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@zytor.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://patch.msgid.link/20251216212606.1325678-11-hpa@zytor.com

Author: H. Peter Anvin

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)
 

arch/x86/include/asm/vdso/sys_call.h

@@ -20,7 +20,9 @@
 # define __sys_reg4	"r10"
 # define __sys_reg5	"r8"
 #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"