x86/entry_32: Remove .fixup usage

Where possible, push the .fixup into code, at the tail of functions.

This is hard for macros since they're used in multiple functions,
therefore introduce a new extable handler to pop zeros.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lore.kernel.org/r/20211110101325.245184699@infradead.org

Author: Peter Zijlstra

arch/x86/entry/entry_32.S

@@ -270,17 +270,9 @@
 3:	popl	%fs
 	addl	$(4 + \pop), %esp	/* pop the unused "gs" slot */
 	IRET_FRAME
-.pushsection .fixup, "ax"
-4:	movl	$0, (%esp)
-	jmp	1b
-5:	movl	$0, (%esp)
-	jmp	2b
-6:	movl	$0, (%esp)
-	jmp	3b
-.popsection
-	_ASM_EXTABLE(1b, 4b)
-	_ASM_EXTABLE(2b, 5b)
-	_ASM_EXTABLE(3b, 6b)
+	_ASM_EXTABLE_TYPE(1b, 1b, EX_TYPE_POP_ZERO)
+	_ASM_EXTABLE_TYPE(2b, 2b, EX_TYPE_POP_ZERO)
+	_ASM_EXTABLE_TYPE(3b, 3b, EX_TYPE_POP_ZERO)
 .endm
 
 .macro RESTORE_ALL_NMI cr3_reg:req pop=0
@@ -925,10 +917,8 @@ SYM_FUNC_START(entry_SYSENTER_32)
 	sti
 	sysexit
 
-.pushsection .fixup, "ax"
-2:	movl	$0, PT_FS(%esp)
-	jmp	1b
-.popsection
+2:	movl    $0, PT_FS(%esp)
+	jmp     1b
 	_ASM_EXTABLE(1b, 2b)
 
 .Lsysenter_fix_flags:
@@ -996,8 +986,7 @@ restore_all_switch_stack:
 	 */
 	iret
 
-.section .fixup, "ax"
-SYM_CODE_START(asm_iret_error)
+.Lasm_iret_error:
 	pushl	$0				# no error code
 	pushl	$iret_error
 
@@ -1014,9 +1003,8 @@ SYM_CODE_START(asm_iret_error)
 #endif
 
 	jmp	handle_exception
-SYM_CODE_END(asm_iret_error)
-.previous
-	_ASM_EXTABLE(.Lirq_return, asm_iret_error)
+
+	_ASM_EXTABLE(.Lirq_return, .Lasm_iret_error)
 SYM_FUNC_END(entry_INT80_32)
 
 .macro FIXUP_ESPFIX_STACK

arch/x86/include/asm/extable_fixup_types.h

@@ -19,4 +19,6 @@
 #define	EX_TYPE_DEFAULT_MCE_SAFE	12
 #define	EX_TYPE_FAULT_MCE_SAFE		13
 
+#define	EX_TYPE_POP_ZERO		14
+
 #endif

arch/x86/mm/extable.c

@@ -99,6 +99,18 @@ static bool ex_handler_clear_fs(const struct exception_table_entry *fixup,
 	return ex_handler_default(fixup, regs);
 }
 
+static bool ex_handler_pop_zero(const struct exception_table_entry *fixup,
+				struct pt_regs *regs)
+{
+	/*
+	 * Typically used for when "pop %seg" traps, in which case we'll clear
+	 * the stack slot and re-try the instruction, which will then succeed
+	 * to pop zero.
+	 */
+	*((unsigned long *)regs->sp) = 0;
+	return ex_handler_default(fixup, regs);
+}
+
 int ex_get_fixup_type(unsigned long ip)
 {
 	const struct exception_table_entry *e = search_exception_tables(ip);
@@ -156,6 +168,8 @@ int fixup_exception(struct pt_regs *regs, int trapnr, unsigned long error_code,
 	case EX_TYPE_WRMSR_IN_MCE:
 		ex_handler_msr_mce(regs, true);
 		break;
+	case EX_TYPE_POP_ZERO:
+		return ex_handler_pop_zero(e, regs);
 	}
 	BUG();
 }