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 |  1 +
 gdbstub/user.c      | 95 +++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 105 insertions(+)

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 aeb0d9b5377..56b7c13b750 100644
--- a/gdbstub/internals.h
+++ b/gdbstub/internals.h
@@ -195,6 +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.c b/gdbstub/user.c
index 2ba01c17faf..8f3affbad47 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 +22,20 @@
 #include "trace.h"
 #include "internals.h"
 
+#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.
+     */
+    bool catch_all_syscalls;
+    GDBSyscallsMask catch_syscalls_mask;
 } GDBUserState;
 
 static GDBUserState gdbserver_user_state;
@@ -503,10 +513,95 @@ void gdb_syscall_handling(const char *syscall_packet)
     gdb_handlesig(gdbserver_state.c_cpu, 0);
 }
 
+static bool should_catch_syscall(int num)
+{
+    if (gdbserver_user_state.catch_all_syscalls) {
+        return true;
+    }
+    if (num < 0 || num >= GDB_NR_SYSCALLS) {
+        return false;
+    }
+    return test_bit(num, gdbserver_user_state.catch_syscalls_mask);
+}
+
 void gdb_syscall_entry(CPUState *cs, int num)
 {
+    if (should_catch_syscall(num)) {
+        g_autoptr(GString) reason = g_string_sized_new(24);
+
+        g_string_printf(reason, "syscall_entry:%x;", num);
+        gdb_handlesig_reason(cs, gdb_target_sigtrap(), reason->str);
+    }
 }
 
 void gdb_syscall_return(CPUState *cs, int num)
 {
+    if (should_catch_syscall(num)) {
+        g_autoptr(GString) reason = g_string_sized_new(24);
+
+        g_string_printf(reason, "syscall_return:%x;", num);
+        gdb_handlesig_reason(cs, gdb_target_sigtrap(), reason->str);
+    }
+}
+
+void gdb_handle_set_catch_syscalls(GArray *params, void *user_ctx)
+{
+    const char *param = get_param(params, 0)->data;
+    GDBSyscallsMask catch_syscalls_mask;
+    bool catch_all_syscalls;
+    unsigned int num;
+    const char *p;
+
+    /* "0" means not catching any syscalls. */
+    if (strcmp(param, "0") == 0) {
+        gdbserver_user_state.catch_all_syscalls = false;
+        memset(gdbserver_user_state.catch_syscalls_mask, 0,
+               sizeof(gdbserver_user_state.catch_syscalls_mask));
+        gdb_put_packet("OK");
+        return;
+    }
+
+    /* "1" means catching all syscalls. */
+    if (strcmp(param, "1") == 0) {
+        gdbserver_user_state.catch_all_syscalls = true;
+        gdb_put_packet("OK");
+        return;
+    }
+
+    /*
+     * "1;..." means catching only the specified syscalls.
+     * The syscall list must not be empty.
+     */
+    if (param[0] == '1' && param[1] == ';') {
+        catch_all_syscalls = false;
+        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. Reporting extra
+                 * syscalls is inefficient, but the spec explicitly allows it.
+                 * Keep parsing in case there is a syntax error ahead.
+                 */
+                catch_all_syscalls = true;
+            } else {
+                set_bit(num, catch_syscalls_mask);
+            }
+            if (!*p) {
+                break;
+            }
+        }
+        gdbserver_user_state.catch_all_syscalls = catch_all_syscalls;
+        if (!catch_all_syscalls) {
+            memcpy(gdbserver_user_state.catch_syscalls_mask,
+                   catch_syscalls_mask, sizeof(catch_syscalls_mask));
+        }
+        gdb_put_packet("OK");
+        return;
+    }
+
+err:
+    gdb_put_packet("E00");
 }
-- 
2.43.0