linux-user/elfload: use MAP_FIXED_NOREPLACE in pgb_reserved_va

Given we assert the requested address matches what we asked we should
also make that clear in the mmap flags. Otherwise we see failures in
the GitLab environment for some currently unknown but allowable
reason. We use MAP_FIXED_NOREPLACE if we can so we don't just clobber
an existing mapping. Also include the strerror string for a bit more
info on failure.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

Message-Id: <20200701135652.1366-34-alex.bennee@linaro.org>

linux-user/elfload.c

@@ -2294,7 +2294,7 @@ static void pgb_dynamic(const char *image_name, long align)
 static void pgb_reserved_va(const char *image_name, abi_ulong guest_loaddr,
                             abi_ulong guest_hiaddr, long align)
 {
-    const int flags = MAP_ANONYMOUS | MAP_PRIVATE | MAP_NORESERVE;
+    int flags = MAP_ANONYMOUS | MAP_PRIVATE | MAP_NORESERVE;
     void *addr, *test;
 
     if (guest_hiaddr > reserved_va) {
@@ -2307,15 +2307,19 @@ static void pgb_reserved_va(const char *image_name, abi_ulong guest_loaddr,
     /* Widen the "image" to the entire reserved address space. */
     pgb_static(image_name, 0, reserved_va, align);
 
+#ifdef MAP_FIXED_NOREPLACE
+    flags |= MAP_FIXED_NOREPLACE;
+#endif
+
     /* Reserve the memory on the host. */
     assert(guest_base != 0);
     test = g2h(0);
     addr = mmap(test, reserved_va, PROT_NONE, flags, -1, 0);
     if (addr == MAP_FAILED) {
         error_report("Unable to reserve 0x%lx bytes of virtual address "
-                     "space for use as guest address space (check your "
+                     "space (%s) for use as guest address space (check your "
                      "virtual memory ulimit setting or reserve less "
-                     "using -R option)", reserved_va);
+                     "using -R option)", reserved_va, strerror(errno));
         exit(EXIT_FAILURE);
     }
     assert(addr == test);