[PATCH v2 1/2] gdbstub: Implement catching syscalls

[Ilya Leoshkevich]

GDB supports stopping on syscall entry and exit using the "catch
syscall" command. It relies on 3 packets, which are currently not
supported by QEMU:

* qSupported:QCatchSyscalls+ [1]
* QCatchSyscalls: [2]
* T05syscall_entry: and T05syscall_return: [3]

Implement generation and handling of these packets.

[1] 
https://sourceware.org/gdb/current/onlinedocs/gdb.html/General-Query-Packets.html#qSupported
[2] 
https://sourceware.org/gdb/current/onlinedocs/gdb.html/General-Query-Packets.html#QCatchSyscalls
[3] 
https://sourceware.org/gdb/current/onlinedocs/gdb.html/Stop-Reply-Packets.html

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
---
 gdbstub/gdbstub.c            |   9 +++
 gdbstub/internals.h          |   2 +
 gdbstub/user-target.c        |   5 ++
 gdbstub/user.c               | 104 ++++++++++++++++++++++++++++++++++-
 include/gdbstub/user.h       |  29 +++++++++-
 include/user/syscall-trace.h |   7 ++-
 6 files changed, 151 insertions(+), 5 deletions(-)

diff --git a/gdbstub/gdbstub.c b/gdbstub/gdbstub.c
index 46d752bbc2c..7e73e916bdc 100644
--- a/gdbstub/gdbstub.c
+++ b/gdbstub/gdbstub.c
@@ -1617,6 +1617,7 @@ static void handle_query_supported(GArray *params, void 
*user_ctx)
     if (gdbserver_state.c_cpu->opaque) {
         g_string_append(gdbserver_state.str_buf, ";qXfer:auxv:read+");
     }
+    g_string_append(gdbserver_state.str_buf, ";QCatchSyscalls+");
 #endif
     g_string_append(gdbserver_state.str_buf, ";qXfer:exec-file:read+");
 #endif
@@ -1810,6 +1811,14 @@ static const GdbCmdParseEntry gdb_gen_set_table[] = {
         .schema = "l0"
     },
 #endif
+#if defined(CONFIG_USER_ONLY)
+    {
+        .handler = gdb_handle_set_catch_syscalls,
+        .cmd = "CatchSyscalls:",
+        .cmd_startswith = 1,
+        .schema = "s0",
+    },
+#endif
 };
 
 static void handle_gen_query(GArray *params, void *user_ctx)
diff --git a/gdbstub/internals.h b/gdbstub/internals.h
index 5c0c725e54c..56b7c13b750 100644
--- a/gdbstub/internals.h
+++ b/gdbstub/internals.h
@@ -136,6 +136,7 @@ void gdb_append_thread_id(CPUState *cpu, GString *buf);
 int gdb_get_cpu_index(CPUState *cpu);
 unsigned int gdb_get_max_cpus(void); /* both */
 bool gdb_can_reverse(void); /* softmmu, stub for user */
+int gdb_target_sigtrap(void); /* user */
 
 void gdb_create_default_process(GDBState *s);
 
@@ -194,6 +195,7 @@ void gdb_handle_v_file_close(GArray *params, void 
*user_ctx); /* user */
 void gdb_handle_v_file_pread(GArray *params, void *user_ctx); /* user */
 void gdb_handle_v_file_readlink(GArray *params, void *user_ctx); /* user */
 void gdb_handle_query_xfer_exec_file(GArray *params, void *user_ctx); /* user 
*/
+void gdb_handle_set_catch_syscalls(GArray *params, void *user_ctx); /* user */
 
 void gdb_handle_query_attached(GArray *params, void *user_ctx); /* both */
 
diff --git a/gdbstub/user-target.c b/gdbstub/user-target.c
index c4bba4c72c7..b7d4c37cd81 100644
--- a/gdbstub/user-target.c
+++ b/gdbstub/user-target.c
@@ -418,3 +418,8 @@ void gdb_handle_query_xfer_exec_file(GArray *params, void 
*user_ctx)
                     ts->bprm->filename + offset);
     gdb_put_strbuf();
 }
+
+int gdb_target_sigtrap(void)
+{
+    return TARGET_SIGTRAP;
+}
diff --git a/gdbstub/user.c b/gdbstub/user.c
index dbe1d9b8875..01dd7169258 100644
--- a/gdbstub/user.c
+++ b/gdbstub/user.c
@@ -10,6 +10,7 @@
  */
 
 #include "qemu/osdep.h"
+#include "qemu/bitops.h"
 #include "qemu/cutils.h"
 #include "qemu/sockets.h"
 #include "exec/hwaddr.h"
@@ -21,11 +22,25 @@
 #include "trace.h"
 #include "internals.h"
 
+enum GDBCatchSyscallsState {
+    GDB_CATCH_SYSCALLS_NONE,
+    GDB_CATCH_SYSCALLS_ALL,
+    GDB_CATCH_SYSCALLS_SELECTED,
+};
+#define GDB_NR_SYSCALLS 1024
+typedef unsigned long GDBSyscallsMask[BITS_TO_LONGS(GDB_NR_SYSCALLS)];
+
 /* User-mode specific state */
 typedef struct {
     int fd;
     char *socket_path;
     int running_state;
+    /*
+     * Store syscalls mask without memory allocation in order to avoid
+     * implementing synchronization.
+     */
+    enum GDBCatchSyscallsState catch_syscalls_state;
+    GDBSyscallsMask catch_syscalls_mask;
 } GDBUserState;
 
 static GDBUserState gdbserver_user_state;
@@ -121,7 +136,7 @@ void gdb_qemu_exit(int code)
     exit(code);
 }
 
-int gdb_handlesig(CPUState *cpu, int sig)
+int gdb_handlesig_reason(CPUState *cpu, int sig, const char *reason)
 {
     char buf[256];
     int n;
@@ -141,6 +156,9 @@ int gdb_handlesig(CPUState *cpu, int sig)
                             "T%02xthread:", gdb_target_signal_to_gdb(sig));
             gdb_append_thread_id(cpu, gdbserver_state.str_buf);
             g_string_append_c(gdbserver_state.str_buf, ';');
+            if (reason) {
+                g_string_append(gdbserver_state.str_buf, reason);
+            }
             gdb_put_strbuf();
             gdbserver_state.allow_stop_reply = false;
         }
@@ -499,3 +517,87 @@ void gdb_syscall_handling(const char *syscall_packet)
     gdb_put_packet(syscall_packet);
     gdb_handlesig(gdbserver_state.c_cpu, 0);
 }
+
+static bool should_catch_syscall(int num)
+{
+    switch (gdbserver_user_state.catch_syscalls_state) {
+    case GDB_CATCH_SYSCALLS_NONE:
+        return false;
+    case GDB_CATCH_SYSCALLS_ALL:
+        return true;
+    case GDB_CATCH_SYSCALLS_SELECTED:
+        if (num < 0 || num >= GDB_NR_SYSCALLS) {
+            return false;
+        } else {
+            return test_bit(num, gdbserver_user_state.catch_syscalls_mask);
+        }
+    default:
+        g_assert_not_reached();
+    }
+}
+
+void gdb_syscall_entry(CPUState *cs, int num)
+{
+    char reason[32];
+
+    if (should_catch_syscall(num)) {
+        snprintf(reason, sizeof(reason), "syscall_entry:%x;", num);
+        gdb_handlesig_reason(cs, gdb_target_sigtrap(), reason);
+    }
+}
+
+void gdb_syscall_return(CPUState *cs, int num)
+{
+    char reason[32];
+
+    if (should_catch_syscall(num)) {
+        snprintf(reason, sizeof(reason), "syscall_return:%x;", num);
+        gdb_handlesig_reason(cs, gdb_target_sigtrap(), reason);
+    }
+}
+
+void gdb_handle_set_catch_syscalls(GArray *params, void *user_ctx)
+{
+    enum GDBCatchSyscallsState catch_syscalls_state;
+    const char *param = get_param(params, 0)->data;
+    GDBSyscallsMask catch_syscalls_mask;
+    bool catch_syscalls_none;
+    unsigned int num;
+    const char *p;
+
+    catch_syscalls_none = strcmp(param, "0") == 0;
+    if (catch_syscalls_none || strcmp(param, "1") == 0) {
+        gdbserver_user_state.catch_syscalls_state =
+            catch_syscalls_none ? GDB_CATCH_SYSCALLS_NONE :
+                                  GDB_CATCH_SYSCALLS_ALL;
+        gdb_put_packet("OK");
+        return;
+    }
+
+    if (param[0] == '1' && param[1] == ';') {
+        catch_syscalls_state = GDB_CATCH_SYSCALLS_SELECTED;
+        memset(catch_syscalls_mask, 0, sizeof(catch_syscalls_mask));
+        for (p = &param[2];; p++) {
+            if (qemu_strtoui(p, &p, 16, &num) || (*p && *p != ';')) {
+                goto err;
+            }
+            if (num >= GDB_NR_SYSCALLS) {
+                /* Fall back to reporting all syscalls. */
+                catch_syscalls_state = GDB_CATCH_SYSCALLS_ALL;
+            } else {
+                set_bit(num, catch_syscalls_mask);
+            }
+            if (!*p) {
+                break;
+            }
+        }
+        gdbserver_user_state.catch_syscalls_state = catch_syscalls_state;
+        memcpy(gdbserver_user_state.catch_syscalls_mask, catch_syscalls_mask,
+               sizeof(catch_syscalls_mask));
+        gdb_put_packet("OK");
+        return;
+    }
+
+err:
+    gdb_put_packet("E00");
+}
diff --git a/include/gdbstub/user.h b/include/gdbstub/user.h
index d392e510c59..68b6534130c 100644
--- a/include/gdbstub/user.h
+++ b/include/gdbstub/user.h
@@ -10,9 +10,10 @@
 #define GDBSTUB_USER_H
 
 /**
- * gdb_handlesig() - yield control to gdb
+ * gdb_handlesig_reason() - yield control to gdb
  * @cpu: CPU
  * @sig: if non-zero, the signal number which caused us to stop
+ * @reason: stop reason for stop reply packet or NULL
  *
  * This function yields control to gdb, when a user-mode-only target
  * needs to stop execution. If @sig is non-zero, then we will send a
@@ -24,7 +25,18 @@
  * or 0 if no signal should be delivered, ie the signal that caused
  * us to stop should be ignored.
  */
-int gdb_handlesig(CPUState *, int);
+int gdb_handlesig_reason(CPUState *, int, const char *);
+
+/**
+ * gdb_handlesig() - yield control to gdb
+ * @cpu CPU
+ * @sig: if non-zero, the signal number which caused us to stop
+ * @see gdb_handlesig_reason()
+ */
+static inline int gdb_handlesig(CPUState *cpu, int sig)
+{
+    return gdb_handlesig_reason(cpu, sig, NULL);
+}
 
 /**
  * gdb_signalled() - inform remote gdb of sig exit
@@ -39,5 +51,18 @@ void gdb_signalled(CPUArchState *as, int sig);
  */
 void gdbserver_fork(CPUState *cs);
 
+/**
+ * gdb_syscall_entry() - inform gdb of syscall entry and yield control to it
+ * @cs: CPU
+ * @num: syscall number
+ */
+void gdb_syscall_entry(CPUState *cs, int num);
+
+/**
+ * gdb_syscall_entry() - inform gdb of syscall return and yield control to it
+ * @cs: CPU
+ * @num: syscall number
+ */
+void gdb_syscall_return(CPUState *cs, int num);
 
 #endif /* GDBSTUB_USER_H */
diff --git a/include/user/syscall-trace.h b/include/user/syscall-trace.h
index 557f881a79b..b48b2b2d0ae 100644
--- a/include/user/syscall-trace.h
+++ b/include/user/syscall-trace.h
@@ -11,6 +11,7 @@
 #define SYSCALL_TRACE_H
 
 #include "exec/user/abitypes.h"
+#include "gdbstub/user.h"
 #include "qemu/plugin.h"
 #include "trace/trace-root.h"
 
@@ -20,7 +21,7 @@
  * could potentially unify the -strace code here as well.
  */
 
-static inline void record_syscall_start(void *cpu, int num,
+static inline void record_syscall_start(CPUState *cpu, int num,
                                         abi_long arg1, abi_long arg2,
                                         abi_long arg3, abi_long arg4,
                                         abi_long arg5, abi_long arg6,
@@ -29,11 +30,13 @@ static inline void record_syscall_start(void *cpu, int num,
     qemu_plugin_vcpu_syscall(cpu, num,
                              arg1, arg2, arg3, arg4,
                              arg5, arg6, arg7, arg8);
+    gdb_syscall_entry(cpu, num);
 }
 
-static inline void record_syscall_return(void *cpu, int num, abi_long ret)
+static inline void record_syscall_return(CPUState *cpu, int num, abi_long ret)
 {
     qemu_plugin_vcpu_syscall_ret(cpu, num, ret);
+    gdb_syscall_return(cpu, num);
 }
 
 
-- 
2.43.0