plugins: prohibit writing to read-only registers

The opaque register handle encodes whether a register is read-only in
the lowest bit and prevents writing to the register via the plugin API
in this case.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Signed-off-by: Florian Hofhammer <florian.hofhammer@epfl.ch>
Link: https://lore.kernel.org/qemu-devel/20260305-setpc-v5-v7-7-4c3adba52403@epfl.ch
Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>

plugins/api.c

@@ -424,6 +424,7 @@ static GArray *create_register_handles(GArray *gdbstub_regs)
     for (int i = 0; i < gdbstub_regs->len; i++) {
         GDBRegDesc *grd = &g_array_index(gdbstub_regs, GDBRegDesc, i);
         qemu_plugin_reg_descriptor desc;
+        gint plugin_ro_bit = 0;
 
         /* skip "un-named" regs */
         if (!grd->name) {
@@ -431,7 +432,6 @@ static GArray *create_register_handles(GArray *gdbstub_regs)
         }
 
         /* Create a record for the plugin */
-        desc.handle = GINT_TO_POINTER(grd->gdb_reg + 1);
         desc.name = g_intern_string(grd->name);
         desc.is_readonly = false;
         if (g_strcmp0(desc.name, pc_str) == 0
@@ -442,7 +442,9 @@ static GArray *create_register_handles(GArray *gdbstub_regs)
             || g_strcmp0(desc.name, rpc_str) == 0
            ) {
             desc.is_readonly = true;
+            plugin_ro_bit = 1;
         }
+        desc.handle = GINT_TO_POINTER((grd->gdb_reg << 1) | plugin_ro_bit);
         desc.feature = g_intern_string(grd->feature_name);
         g_array_append_val(find_data, desc);
     }
@@ -467,7 +469,7 @@ bool qemu_plugin_read_register(struct qemu_plugin_register *reg,
         return false;
     }
 
-    return (gdb_read_register(current_cpu, buf, GPOINTER_TO_INT(reg) - 1) > 0);
+    return (gdb_read_register(current_cpu, buf, GPOINTER_TO_INT(reg) >> 1) > 0);
 }
 
 bool qemu_plugin_write_register(struct qemu_plugin_register *reg,
@@ -475,13 +477,16 @@ bool qemu_plugin_write_register(struct qemu_plugin_register *reg,
 {
     g_assert(current_cpu);
 
+    /* Read-only property is encoded in least significant bit */
+    g_assert((GPOINTER_TO_INT(reg) & 1) == 0);
+
     if (buf->len == 0 ||
         (qemu_plugin_get_cb_flags() != QEMU_PLUGIN_CB_RW_REGS &&
          qemu_plugin_get_cb_flags() != QEMU_PLUGIN_CB_RW_REGS_PC)) {
         return false;
     }
 
-    return (gdb_write_register(current_cpu, buf->data, GPOINTER_TO_INT(reg) - 1) > 0);
+    return (gdb_write_register(current_cpu, buf->data, GPOINTER_TO_INT(reg) >> 1) > 0);
 }
 
 void qemu_plugin_set_pc(uint64_t vaddr)