gdbstub: Fix handle_query_xfer_auxv

The main problem was that we were treating a guest address
as a host address with a mere cast.

Use the correct interface for accessing guest memory.  Do not
allow offset == auxv_len, which would result in an empty packet.

Fixes: 51c623b0de ("gdbstub: add support to Xfer:auxv:read: packet")
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20210128201831.534033-1-richard.henderson@linaro.org>
Message-Id: <20210202134001.25738-11-alex.bennee@linaro.org>

gdbstub.c

@@ -2245,7 +2245,6 @@ static void handle_query_xfer_auxv(GdbCmdContext *gdb_ctx, void *user_ctx)
 {
     TaskState *ts;
     unsigned long offset, len, saved_auxv, auxv_len;
-    const char *mem;
 
     if (gdb_ctx->num_params < 2) {
         put_packet("E22");
@@ -2257,8 +2256,8 @@ static void handle_query_xfer_auxv(GdbCmdContext *gdb_ctx, void *user_ctx)
     ts = gdbserver_state.c_cpu->opaque;
     saved_auxv = ts->info->saved_auxv;
     auxv_len = ts->info->auxv_len;
-    mem = (const char *)(saved_auxv + offset);
-    if (offset > auxv_len) {
+
+    if (offset >= auxv_len) {
         put_packet("E00");
         return;
     }
@@ -2269,12 +2268,20 @@ static void handle_query_xfer_auxv(GdbCmdContext *gdb_ctx, void *user_ctx)
 
     if (len < auxv_len - offset) {
         g_string_assign(gdbserver_state.str_buf, "m");
-        memtox(gdbserver_state.str_buf, mem, len);
     } else {
         g_string_assign(gdbserver_state.str_buf, "l");
-        memtox(gdbserver_state.str_buf, mem, auxv_len - offset);
+        len = auxv_len - offset;
+    }
+
+    g_byte_array_set_size(gdbserver_state.mem_buf, len);
+    if (target_memory_rw_debug(gdbserver_state.g_cpu, saved_auxv + offset,
+                               gdbserver_state.mem_buf->data, len, false)) {
+        put_packet("E14");
+        return;
     }
 
+    memtox(gdbserver_state.str_buf,
+           (const char *)gdbserver_state.mem_buf->data, len);
     put_packet_binary(gdbserver_state.str_buf->str,
                       gdbserver_state.str_buf->len, true);
 }