linux-user/x86_64: Allocate vsyscall page as a commpage

We're about to start validating PAGE_EXEC, which means that we've
got to mark the vsyscall page executable.  We had been special
casing this entirely within translate.

Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
Tested-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

linux-user/elfload.c

@@ -195,6 +195,27 @@ static void elf_core_copy_regs(target_elf_gregset_t *regs, const CPUX86State *en
     (*regs)[26] = tswapreg(env->segs[R_GS].selector & 0xffff);
 }
 
+#if ULONG_MAX >= TARGET_VSYSCALL_PAGE
+#define INIT_GUEST_COMMPAGE
+static bool init_guest_commpage(void)
+{
+    /*
+     * The vsyscall page is at a high negative address aka kernel space,
+     * which means that we cannot actually allocate it with target_mmap.
+     * We still should be able to use page_set_flags, unless the user
+     * has specified -R reserved_va, which would trigger an assert().
+     */
+    if (reserved_va != 0 &&
+        TARGET_VSYSCALL_PAGE + TARGET_PAGE_SIZE >= reserved_va) {
+        error_report("Cannot allocate vsyscall page");
+        exit(EXIT_FAILURE);
+    }
+    page_set_flags(TARGET_VSYSCALL_PAGE,
+                   TARGET_VSYSCALL_PAGE + TARGET_PAGE_SIZE,
+                   PAGE_EXEC | PAGE_VALID);
+    return true;
+}
+#endif
 #else
 
 #define ELF_START_MMAP 0x80000000
@@ -2360,8 +2381,10 @@ static abi_ulong create_elf_tables(abi_ulong p, int argc, int envc,
 #else
 #define HI_COMMPAGE 0
 #define LO_COMMPAGE -1
+#ifndef INIT_GUEST_COMMPAGE
 #define init_guest_commpage() true
 #endif
+#endif
 
 static void pgb_fail_in_use(const char *image_name)
 {