plugins: Add enforcement of QEMU_PLUGIN_CB flags in register R/W callbacks

This patch adds functionality to enforce the requested QEMU_PLUGIN_CB_
flags level passed when registering a callback function using the
plugins API. Each time a callback is about to be invoked, a thread-local
variable will be updated with the level that callback requested. Then,
called API functions (in particular, the register read and write API)
will call qemu_plugin_get_cb_flags() to check the level is at least the
level they require.

Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Signed-off-by: Rowan Hart <rowanbhart@gmail.com>
Message-ID: <20250624175351.440780-4-rowanbhart@gmail.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-ID: <20250627112512.1880708-8-alex.bennee@linaro.org>

6 files changed, 96 insertions, 6 deletions

diff --git a/accel/tcg/plugin-gen.c b/accel/tcg/plugin-gen.c
index c1da753..9920381 100644
--- a/accel/tcg/plugin-gen.c
+++ b/accel/tcg/plugin-gen.c
@@ -117,10 +117,20 @@ static TCGv_i32 gen_cpu_index(void)
 static void gen_udata_cb(struct qemu_plugin_regular_cb *cb)
 {
     TCGv_i32 cpu_index = gen_cpu_index();
+    enum qemu_plugin_cb_flags cb_flags =
+        tcg_call_to_qemu_plugin_cb_flags(cb->info->flags);
+    TCGv_i32 flags = tcg_constant_i32(cb_flags);
+    TCGv_i32 clear_flags = tcg_constant_i32(QEMU_PLUGIN_CB_NO_REGS);
+    tcg_gen_st_i32(flags, tcg_env,
+           offsetof(CPUState, neg.plugin_cb_flags) - sizeof(CPUState));
     tcg_gen_call2(cb->f.vcpu_udata, cb->info, NULL,
                   tcgv_i32_temp(cpu_index),
                   tcgv_ptr_temp(tcg_constant_ptr(cb->userp)));
+    tcg_gen_st_i32(clear_flags, tcg_env,
+           offsetof(CPUState, neg.plugin_cb_flags) - sizeof(CPUState));
     tcg_temp_free_i32(cpu_index);
+    tcg_temp_free_i32(flags);
+    tcg_temp_free_i32(clear_flags);
 }
 
 static TCGv_ptr gen_plugin_u64_ptr(qemu_plugin_u64 entry)
@@ -173,10 +183,20 @@ static void gen_udata_cond_cb(struct qemu_plugin_conditional_cb *cb)
     tcg_gen_ld_i64(val, ptr, 0);
     tcg_gen_brcondi_i64(cond, val, cb->imm, after_cb);
     TCGv_i32 cpu_index = gen_cpu_index();
+    enum qemu_plugin_cb_flags cb_flags =
+        tcg_call_to_qemu_plugin_cb_flags(cb->info->flags);
+    TCGv_i32 flags = tcg_constant_i32(cb_flags);
+    TCGv_i32 clear_flags = tcg_constant_i32(QEMU_PLUGIN_CB_NO_REGS);
+    tcg_gen_st_i32(flags, tcg_env,
+           offsetof(CPUState, neg.plugin_cb_flags) - sizeof(CPUState));
     tcg_gen_call2(cb->f.vcpu_udata, cb->info, NULL,
                   tcgv_i32_temp(cpu_index),
                   tcgv_ptr_temp(tcg_constant_ptr(cb->userp)));
+    tcg_gen_st_i32(clear_flags, tcg_env,
+           offsetof(CPUState, neg.plugin_cb_flags) - sizeof(CPUState));
     tcg_temp_free_i32(cpu_index);
+    tcg_temp_free_i32(flags);
+    tcg_temp_free_i32(clear_flags);
     gen_set_label(after_cb);
 
     tcg_temp_free_i64(val);
@@ -210,12 +230,22 @@ static void gen_mem_cb(struct qemu_plugin_regular_cb *cb,
                        qemu_plugin_meminfo_t meminfo, TCGv_i64 addr)
 {
     TCGv_i32 cpu_index = gen_cpu_index();
+    enum qemu_plugin_cb_flags cb_flags =
+        tcg_call_to_qemu_plugin_cb_flags(cb->info->flags);
+    TCGv_i32 flags = tcg_constant_i32(cb_flags);
+    TCGv_i32 clear_flags = tcg_constant_i32(QEMU_PLUGIN_CB_NO_REGS);
+    tcg_gen_st_i32(flags, tcg_env,
+           offsetof(CPUState, neg.plugin_cb_flags) - sizeof(CPUState));
     tcg_gen_call4(cb->f.vcpu_mem, cb->info, NULL,
                   tcgv_i32_temp(cpu_index),
                   tcgv_i32_temp(tcg_constant_i32(meminfo)),
                   tcgv_i64_temp(addr),
                   tcgv_ptr_temp(tcg_constant_ptr(cb->userp)));
+    tcg_gen_st_i32(clear_flags, tcg_env,
+           offsetof(CPUState, neg.plugin_cb_flags) - sizeof(CPUState));
     tcg_temp_free_i32(cpu_index);
+    tcg_temp_free_i32(flags);
+    tcg_temp_free_i32(clear_flags);
 }
 
 static void inject_cb(struct qemu_plugin_dyn_cb *cb)
diff --git a/include/hw/core/cpu.h b/include/hw/core/cpu.h
index 33296a1..162a56a 100644
--- a/include/hw/core/cpu.h
+++ b/include/hw/core/cpu.h
@@ -368,6 +368,7 @@ typedef struct CPUNegativeOffsetState {
     GArray *plugin_mem_cbs;
     uint64_t plugin_mem_value_low;
     uint64_t plugin_mem_value_high;
+    int32_t plugin_cb_flags;
 #endif
     IcountDecr icount_decr;
     bool can_do_io;
diff --git a/include/qemu/plugin.h b/include/qemu/plugin.h
index 9726a9e..f355c7c 100644
--- a/include/qemu/plugin.h
+++ b/include/qemu/plugin.h
@@ -209,6 +209,21 @@ void qemu_plugin_user_prefork_lock(void);
  */
 void qemu_plugin_user_postfork(bool is_child);
 
+enum qemu_plugin_cb_flags tcg_call_to_qemu_plugin_cb_flags(int flags);
+
+static inline void qemu_plugin_set_cb_flags(CPUState *cpu,
+                                            enum qemu_plugin_cb_flags flags)
+{
+    assert(cpu);
+    cpu->neg.plugin_cb_flags = flags;
+}
+
+static inline enum qemu_plugin_cb_flags qemu_plugin_get_cb_flags(void)
+{
+    assert(current_cpu);
+    return current_cpu->neg.plugin_cb_flags;
+}
+
 #else /* !CONFIG_PLUGIN */
 
 static inline void qemu_plugin_add_opts(void)
diff --git a/include/qemu/qemu-plugin.h b/include/qemu/qemu-plugin.h
index cfe1692..9c9ebf6 100644
--- a/include/qemu/qemu-plugin.h
+++ b/include/qemu/qemu-plugin.h
@@ -254,9 +254,6 @@ typedef struct {
  * @QEMU_PLUGIN_CB_NO_REGS: callback does not access the CPU's regs
  * @QEMU_PLUGIN_CB_R_REGS: callback reads the CPU's regs
  * @QEMU_PLUGIN_CB_RW_REGS: callback reads and writes the CPU's regs
- *
- * Note: currently QEMU_PLUGIN_CB_RW_REGS is unused, plugins cannot change
- * system register state.
  */
 enum qemu_plugin_cb_flags {
     QEMU_PLUGIN_CB_NO_REGS,
@@ -901,7 +898,12 @@ GArray *qemu_plugin_get_registers(void);
  * @buf: A GByteArray for the data owned by the plugin
  *
  * This function is only available in a context that register read access is
- * explicitly requested via the QEMU_PLUGIN_CB_R_REGS flag.
+ * explicitly requested via the QEMU_PLUGIN_CB_R_REGS flag, if called inside a
+ * callback that can be registered with a qemu_plugin_cb_flags argument. This
+ * function can also be used in any callback context that does not use a flags
+ * argument, such as in a callback registered with
+ * qemu_plugin_register_vcpu_init_cb(), except for callbacks registered with
+ * qemu_plugin_register_atexit_cb() and qemu_plugin_register_flush_cb().
  *
  * Returns the size of the read register. The content of @buf is in target byte
  * order. On failure returns -1.
@@ -916,8 +918,13 @@ int qemu_plugin_read_register(struct qemu_plugin_register *handle,
  * @handle: a @qemu_plugin_reg_handle handle
  * @buf: A GByteArray for the data owned by the plugin
  *
- * This function is only available in a context that register write access is
- * explicitly requested via the QEMU_PLUGIN_CB_RW_REGS flag.
+ * This function is only available in a context that register read access is
+ * explicitly requested via the QEMU_PLUGIN_CB_RW_REGS flag, if called inside a
+ * callback that can be registered with a qemu_plugin_cb_flags argument. This
+ * function can also be used in any callback context that does not use a flags
+ * argument, such as in a callback registered with
+ * qemu_plugin_register_vcpu_init_cb(), except for callbacks registered with
+ * qemu_plugin_register_atexit_cb() and qemu_plugin_register_flush_cb().
  *
  * The size of @buf must be at least the size of the requested register.
  * Attempting to write a register with @buf smaller than the register size
diff --git a/plugins/api.c b/plugins/api.c
index 6514f2c..3f04399 100644
--- a/plugins/api.c
+++ b/plugins/api.c
@@ -437,6 +437,10 @@ int qemu_plugin_read_register(struct qemu_plugin_register *reg, GByteArray *buf)
 {
     g_assert(current_cpu);
 
+    if (qemu_plugin_get_cb_flags() == QEMU_PLUGIN_CB_NO_REGS) {
+        return -1;
+    }
+
     return gdb_read_register(current_cpu, buf, GPOINTER_TO_INT(reg) - 1);
 }
 
diff --git a/plugins/core.c b/plugins/core.c
index eb9281f..c6e9ef1 100644
--- a/plugins/core.c
+++ b/plugins/core.c
@@ -15,6 +15,7 @@
 #include "qemu/lockable.h"
 #include "qemu/option.h"
 #include "qemu/plugin.h"
+#include "qemu/qemu-plugin.h"
 #include "qemu/queue.h"
 #include "qemu/rcu_queue.h"
 #include "qemu/rcu.h"
@@ -266,7 +267,9 @@ static void qemu_plugin_vcpu_init__async(CPUState *cpu, run_on_cpu_data unused)
     plugin_grow_scoreboards__locked(cpu);
     qemu_rec_mutex_unlock(&plugin.lock);
 
+    qemu_plugin_set_cb_flags(cpu, QEMU_PLUGIN_CB_RW_REGS);
     plugin_vcpu_cb__simple(cpu, QEMU_PLUGIN_EV_VCPU_INIT);
+    qemu_plugin_set_cb_flags(cpu, QEMU_PLUGIN_CB_NO_REGS);
 }
 
 void qemu_plugin_vcpu_init_hook(CPUState *cpu)
@@ -279,7 +282,9 @@ void qemu_plugin_vcpu_exit_hook(CPUState *cpu)
 {
     bool success;
 
+    qemu_plugin_set_cb_flags(cpu, QEMU_PLUGIN_CB_RW_REGS);
     plugin_vcpu_cb__simple(cpu, QEMU_PLUGIN_EV_VCPU_EXIT);
+    qemu_plugin_set_cb_flags(cpu, QEMU_PLUGIN_CB_NO_REGS);
 
     assert(cpu->cpu_index != UNASSIGNED_CPU_INDEX);
     qemu_rec_mutex_lock(&plugin.lock);
@@ -367,6 +372,7 @@ void plugin_register_dyn_cb__udata(GArray **arr,
     static TCGHelperInfo info[3] = {
         [QEMU_PLUGIN_CB_NO_REGS].flags = TCG_CALL_NO_RWG,
         [QEMU_PLUGIN_CB_R_REGS].flags = TCG_CALL_NO_WG,
+        [QEMU_PLUGIN_CB_RW_REGS].flags = 0,
         /*
          * Match qemu_plugin_vcpu_udata_cb_t:
          *   void (*)(uint32_t, void *)
@@ -396,6 +402,7 @@ void plugin_register_dyn_cond_cb__udata(GArray **arr,
     static TCGHelperInfo info[3] = {
         [QEMU_PLUGIN_CB_NO_REGS].flags = TCG_CALL_NO_RWG,
         [QEMU_PLUGIN_CB_R_REGS].flags = TCG_CALL_NO_WG,
+        [QEMU_PLUGIN_CB_RW_REGS].flags = 0,
         /*
          * Match qemu_plugin_vcpu_udata_cb_t:
          *   void (*)(uint32_t, void *)
@@ -434,6 +441,7 @@ void plugin_register_vcpu_mem_cb(GArray **arr,
     static TCGHelperInfo info[3] = {
         [QEMU_PLUGIN_CB_NO_REGS].flags = TCG_CALL_NO_RWG,
         [QEMU_PLUGIN_CB_R_REGS].flags = TCG_CALL_NO_WG,
+        [QEMU_PLUGIN_CB_RW_REGS].flags = 0,
         /*
          * Match qemu_plugin_vcpu_mem_cb_t:
          *   void (*)(uint32_t, qemu_plugin_meminfo_t, uint64_t, void *)
@@ -473,7 +481,9 @@ void qemu_plugin_tb_trans_cb(CPUState *cpu, struct qemu_plugin_tb *tb)
     QLIST_FOREACH_SAFE_RCU(cb, &plugin.cb_lists[ev], entry, next) {
         qemu_plugin_vcpu_tb_trans_cb_t func = cb->f.vcpu_tb_trans;
 
+        qemu_plugin_set_cb_flags(cpu, QEMU_PLUGIN_CB_RW_REGS);
         func(cb->ctx->id, tb);
+        qemu_plugin_set_cb_flags(cpu, QEMU_PLUGIN_CB_NO_REGS);
     }
 }
 
@@ -498,7 +508,9 @@ qemu_plugin_vcpu_syscall(CPUState *cpu, int64_t num, uint64_t a1, uint64_t a2,
     QLIST_FOREACH_SAFE_RCU(cb, &plugin.cb_lists[ev], entry, next) {
         qemu_plugin_vcpu_syscall_cb_t func = cb->f.vcpu_syscall;
 
+        qemu_plugin_set_cb_flags(cpu, QEMU_PLUGIN_CB_RW_REGS);
         func(cb->ctx->id, cpu->cpu_index, num, a1, a2, a3, a4, a5, a6, a7, a8);
+        qemu_plugin_set_cb_flags(cpu, QEMU_PLUGIN_CB_NO_REGS);
     }
 }
 
@@ -520,7 +532,9 @@ void qemu_plugin_vcpu_syscall_ret(CPUState *cpu, int64_t num, int64_t ret)
     QLIST_FOREACH_SAFE_RCU(cb, &plugin.cb_lists[ev], entry, next) {
         qemu_plugin_vcpu_syscall_ret_cb_t func = cb->f.vcpu_syscall_ret;
 
+        qemu_plugin_set_cb_flags(cpu, QEMU_PLUGIN_CB_RW_REGS);
         func(cb->ctx->id, cpu->cpu_index, num, ret);
+        qemu_plugin_set_cb_flags(cpu, QEMU_PLUGIN_CB_NO_REGS);
     }
 }
 
@@ -528,14 +542,18 @@ void qemu_plugin_vcpu_idle_cb(CPUState *cpu)
 {
     /* idle and resume cb may be called before init, ignore in this case */
     if (cpu->cpu_index < plugin.num_vcpus) {
+        qemu_plugin_set_cb_flags(cpu, QEMU_PLUGIN_CB_RW_REGS);
         plugin_vcpu_cb__simple(cpu, QEMU_PLUGIN_EV_VCPU_IDLE);
+        qemu_plugin_set_cb_flags(cpu, QEMU_PLUGIN_CB_NO_REGS);
     }
 }
 
 void qemu_plugin_vcpu_resume_cb(CPUState *cpu)
 {
     if (cpu->cpu_index < plugin.num_vcpus) {
+        qemu_plugin_set_cb_flags(cpu, QEMU_PLUGIN_CB_RW_REGS);
         plugin_vcpu_cb__simple(cpu, QEMU_PLUGIN_EV_VCPU_RESUME);
+        qemu_plugin_set_cb_flags(cpu, QEMU_PLUGIN_CB_NO_REGS);
     }
 }
 
@@ -615,9 +633,13 @@ void qemu_plugin_vcpu_mem_cb(CPUState *cpu, uint64_t vaddr,
         switch (cb->type) {
         case PLUGIN_CB_MEM_REGULAR:
             if (rw & cb->regular.rw) {
+                qemu_plugin_set_cb_flags(cpu,
+                    tcg_call_to_qemu_plugin_cb_flags(cb->regular.info->flags));
+
                 cb->regular.f.vcpu_mem(cpu->cpu_index,
                                        make_plugin_meminfo(oi, rw),
                                        vaddr, cb->regular.userp);
+                qemu_plugin_set_cb_flags(cpu, QEMU_PLUGIN_CB_NO_REGS);
             }
             break;
         case PLUGIN_CB_INLINE_ADD_U64:
@@ -760,3 +782,14 @@ void plugin_scoreboard_free(struct qemu_plugin_scoreboard *score)
     g_array_free(score->data, TRUE);
     g_free(score);
 }
+
+enum qemu_plugin_cb_flags tcg_call_to_qemu_plugin_cb_flags(int flags)
+{
+    if (flags & TCG_CALL_NO_RWG) {
+        return QEMU_PLUGIN_CB_NO_REGS;
+    } else if (flags & TCG_CALL_NO_WG) {
+        return QEMU_PLUGIN_CB_R_REGS;
+    } else {
+        return QEMU_PLUGIN_CB_RW_REGS;
+    }
+}