[PATCH v4 08/17] x86/insn-eval: Add functions to get default operand and address sizes

[Ricardo Neri]

These functions read the default values of the address and operand sizes
as specified in the segment descriptor. This information is determined
from the D and L bits. Hence, it can be used for both IA-32e 64-bit and
32-bit legacy modes. For virtual-8086 mode, the default address and
operand sizes are always 2 bytes.

The D bit is only meaningful for code segments. Thus, these functions
always use the code segment selector contained in regs.

Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Adam Buchbinder <adam.buchbinder@xxxxxxxxx>
Cc: Colin Ian King <colin.king@xxxxxxxxxxxxx>
Cc: Lorenzo Stoakes <lstoakes@xxxxxxxxx>
Cc: Qiaowei Ren <qiaowei.ren@xxxxxxxxx>
Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Cc: Adrian Hunter <adrian.hunter@xxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Thomas Garnier <thgarnie@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxx>
Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Cc: Ravi V. Shankar <ravi.v.shankar@xxxxxxxxx>
Cc: x86@xxxxxxxxxx
Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
---
 arch/x86/include/asm/insn-eval.h |  2 +
 arch/x86/lib/insn-eval.c         | 80 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 82 insertions(+)

diff --git a/arch/x86/include/asm/insn-eval.h b/arch/x86/include/asm/insn-eval.h
index 0de3083..cd4008251 100644
--- a/arch/x86/include/asm/insn-eval.h
+++ b/arch/x86/include/asm/insn-eval.h
@@ -15,6 +15,8 @@ void __user *insn_get_addr_ref(struct insn *insn, struct pt_regs *regs);
 int insn_get_reg_offset_modrm_rm(struct insn *insn, struct pt_regs *regs);
 int insn_get_reg_offset_sib_base(struct insn *insn, struct pt_regs *regs);
 int insn_get_reg_offset_sib_base(struct insn *insn, struct pt_regs *regs);
+unsigned char insn_get_seg_default_address_bytes(struct pt_regs *regs);
+unsigned char insn_get_seg_default_operand_bytes(struct pt_regs *regs);
 unsigned long insn_get_seg_base(struct pt_regs *regs, struct insn *insn,
 				int regoff);
 
diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
index 4e3f797..3fe4ddb 100644
--- a/arch/x86/lib/insn-eval.c
+++ b/arch/x86/lib/insn-eval.c
@@ -365,6 +365,86 @@ unsigned long insn_get_seg_base(struct pt_regs *regs, struct insn *insn,
 }
 
 /**
+ * insn_get_seg_default_address_bytes - Obtain default address size of segment
+ * @regs:	Set of registers containing the segment selector
+ *
+ * Obtain the default address size as indicated in the segment descriptor
+ * selected in regs' code segment selector. In protected mode, the default
+ * address is determined by inspecting the L and D bits of the segment
+ * descriptor. In virtual-8086 mode, the default is always two bytes.
+ *
+ * Return: Default address size of segment
+ */
+unsigned char insn_get_seg_default_address_bytes(struct pt_regs *regs)
+{
+	struct desc_struct *desc;
+	unsigned short seg;
+	int ret;
+
+	if (v8086_mode(regs))
+		return 2;
+
+	seg = (unsigned short)regs->cs;
+
+	ret = get_desc(seg, &desc);
+	if (ret)
+		return 0;
+
+	switch ((desc->l << 1) | desc->d) {
+	case 0: /* Legacy mode. 16-bit addresses. CS.L=0, CS.D=0 */
+		return 2;
+	case 1: /* Legacy mode. 32-bit addresses. CS.L=0, CS.D=1 */
+		return 4;
+	case 2: /* IA-32e 64-bit mode. 64-bit addresses. CS.L=1, CS.D=0 */
+		return 8;
+	case 3: /* Invalid setting. CS.L=1, CS.D=1 */
+		/* fall through */
+	default:
+		return 0;
+	}
+}
+
+/**
+ * insn_get_seg_default_operand_bytes - Obtain default operand size of segment
+ * @regs:	Set of registers containing the segment selector
+ *
+ * Obtain the default operand size as indicated in the segment descriptor
+ * selected in regs' code segment selector. In protected mode, the default
+ * operand size is determined by inspecting the L and D bits of the segment
+ * descriptor. In virtual-8086 mode, the default is always two bytes.
+ *
+ * Return: Default operand size of segment
+ */
+unsigned char insn_get_seg_default_operand_bytes(struct pt_regs *regs)
+{
+	struct desc_struct *desc;
+	unsigned short seg;
+	int ret;
+
+	if (v8086_mode(regs))
+		return 2;
+
+	seg = (unsigned short)regs->cs;
+
+	ret = get_desc(seg, &desc);
+	if (ret)
+		return 0;
+
+	switch ((desc->l << 1) | desc->d) {
+	case 0: /* Legacy mode. 16-bit or 8-bit operands CS.L=0, CS.D=0 */
+		return 2;
+	case 1: /* Legacy mode. 32- or 8 bit operands CS.L=0, CS.D=1 */
+		/* fall through */
+	case 2: /* IA-32e 64-bit mode. 32- or 8-bit opnds. CS.L=1, CS.D=0 */
+		return 4;
+	case 3: /* Invalid setting. CS.L=1, CS.D=1 */
+		/* fall through */
+	default:
+		return 0;
+	}
+}
+
+/**
  * insn_get_reg_offset_modrm_rm - Obtain register in r/m part of ModRM byte
  * @insn:	Instruction structure containing the ModRM byte
  * @regs:	Set of registers indicated by the ModRM byte
-- 
2.9.3