/* Enter using int $0x80 */
+	/* Re-enter using int $0x80 */
 	int	$0x80
 SYM_INNER_LABEL(int80_landing_pad, SYM_L_GLOBAL)
 
diff --git a/arch/x86/include/asm/vdso/sys_call.h b/arch/x86/include/asm/vdso/sys_call.h
index 6b1fbcdcbd5c..603ad8a83c66 100644
--- a/arch/x86/include/asm/vdso/sys_call.h
+++ b/arch/x86/include/asm/vdso/sys_call.h
@@ -27,7 +27,9 @@
 #define __sys_reg5	"r8"
 #define __sys_reg6	"r9"
 #else
-#define __sys_instr	"call __kernel_vsyscall"
+#define __sys_instr	ALTERNATIVE("ds;ds;ds;int $0x80",	\
+				    "call __kernel_vsyscall",	\
+				    X86_FEATURE_SYSFAST32)
 #define __sys_clobber	"memory"
 #define __sys_nr(x,y)	__NR_ ## x ## y
 #define __sys_reg1	"ebx"
-- 
2.51.1