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Subject[PATCH v9 16/29] x86/insn-eval: Add function to get default params of code segment
Date
Obtain the default values of the address and operand sizes as specified in
the D and L bits of the the segment descriptor selected by the register
CS. The function can be used for both protected and long modes.
For virtual-8086 mode, the default address and operand sizes are always 2
bytes.

The returned parameters are encoded in a signed 8-bit data type. Auxiliar
macros are provided to encode and decode such values.

Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Adam Buchbinder <adam.buchbinder@gmail.com>
Cc: Colin Ian King <colin.king@canonical.com>
Cc: Lorenzo Stoakes <lstoakes@gmail.com>
Cc: Qiaowei Ren <qiaowei.ren@intel.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Thomas Garnier <thgarnie@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Borislav Petkov <bp@suse.de>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Ravi V. Shankar <ravi.v.shankar@intel.com>
Cc: x86@kernel.org
Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
---
arch/x86/include/asm/insn-eval.h | 5 ++++
arch/x86/lib/insn-eval.c | 64 ++++++++++++++++++++++++++++++++++++++++
2 files changed, 69 insertions(+)

diff --git a/arch/x86/include/asm/insn-eval.h b/arch/x86/include/asm/insn-eval.h
index 25d6e44..a5886ecc 100644
--- a/arch/x86/include/asm/insn-eval.h
+++ b/arch/x86/include/asm/insn-eval.h
@@ -11,8 +11,13 @@
#include <linux/err.h>
#include <asm/ptrace.h>

+#define INSN_CODE_SEG_ADDR_SZ(params) ((params >> 4) & 0xf)
+#define INSN_CODE_SEG_OPND_SZ(params) (params & 0xf)
+#define INSN_CODE_SEG_PARAMS(oper_sz, addr_sz) (oper_sz | (addr_sz << 4))
+
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, int seg_reg_idx);
+char 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 02c4498..cb2734a 100644
--- a/arch/x86/lib/insn-eval.c
+++ b/arch/x86/lib/insn-eval.c
@@ -624,6 +624,70 @@ static unsigned long get_seg_limit(struct pt_regs *regs, int seg_reg_idx)
}

/**
+ * 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.
+ *
+ * Returns:
+ *
+ * A signed 8-bit value containing the default parameters on success.
+ *
+ * -EINVAL on error.
+ */
+char insn_get_code_seg_defaults(struct pt_regs *regs)
+{
+ struct desc_struct *desc;
+ short sel;
+
+ if (v8086_mode(regs))
+ /* Address and operand size are both 16-bit. */
+ return INSN_CODE_SEG_PARAMS(2, 2);
+
+ sel = get_segment_selector(regs, INAT_SEG_REG_CS);
+ if (sel < 0)
+ return -1L;
+
+ desc = get_desc(sel);
+ if (!desc)
+ return -EINVAL;
+
+ /*
+ * The most significant byte of the Type field of the segment descriptor
+ * determines whether a segment contains data or code. If this is a data
+ * segment, return error.
+ */
+ if (!(desc->type & BIT(3)))
+ return -EINVAL;
+
+ switch ((desc->l << 1) | desc->d) {
+ case 0: /*
+ * Legacy mode. CS.L=0, CS.D=0. Address and operand size are
+ * both 16-bit.
+ */
+ return INSN_CODE_SEG_PARAMS(2, 2);
+ case 1: /*
+ * Legacy mode. CS.L=0, CS.D=1. Address and operand size are
+ * both 32-bit.
+ */
+ return INSN_CODE_SEG_PARAMS(4, 4);
+ case 2: /*
+ * IA-32e 64-bit mode. CS.L=1, CS.D=0. Address size is 64-bit;
+ * operand size is 32-bit.
+ */
+ return INSN_CODE_SEG_PARAMS(4, 8);
+ case 3: /* Invalid setting. CS.L=1, CS.D=1 */
+ /* fall through */
+ default:
+ return -EINVAL;
+ }
+}
+
+/**
* insn_get_modrm_rm_off() - Obtain register in r/m part of the ModRM byte
* @insn: Instruction containing the ModRM byte
* @regs: Register values as seen when entering kernel mode
--
2.7.4
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