[PATCH 2/6] x86/entry: Move SYSENTER's regs->sp and regs->flags fixups into C

[Andy Lutomirski]

The SYSENTER asm (32-bit and compat) contains fixups for regs->sp
and regs->flags.  Move the fixups into C and fix some comments while
we're at it.

IMO this is a valid cleanup all by itself, and it also simplifies
the subsequent patch that will fix Xen PV SYSENTER.

Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
---
 arch/x86/entry/common.c          | 12 ++++++++++++
 arch/x86/entry/entry_32.S        |  5 ++---
 arch/x86/entry/entry_64_compat.S | 11 +++++------
 3 files changed, 19 insertions(+), 9 deletions(-)

diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index ed8ccc820995..f392a8bcd1c3 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -522,6 +522,18 @@ __visible noinstr long do_fast_syscall_32(struct pt_regs *regs)
 		(regs->flags & (X86_EFLAGS_RF | X86_EFLAGS_TF | X86_EFLAGS_VM)) == 0;
 #endif
 }
+
+/* Returns 0 to return using IRET or 1 to return using SYSEXIT/SYSRETL. */
+__visible noinstr long do_SYSENTER_32(struct pt_regs *regs)
+{
+	/* SYSENTER loses RSP, but the vDSO saved it in RBP. */
+	regs->sp = regs->bp;
+
+	/* SYSENTER clobbers EFLAGS.IF.  Assume it was set in usermode. */
+	regs->flags |= X86_EFLAGS_IF;
+
+	return do_fast_syscall_32(regs);
+}
 #endif
 
 SYSCALL_DEFINE0(ni_syscall)
diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index 024d7d276cd4..2d0bd5d5f032 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -933,9 +933,8 @@ SYM_FUNC_START(entry_SYSENTER_32)
 
 .Lsysenter_past_esp:
 	pushl	$__USER_DS		/* pt_regs->ss */
-	pushl	%ebp			/* pt_regs->sp (stashed in bp) */
+	pushl	$0			/* pt_regs->sp (placeholder) */
 	pushfl				/* pt_regs->flags (except IF = 0) */
-	orl	$X86_EFLAGS_IF, (%esp)	/* Fix IF */
 	pushl	$__USER_CS		/* pt_regs->cs */
 	pushl	$0			/* pt_regs->ip = 0 (placeholder) */
 	pushl	%eax			/* pt_regs->orig_ax */
@@ -965,7 +964,7 @@ SYM_FUNC_START(entry_SYSENTER_32)
 .Lsysenter_flags_fixed:
 
 	movl	%esp, %eax
-	call	do_fast_syscall_32
+	call	do_SYSENTER_32
 	/* XEN PV guests always use IRET path */
 	ALTERNATIVE "testl %eax, %eax; jz .Lsyscall_32_done", \
 		    "jmp .Lsyscall_32_done", X86_FEATURE_XENPV
diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
index 0f974ae01e62..7b9d8150f652 100644
--- a/arch/x86/entry/entry_64_compat.S
+++ b/arch/x86/entry/entry_64_compat.S
@@ -68,16 +68,15 @@ SYM_CODE_START(entry_SYSENTER_compat)
 
 	/* Construct struct pt_regs on stack */
 	pushq	$__USER32_DS		/* pt_regs->ss */
-	pushq	%rbp			/* pt_regs->sp (stashed in bp) */
+	pushq	$0			/* pt_regs->sp = 0 (placeholder) */
 
 	/*
 	 * Push flags.  This is nasty.  First, interrupts are currently
-	 * off, but we need pt_regs->flags to have IF set.  Second, even
-	 * if TF was set when SYSENTER started, it's clear by now.  We fix
-	 * that later using TIF_SINGLESTEP.
+	 * off, but we need pt_regs->flags to have IF set.  Second, if TS
+	 * was set in usermode, it's still set, and we're singlestepping
+	 * through this code.  do_SYSENTER_32() will fix up IF.
 	 */
 	pushfq				/* pt_regs->flags (except IF = 0) */
-	orl	$X86_EFLAGS_IF, (%rsp)	/* Fix saved flags */
 	pushq	$__USER32_CS		/* pt_regs->cs */
 	pushq	$0			/* pt_regs->ip = 0 (placeholder) */
 	pushq	%rax			/* pt_regs->orig_ax */
@@ -135,7 +134,7 @@ SYM_CODE_START(entry_SYSENTER_compat)
 .Lsysenter_flags_fixed:
 
 	movq	%rsp, %rdi
-	call	do_fast_syscall_32
+	call	do_SYSENTER_32
 	/* XEN PV guests always use IRET path */
 	ALTERNATIVE "testl %eax, %eax; jz swapgs_restore_regs_and_return_to_usermode", \
 		    "jmp swapgs_restore_regs_and_return_to_usermode", X86_FEATURE_XENPV
-- 
2.25.4