linux-user: Use MAP_FIXED_NOREPLACE for do_brk()

MAP_FIXED_NOREPLACE can ensure the mapped address is fixed without
concerning that the new mapping overwrites something else.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Message-Id: <20230802071754.14876-5-akihiko.odaki@daynix.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

linux-user/syscall.c

@@ -854,17 +854,12 @@ abi_long do_brk(abi_ulong brk_val)
         return target_brk;
     }
 
-    /* We need to allocate more memory after the brk... Note that
-     * we don't use MAP_FIXED because that will map over the top of
-     * any existing mapping (like the one with the host libc or qemu
-     * itself); instead we treat "mapped but at wrong address" as
-     * a failure and unmap again.
-     */
     if (new_host_brk_page > brk_page) {
         new_alloc_size = new_host_brk_page - brk_page;
         mapped_addr = target_mmap(brk_page, new_alloc_size,
                                   PROT_READ | PROT_WRITE,
-                                  MAP_ANON | MAP_PRIVATE, -1, 0);
+                                  MAP_FIXED_NOREPLACE | MAP_ANON | MAP_PRIVATE,
+                                  -1, 0);
     } else {
         new_alloc_size = 0;
         mapped_addr = brk_page;
@@ -883,12 +878,6 @@ abi_long do_brk(abi_ulong brk_val)
         target_brk = brk_val;
         brk_page = new_host_brk_page;
         return target_brk;
-    } else if (mapped_addr != -1) {
-        /* Mapped but at wrong address, meaning there wasn't actually
-         * enough space for this brk.
-         */
-        target_munmap(mapped_addr, new_alloc_size);
-        mapped_addr = -1;
     }
 
 #if defined(TARGET_ALPHA)