[PATCH] [gdb/tui] Fix crash with debuginfod query

[Tom de Vries <tdevries@suse.de>]

PR tui/31449 reports a SIGFPE when the debuginfod query happens while enabling
TUI using the "tui enable" command:
...
Thread 1 "gdb" received signal SIGFPE, Arithmetic exception.
0x0000000001021084 in tui_inject_newline_into_command_window ()
    at /data/vries/gdb/src/gdb/tui/tui-io.c:1096
1096	      py += px / tui_cmd_win ()->width;
...
due to divide-by-zero because tui_cmd_win ()->width == 0.

The corresponding backtrace is:
...
 (gdb) bt
 #0  0x0000000001021084 in tui_inject_newline_into_command_window ()
     at gdb/tui/tui-io.c:1096
 #1  0x0000000000fe65fd in gdb_readline_wrapper_line (line=...) at gdb/top.c:939
 #2  0x0000000000944eef in gdb_rl_callback_handler (rl=0x2cc865a0 "n")
     at gdb/event-top.c:288
 #3  0x0000000001175779 in rl_callback_read_char ()
     at readline/readline/callback.c:302
 #4  0x0000000000944bc3 in gdb_rl_callback_read_char_wrapper_sjlj ()
     at gdb/event-top.c:197
 #5  0x0000000000944cd4 in gdb_rl_callback_read_char_wrapper_noexcept ()
     at gdb/event-top.c:240
 #6  0x0000000000944d52 in gdb_rl_callback_read_char_wrapper (...)
     at gdb/event-top.c:252
 #7  0x0000000001062352 in stdin_event_handler (error=0, client_data=0x2c865150)
     at gdb/ui.c:154
 #8  0x0000000001a04edf in handle_file_event (file_ptr=0x2ccf8850, ready_mask=1)
     at gdbsupport/event-loop.cc:551
 #9  0x0000000001a05522 in gdb_wait_for_event (block=1)
     at gdbsupport/event-loop.cc:672
 #10 0x0000000001a043ff in gdb_do_one_event (mstimeout=-1)
     at gdbsupport/event-loop.cc:263
 #11 0x00000000006d5480 in interp::do_one_event (this=0x2cc2af20, mstimeout=-1)
     at gdb/interps.h:93
 #12 0x0000000000fe670d in gdb_readline_wrapper (
     prompt=0x2ccca4e0 "Enable debuginfod for this session? (y or [n]) ")
     at gdb/top.c:1033
 #13 0x00000000010c6853 in defaulted_query(...) (...)
     at gdb/utils.c:844
 #14 0x00000000010c6b8a in nquery (...)
     at gdb/utils.c:901
 #15 0x00000000007a9324 in debuginfod_is_enabled ()
     at gdb/debuginfod-support.c:268
 #16 0x00000000007a950d in debuginfod_source_query (...)
     at gdb/debuginfod-support.c:311
 #17 0x0000000000efc2c7 in open_source_file (s=0x2cc8f4b0) at gdb/source.c:1152
 #18 0x0000000000efc619 in symtab_to_fullname (...) at gdb/source.c:1214
 #19 0x0000000000f5ebb3 in find_line_symtab (...)
     at gdb/symtab.c:3287
 #20 0x0000000000f5f0e5 in find_pc_for_line (...)
     at gdb/symtab.c:3391
 #21 0x0000000001011f54 in tui_get_begin_asm_address (...)
     at gdb/tui/tui-disasm.c:404
 #22 0x000000000104888d in tui_source_window_base::rerender (this=0x2cbdc570)
     at gdb/tui/tui-winsource.c:474
 #23 0x0000000001028e81 in tui_win_info::resize (this=0x2cbdc570,
     height_=21, width_=127, origin_x_=0,
     origin_y_=0) at gdb/tui/tui-layout.c:299
 #24 0x00000000010297d0 in tui_layout_window::apply (this=0x2cc50350,
     x_=0, y_=0, width_=127,
     height_=21, preserve_cmd_win_size_p=false) at gdb/tui/tui-layout.c:432
 #25 0x000000000102bfea in tui_layout_split::apply (this=0x2caea920,
     x_=0, y_=0, width_=127,
     height_=33, preserve_cmd_win_size_p=false) at gdb/tui/tui-layout.c:1026
 #26 0x0000000001028267 in tui_apply_current_layout (...)
     at gdb/tui/tui-layout.c:68
 #27 0x0000000001028737 in tui_set_layout (layout=0x2c9b9e90)
     at gdb/tui/tui-layout.c:133
 #28 0x0000000001028af5 in tui_set_initial_layout () at gdb/tui/tui-layout.c:209
 #29 0x000000000104b795 in tui_enable () at gdb/tui/tui.c:496
 #30 0x000000000104bab3 in tui_enable_command (args=0x0, from_tty=1)
     at gdb/tui/tui.c:591
 #31 0x00000000006c5ffe in do_simple_func (args=0x0, from_tty=1, c=0x2c9bb2f0)
     at gdb/cli/cli-decode.c:94
 #32 0x00000000006cc94f in cmd_func (cmd=0x2c9bb2f0, args=0x0, from_tty=1)
     at gdb/cli/cli-decode.c:2831
 #33 0x0000000000fe53ad in execute_command (p=0x2c86699a "", from_tty=1)
     at gdb/top.c:563
 #34 0x000000000094584d in command_handler (command=0x2c866990 "tui enable")
     at gdb/event-top.c:611
 #35 0x0000000000945dfe in command_line_handler (rl=...) at gdb/event-top.c:844
 #36 0x000000000101e916 in tui_command_line_handler (rl=...)
     at gdb/tui/tui-interp.c:101
 #37 0x0000000000944eef in gdb_rl_callback_handler (rl=0x2cc86a30 "tui enable")
     at gdb/event-top.c:288
 #38 0x0000000001175779 in rl_callback_read_char ()
     at readline/readline/callback.c:302
 #39 0x0000000000944bc3 in gdb_rl_callback_read_char_wrapper_sjlj ()
     at gdb/event-top.c:197
 #40 0x0000000000944cd4 in gdb_rl_callback_read_char_wrapper_noexcept ()
     at gdb/event-top.c:240
 #41 0x0000000000944d52 in gdb_rl_callback_read_char_wrapper (...)
     at gdb/event-top.c:252
 #42 0x0000000001062352 in stdin_event_handler (error=0, client_data=0x2c865150)
     at gdb/ui.c:154
 #43 0x0000000001a04edf in handle_file_event (file_ptr=0x2ccf8850, ready_mask=1)
     at gdbsupport/event-loop.cc:551
 #44 0x0000000001a05522 in gdb_wait_for_event (block=1)
     at gdbsupport/event-loop.cc:672
 #45 0x0000000001a043ff in gdb_do_one_event (mstimeout=-1)
     at gdbsupport/event-loop.cc:263
 #46 0x00000000006d5480 in interp::do_one_event (this=0x2cc2af20, mstimeout=-1)
     at gdb/interps.h:93
 #47 0x0000000000b77f25 in start_event_loop () at gdb/main.c:403
 #48 0x0000000000b78113 in captured_command_loop () at gdb/main.c:468
 #49 0x0000000000b7a07c in captured_main (context=0x7fff660b9e60)
     at gdb/main.c:1381
 #50 0x0000000000b7a178 in gdb_main (args=0x7fff660b9e60) at gdb/main.c:1400
 #51 0x0000000000419705 in main (argc=5, argv=0x7fff660b9f98) at gdb/gdb.c:38
 (gdb)
...

The problem is that the command window is used (during initialization of the
source window) before it's properly initialized.

A patch [1] was posted for this PR, which fixes the problem by calling
a function that will force the debuginfod query to happen before enabling the
TUI:
...
 static void
 tui_enable_command (const char *args, int from_tty)
 {
+  /* Trigger any debuginfod-related y/n prompts now to avoid having
+     it occur during tui initialization.  Handling the prompt while
+     tui windows are initializing can cause crashes.  */
+  debuginfod_is_enabled ();
+
   tui_enable ();
 }
...

While that approach works fine, it requires us to hard-code each query that
may happen.

[ A review comment reported a problem when using C-x C-a to enable TUI instead
of "tui enable", not fixed by the previous patch.  It's also not fixed by this
patch. Filed as PR33794. ]

This patch takes another approach: add a layout with only the command window,
and activate that layout first, making sure that we finalize the
initialization of the command window before we activate the initial layout.

While that fixes the divide-by-zero, the query still looks slightly odd
because '\n' does a line feed but no a carriage return, so each new line has
more leading spaces.

Fix this by moving the activation of the initial layout to after tui_setup_io.

Tested on x86_64-linux.  [ Doesn't reproduce on aarch64-linux, because there a
divide-by-zero just returns zero. ]

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31449

[1] https://sourceware.org/pipermail/gdb-patches/2024-March/207224.html
---
 gdb/testsuite/gdb.tui/query.exp | 53 +++++++++++++++++++++++++++++++++
 gdb/tui/tui-layout.c            | 12 ++++++++
 gdb/tui/tui-layout.h            |  3 ++
 gdb/tui/tui.c                   | 20 ++++++++++---
 4 files changed, 84 insertions(+), 4 deletions(-)
 create mode 100644 gdb/testsuite/gdb.tui/query.exp

diff --git a/gdb/testsuite/gdb.tui/query.exp b/gdb/testsuite/gdb.tui/query.exp
new file mode 100644
index 00000000000..913a424f4a5
--- /dev/null
+++ b/gdb/testsuite/gdb.tui/query.exp
@@ -0,0 +1,53 @@
+# Copyright 2026 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+require allow_tui_tests
+
+tuiterm_env
+
+standard_testfile tui-layout.c
+
+if { [prepare_for_testing "failed to prepare" $testfile $srcfile nodebug] } {
+    return
+}
+
+set l {}
+lappend l "command"
+# Disabled due to PR33794.
+#lappend l "keys"
+
+foreach_with_prefix how $l {
+    save_vars { env(DEBUGINFOD_URLS) } {
+	setenv DEBUGINFOD_URLS "foo"
+	Term::clean_restart 24 120 $testfile
+    }
+
+    if {![Term::prepare_for_tui]} {
+	return 0
+    }
+
+    Term::gen_prompt
+
+    if { $how == "command" } {
+	send_gdb "tui enable\n"
+    } else {
+	send_gdb "\030\001"
+    }
+    send_gdb "n\n"
+    gdb_assert { [Term::wait_for ""] } "prompt after query"
+
+    # Check that prompt is responsive.
+    gdb_assert { [Term::command "print 1"] } "responsive prompt"
+}
diff --git a/gdb/tui/tui-layout.c b/gdb/tui/tui-layout.c
index 272f5134d85..e28099f7970 100644
--- a/gdb/tui/tui-layout.c
+++ b/gdb/tui/tui-layout.c
@@ -201,6 +201,14 @@ tui_next_layout_command (const char *arg, int from_tty)
   tui_next_layout ();
 }
 
+static tui_layout_split *cmd_only_layout;
+
+void
+tui_set_cmd_only_layout ()
+{
+  tui_set_layout (cmd_only_layout);
+}
+
 /* See tui-layout.h.  */
 
 void
@@ -1154,6 +1162,10 @@ initialize_layouts ()
 {
   tui_layout_split *layout;
 
+  layout = new tui_layout_split ();
+  layout->add_window (CMD_NAME, 1);
+  cmd_only_layout = layout;
+
   layout = new tui_layout_split ();
   layout->add_window (SRC_NAME, 2);
   layout->add_window (STATUS_NAME, 0);
diff --git a/gdb/tui/tui-layout.h b/gdb/tui/tui-layout.h
index ca9afbabae6..2da99b5930c 100644
--- a/gdb/tui/tui-layout.h
+++ b/gdb/tui/tui-layout.h
@@ -334,6 +334,9 @@ class tui_layout_split : public tui_layout_base
    way.  */
 extern void tui_add_win_to_layout (enum tui_win_type);
 
+/* Set the cmd-only layout.  */
+extern void tui_set_cmd_only_layout ();
+
 /* Set the initial layout.  */
 extern void tui_set_initial_layout ();
 
diff --git a/gdb/tui/tui.c b/gdb/tui/tui.c
index fb94823f0fe..298ff832477 100644
--- a/gdb/tui/tui.c
+++ b/gdb/tui/tui.c
@@ -492,12 +492,10 @@ tui_enable (void)
       tui_set_term_width_to (COLS);
       def_prog_mode ();
 
-      tui_show_frame_info (deprecated_safe_get_selected_frame ());
-      tui_set_initial_layout ();
-      tui_set_win_focus_to (tui_src_win ());
+      /* Setup the command window, using a preliminary layout.  */
+      tui_set_cmd_only_layout ();
       keypad (tui_cmd_win ()->handle.get (), TRUE);
       wrefresh (tui_cmd_win ()->handle.get ());
-      tui_finish_init = TRIBOOL_FALSE;
     }
   else
     {
@@ -516,6 +514,20 @@ tui_enable (void)
 
   tui_setup_io (1);
 
+  if (tui_finish_init == TRIBOOL_UNKNOWN)
+    {
+      tui_finish_init = TRIBOOL_FALSE;
+
+      tui_show_frame_info (deprecated_safe_get_selected_frame ());
+
+      /* Now that the command window and io is setup, we switch to the initial
+	 layout.  Doing so may produce queries (for instance for
+	 debuginfo enabled == ask), but we should be able to handle those at
+	 this point.  */
+      tui_set_initial_layout ();
+      tui_set_win_focus_to (tui_src_win ());
+    }
+
   /* Resize windows before anything might display/refresh a
      window.  */
   if (tui_win_resized ())

base-commit: 18d0fe3ddb9dac96b23b446fbab60f1e6aa39c52
-- 
2.51.0