[PATCH] x86: xen: insn: Decode XEN_EMULATE_PREFIX correctly

[Masami Hiramatsu]

Add XEN_EMULATE_PREFIX prefix support to x86 insn decoder.
This treats a special sequence of instructions of XEN_EMULATE_PREFIX
as a prefix bytes in x86 insn decoder only if CONFIG_XEN_PVHVM=y.
Note that this prefix is treated as just a dummy code.

Reported-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Signed-off-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
---
 arch/x86/include/asm/xen/interface.h |  8 +++++--
 arch/x86/lib/insn.c                  | 35 ++++++++++++++++++++++++++++
 2 files changed, 41 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/xen/interface.h b/arch/x86/include/asm/xen/interface.h
index 62ca03ef5c65..fbee520b1f07 100644
--- a/arch/x86/include/asm/xen/interface.h
+++ b/arch/x86/include/asm/xen/interface.h
@@ -27,6 +27,8 @@
 #ifndef _ASM_X86_XEN_INTERFACE_H
 #define _ASM_X86_XEN_INTERFACE_H
 
+#include <linux/stringify.h>
+
 /*
  * XEN_GUEST_HANDLE represents a guest pointer, when passed as a field
  * in a struct in memory.
@@ -379,11 +381,13 @@ struct xen_pmu_arch {
  * Prefix forces emulation of some non-trapping instructions.
  * Currently only CPUID.
  */
+#define __XEN_EMULATE_PREFIX  0x0f,0x0b,0x78,0x65,0x6e
+#define __XEN_EMULATE_PREFIX_STR  __stringify(__XEN_EMULATE_PREFIX)
 #ifdef __ASSEMBLY__
-#define XEN_EMULATE_PREFIX .byte 0x0f,0x0b,0x78,0x65,0x6e ;
+#define XEN_EMULATE_PREFIX .byte __XEN_EMULATE_PREFIX ;
 #define XEN_CPUID          XEN_EMULATE_PREFIX cpuid
 #else
-#define XEN_EMULATE_PREFIX ".byte 0x0f,0x0b,0x78,0x65,0x6e ; "
+#define XEN_EMULATE_PREFIX ".byte " __XEN_EMULATE_PREFIX_STR " ; "
 #define XEN_CPUID          XEN_EMULATE_PREFIX "cpuid"
 #endif
 
diff --git a/arch/x86/lib/insn.c b/arch/x86/lib/insn.c
index 0b5862ba6a75..2401a6fc9509 100644
--- a/arch/x86/lib/insn.c
+++ b/arch/x86/lib/insn.c
@@ -7,6 +7,9 @@
 
 #ifdef __KERNEL__
 #include <linux/string.h>
+#include <linux/kernel.h>
+/* For special Xen prefix */
+#include <asm/xen/interface.h>
 #else
 #include <string.h>
 #endif
@@ -58,6 +61,34 @@ void insn_init(struct insn *insn, const void *kaddr, int buf_len, int x86_64)
 		insn->addr_bytes = 4;
 }
 
+#ifdef CONFIG_XEN_PVHVM
+static const insn_byte_t xen_prefix[] = { XEN_EMULATE_PREFIX };
+
+static int insn_xen_prefix(struct insn *insn, insn_byte_t b)
+{
+	struct insn_field *prefixes = &insn->prefixes;
+	int i = 0;
+
+	while (i < ARRAY_SIZE(xen_prefix) && b == xen_prefix[i])
+		b = peek_nbyte_next(insn_byte_t, insn, ++i);
+
+	if (unlikely(i == ARRAY_SIZE(xen_prefix))) {
+		memcpy(prefixes->bytes, xen_prefix, 3);
+		prefixes->bytes[3] = xen_prefix[ARRAY_SIZE(xen_prefix) - 1];
+		prefixes->nbytes = ARRAY_SIZE(xen_prefix);
+		insn->next_byte += prefixes->nbytes;
+		prefixes->got = 1;
+
+		return 1;
+	}
+
+err_out:
+	return 0;
+}
+#else
+#define insn_xen_prefix(insn,b)	(0)
+#endif
+
 /**
  * insn_get_prefixes - scan x86 instruction prefix bytes
  * @insn:	&struct insn containing instruction
@@ -79,6 +110,10 @@ void insn_get_prefixes(struct insn *insn)
 	nb = 0;
 	lb = 0;
 	b = peek_next(insn_byte_t, insn);
+
+	if (insn_xen_prefix(insn, b))
+		return;
+
 	attr = inat_get_opcode_attribute(b);
 	while (inat_is_legacy_prefix(attr)) {
 		/* Skip if same prefix */
-- 
2.20.1


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