x86/extable: Extend extable functionality

In order to remove further .fixup usage, extend the extable
infrastructure to take additional information from the extable entry
sites.

Specifically add _ASM_EXTABLE_TYPE_REG() and EX_TYPE_IMM_REG that
extend the existing _ASM_EXTABLE_TYPE() by taking an additional
register argument and encoding that and an s16 immediate into the
existing s32 type field. This limits the actual types to the first
byte, 255 seem plenty.

Also add a few flags into the type word, specifically CLEAR_AX and
CLEAR_DX which clear the return and extended return register.

Notes:
 - due to the % in our register names it's hard to make it more
   generally usable as arm64 did.
 - the s16 is far larger than used in these patches, future extentions
   can easily shrink this to get more bits.
 - without the bitfield fix this will not compile, because: 0xFF > -1
   and we can't even extract the TYPE field.

[nathanchance: Build fix for clang-lto builds:
 https://lkml.kernel.org/r/20211210234953.3420108-1-nathan@kernel.org
]

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Josh Poimboeuf <jpoimboe@redhat.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Link: https://lore.kernel.org/r/20211110101325.303890153@infradead.org

1 files changed, 36 insertions, 4 deletions

diff --git a/arch/x86/mm/extable.c b/arch/x86/mm/extable.c
index fb0c4752df1a..cef8901b3e6f 100644
--- a/arch/x86/mm/extable.c
+++ b/arch/x86/mm/extable.c
@@ -2,12 +2,25 @@
 #include <linux/extable.h>
 #include <linux/uaccess.h>
 #include <linux/sched/debug.h>
+#include <linux/bitfield.h>
 #include <xen/xen.h>
 
 #include <asm/fpu/api.h>
 #include <asm/sev.h>
 #include <asm/traps.h>
 #include <asm/kdebug.h>
+#include <asm/insn-eval.h>
+
+static inline unsigned long *pt_regs_nr(struct pt_regs *regs, int nr)
+{
+	int reg_offset = pt_regs_offset(regs, nr);
+	static unsigned long __dummy;
+
+	if (WARN_ON_ONCE(reg_offset < 0))
+		return &__dummy;
+
+	return (unsigned long *)((unsigned long)regs + reg_offset);
+}
 
 static inline unsigned long
 ex_fixup_addr(const struct exception_table_entry *x)
@@ -15,10 +28,15 @@ ex_fixup_addr(const struct exception_table_entry *x)
 	return (unsigned long)&x->fixup + x->fixup;
 }
 
-static bool ex_handler_default(const struct exception_table_entry *fixup,
+static bool ex_handler_default(const struct exception_table_entry *e,
 			       struct pt_regs *regs)
 {
-	regs->ip = ex_fixup_addr(fixup);
+	if (e->data & EX_FLAG_CLEAR_AX)
+		regs->ax = 0;
+	if (e->data & EX_FLAG_CLEAR_DX)
+		regs->dx = 0;
+
+	regs->ip = ex_fixup_addr(e);
 	return true;
 }
 
@@ -111,17 +129,25 @@ static bool ex_handler_pop_zero(const struct exception_table_entry *fixup,
 	return ex_handler_default(fixup, regs);
 }
 
+static bool ex_handler_imm_reg(const struct exception_table_entry *fixup,
+			       struct pt_regs *regs, int reg, int imm)
+{
+	*pt_regs_nr(regs, reg) = (long)imm;
+	return ex_handler_default(fixup, regs);
+}
+
 int ex_get_fixup_type(unsigned long ip)
 {
 	const struct exception_table_entry *e = search_exception_tables(ip);
 
-	return e ? e->type : EX_TYPE_NONE;
+	return e ? FIELD_GET(EX_DATA_TYPE_MASK, e->data) : EX_TYPE_NONE;
 }
 
 int fixup_exception(struct pt_regs *regs, int trapnr, unsigned long error_code,
 		    unsigned long fault_addr)
 {
 	const struct exception_table_entry *e;
+	int type, reg, imm;
 
 #ifdef CONFIG_PNPBIOS
 	if (unlikely(SEGMENT_IS_PNP_CODE(regs->cs))) {
@@ -141,7 +167,11 @@ int fixup_exception(struct pt_regs *regs, int trapnr, unsigned long error_code,
 	if (!e)
 		return 0;
 
-	switch (e->type) {
+	type = FIELD_GET(EX_DATA_TYPE_MASK, e->data);
+	reg  = FIELD_GET(EX_DATA_REG_MASK,  e->data);
+	imm  = FIELD_GET(EX_DATA_IMM_MASK,  e->data);
+
+	switch (type) {
 	case EX_TYPE_DEFAULT:
 	case EX_TYPE_DEFAULT_MCE_SAFE:
 		return ex_handler_default(e, regs);
@@ -170,6 +200,8 @@ int fixup_exception(struct pt_regs *regs, int trapnr, unsigned long error_code,
 		break;
 	case EX_TYPE_POP_ZERO:
 		return ex_handler_pop_zero(e, regs);
+	case EX_TYPE_IMM_REG:
+		return ex_handler_imm_reg(e, regs, reg, imm);
 	}
 	BUG();
 }