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[PATCH] gdbstub.c uses incorrect check for active gdb in use_gdb_syscall

From: Keith Packard
Subject: [PATCH] gdbstub.c uses incorrect check for active gdb in use_gdb_syscalls
Date: Wed, 23 Dec 2020 13:27:52 -0800

When checking whether there is a live gdb connection, code shouldn't
use 'gdbserver_state.init' as that value is set when the
gdbserver_state structure is initialized in init_gdbserver_state, not
when the gdb socket has a valid connection.

The 'handle_detach' function appears to use 'gdbserver_state.c_cpu' as
an indication of whether there is a connection, so I've used the same
in use_gdb_syscalls.

This avoids a segfault when qemu is run with the '-s' flag (create a
gdb protocol socket), but without the '-S' flag (delay until 'c'
command is received).

I would like this patch to inform a discussion on whether the numerous
other places using gdbserver_state.init are also incorrect (most of
them appear to be using it in the same way use_gdb_syscalls does), and
also whether use_gdb_syscalls should cache the result of this check or
whether it should check each time it is called to see if a gdb
connection is currently acive. For the second question, I don't have a
clear idea; mixing gdb and native calls seems problematic for stateful
operations like file open/close.

Signed-off-by: Keith Packard <keithp@keithp.com>
 gdbstub.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/gdbstub.c b/gdbstub.c
index d99bc0bf2e..4e709d16fd 100644
--- a/gdbstub.c
+++ b/gdbstub.c
@@ -460,7 +460,7 @@ int use_gdb_syscalls(void)
     /* -semihosting-config target=auto */
     /* On the first call check if gdb is connected and remember. */
     if (gdb_syscall_mode == GDB_SYS_UNKNOWN) {
-        gdb_syscall_mode = gdbserver_state.init ?
+        gdb_syscall_mode = gdbserver_state.c_cpu != NULL ?
     return gdb_syscall_mode == GDB_SYS_ENABLED;

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