[PATCH v7 13/26] x86/insn-eval: Add function to get default params of code segment

This function returns 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 <[email protected]>
Cc: Adam Buchbinder <[email protected]>
Cc: Colin Ian King <[email protected]>
Cc: Lorenzo Stoakes <[email protected]>
Cc: Qiaowei Ren <[email protected]>
Cc: Arnaldo Carvalho de Melo <[email protected]>
Cc: Masami Hiramatsu <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Thomas Garnier <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Dmitry Vyukov <[email protected]>
Cc: Ravi V. Shankar <[email protected]>
Cc: [email protected]
Signed-off-by: Ricardo Neri <[email protected]>
---
 arch/x86/include/asm/insn-eval.h |  6 ++++
 arch/x86/lib/insn-eval.c         | 65 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 71 insertions(+)

diff --git a/arch/x86/include/asm/insn-eval.h b/arch/x86/include/asm/insn-eval.h
index 7f3c7fe..9ed1c88 100644
--- a/arch/x86/include/asm/insn-eval.h
+++ b/arch/x86/include/asm/insn-eval.h
@@ -11,9 +11,15 @@
 #include <linux/err.h>
 #include <asm/ptrace.h>
 
+struct insn_code_seg_defaults {
+	unsigned char address_bytes;
+	unsigned char operand_bytes;
+};
+
 void __user *insn_get_addr_ref(struct insn *insn, struct pt_regs *regs);
 int insn_get_modrm_rm_off(struct insn *insn, struct pt_regs *regs);
 unsigned long insn_get_seg_base(struct pt_regs *regs, struct insn *insn,
 				int regoff);
+struct insn_code_seg_defaults insn_get_code_seg_defaults(struct pt_regs *regs);
 
 #endif /* _ASM_X86_INSN_EVAL_H */
diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
index c77ed80..693e5a8 100644
--- a/arch/x86/lib/insn-eval.c
+++ b/arch/x86/lib/insn-eval.c
@@ -603,6 +603,71 @@ static unsigned long get_seg_limit(struct pt_regs *regs, struct insn *insn,
 }
 
 /**
+ * insn_get_code_seg_defaults() - Obtain code segment default parameters
+ * @regs:	Structure with register values as seen when entering kernel mode
+ *
+ * Obtain the default parameters of the code segment: address and operand sizes.
+ * The code segment is obtained from the selector contained in the CS register
+ * in regs. 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 for both address and operand sizes.
+ *
+ * Return: A populated insn_code_seg_defaults structure on success. The
+ * structure contains only zeros on failure.
+ */
+struct insn_code_seg_defaults insn_get_code_seg_defaults(struct pt_regs *regs)
+{
+	struct desc_struct *desc;
+	struct insn_code_seg_defaults defs;
+	unsigned short sel;
+	/*
+	 * The most significant byte of AR_TYPE_MASK determines whether a
+	 * segment contains data or code.
+	 */
+	unsigned int type_mask = AR_TYPE_MASK & (1 << 11);
+
+	memset(&defs, 0, sizeof(defs));
+
+	if (v8086_mode(regs)) {
+		defs.address_bytes = 2;
+		defs.operand_bytes = 2;
+		return defs;
+	}
+
+	sel = (unsigned short)regs->cs;
+
+	desc = get_desc(sel);
+	if (!desc)
+		return defs;
+
+	/* if data segment, return */
+	if (!(desc->b & type_mask))
+		return defs;
+
+	switch ((desc->l << 1) | desc->d) {
+	case 0: /* Legacy mode. CS.L=0, CS.D=0 */
+		defs.address_bytes = 2;
+		defs.operand_bytes = 2;
+		break;
+	case 1: /* Legacy mode. CS.L=0, CS.D=1 */
+		defs.address_bytes = 4;
+		defs.operand_bytes = 4;
+		break;
+	case 2: /* IA-32e 64-bit mode. CS.L=1, CS.D=0 */
+		defs.address_bytes = 8;
+		defs.operand_bytes = 4;
+		break;
+	case 3: /* Invalid setting. CS.L=1, CS.D=1 */
+		/* fall through */
+	default:
+		defs.address_bytes = 0;
+		defs.operand_bytes = 0;
+	}
+
+	return defs;
+}
+
+/**
  * insn_get_reg_offset_modrm_rm() - Obtain register in r/m part of ModRM byte
  * @insn:	Instruction structure containing the ModRM byte
  * @regs:	Structure with register values as seen when entering kernel mode
-- 
2.9.3