(Fix #341) SYSENTER instruction is not properly hooked with uc_hook_add in x86 emulation.

helper_sysenter in qemu/target-i386/seg_helper.c didn't check properly if a call interrupt callback was registred.
It has been fixed by copying the helper_syscall behavior.

tests/regress/sysenter_hook_x86.c

@@ -0,0 +1,60 @@
+#include <unicorn/unicorn.h>
+
+// code to be emulated
+#define X86_CODE32 "\x0F\x34" // SYSENTER
+
+// memory address where emulation starts
+#define ADDRESS 0x1000000
+
+int got_sysenter = 0;
+
+void sysenter (uc_engine *uc, void *user) {
+    printf ("SYSENTER hook called.\n");
+    got_sysenter = 1;
+}
+
+int main(int argc, char **argv, char **envp)
+{
+  uc_engine *uc;
+  uc_err err;
+  uc_hook sysenterHook;
+
+  // Initialize emulator in X86-32bit mode
+  err = uc_open(UC_ARCH_X86, UC_MODE_32, &uc);
+  if (err != UC_ERR_OK) {
+    printf("Failed on uc_open() with error returned: %u\n", err);
+    return -1;
+  }
+
+  // map 2MB memory for this emulation
+  uc_mem_map(uc, ADDRESS, 2 * 1024 * 1024, UC_PROT_ALL);
+
+  // write machine code to be emulated to memory
+  if (uc_mem_write(uc, ADDRESS, X86_CODE32, sizeof(X86_CODE32) - 1)) {
+    printf("Failed to write emulation code to memory, quit!\n");
+    return -1;
+  }
+
+  // Hook the SYSENTER instructions
+  if (uc_hook_add (uc, &sysenterHook, UC_HOOK_INSN, sysenter, NULL, UC_X86_INS_SYSENTER) != UC_ERR_OK) {
+      printf ("Cannot hook SYSENTER instruction\n.");
+      return -1;
+  }
+
+  // emulate code in infinite time & unlimited instructions
+  err=uc_emu_start(uc, ADDRESS, ADDRESS + sizeof(X86_CODE32) - 1, 0, 0);
+  if (err) {
+    printf("Failed on uc_emu_start() with error returned %u: %s\n",
+      err, uc_strerror(err));
+  }
+
+  printf("Emulation done.\n");
+  uc_close(uc);
+
+  if (!got_sysenter) {
+    printf ("[!] ERROR : SYSENTER hook not called.\n");
+    return -1;
+  }
+
+  return 0;
+}