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

diff --git a/arch/x86/entry/vdso/vdso32/system_call.S b/arch/x86/entry/vdso/vdso32/system_call.S
index 7b1c0f16e511bfd4d01bc63dcfbb858edd958e45..9157cf9c574936985e48e3057aed2243e9499bbe 100644 (file)
--- 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 dcfd17c6dd57e185149797f21e540e1e94a528d0..5806b1cd6aef0d83621385ab3963a700ebdf58c8 100644 (file)
--- a/arch/x86/include/asm/vdso/sys_call.h
+++ b/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"