[RFC PATCH] gdb: Add more support for debugging on EFI platforms

[Glenn Washburn]

If the configure option --enable-efi-debug is given, then enable the
printing early in EFI startup of the command needed to load symbols for
the GRUB EFI kernel. This is needed because EFI firmware determines where
to load the GRUB EFI at runtime, and so the relevant addresses are not
known ahead of time. This is not printed when secure boot is enabled.

The command is a custom command defined in the gdb_grub GDB script. So
GDB should be started with the script as an argument to the -x option or
sourced into an active GDB session before running the outputted command.

Also a command named "gdbinfo" is enabled which allows the user to print
the gdb command string on-demand, which can be valuable as the printing
early in EFI startup is quickly replaced by other text. So if using a
physical screen it may appear too briefly to be registered.

Co-developed-by: Peter Jones <pjones@redhat.com>
Signed-off-by: Glenn Washburn <development@efficientek.com>
---
This is patch 9 from the v6 "GDB script fixes and improvements" series, with
one modification. Now the gdbinfo command will print the gdb load command
even when the configure option is not enabled (though still not when lockdown
is enabled).

Robbie had 2 concerns with the last patch.

 1. Does this need to be configurable?
   * I responded that this was requested by Daniel because of concerns about
     it breaking silent boot and it seemed reasonable to me, but that I don't
     have a strong opinion. I've left it configurable until Dnaiel weighs in.
 2. Why should the load command not be printed when secure boot is enabled?
   * This was also requested by Daniel, I assume because of infomation leakage
     that may be a security concern.

Glenn
---
 config.h.in                 |  3 +++
 configure.ac                | 11 ++++++++++
 grub-core/Makefile.core.def |  1 +
 grub-core/kern/efi/debug.c  | 38 ++++++++++++++++++++++++++++++++
 grub-core/kern/efi/efi.c    |  4 ++--
 grub-core/kern/efi/init.c   |  7 +++++-
 include/grub/efi/debug.h    | 43 +++++++++++++++++++++++++++++++++++++
 include/grub/efi/efi.h      |  2 +-
 8 files changed, 105 insertions(+), 4 deletions(-)
 create mode 100644 grub-core/kern/efi/debug.c
 create mode 100644 include/grub/efi/debug.h

diff --git a/config.h.in b/config.h.in
index 4d1e50eba79c..9f88be159367 100644
--- a/config.h.in
+++ b/config.h.in
@@ -13,6 +13,9 @@
 #define MM_DEBUG @MM_DEBUG@
 #endif
 
+/* Define to 1 to enable printing of gdb command to load module symbols.  */
+#define PRINT_GDB_SYM_LOAD_CMD @EFI_DEBUG@
+
 /* Define to 1 to enable disk cache statistics.  */
 #define DISK_CACHE_STATS @DISK_CACHE_STATS@
 #define BOOT_TIME_STATS @BOOT_TIME_STATS@
diff --git a/configure.ac b/configure.ac
index 93626b7982d4..4bf477b0729a 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1521,6 +1521,17 @@ else
 fi
 AC_SUBST([MM_DEBUG])
 
+# EFI debugging.
+AC_ARG_ENABLE([efi-debug],
+             AS_HELP_STRING([--enable-efi-debug],
+                             [include aides for debugging the EFI binary]))
+if test x$enable_efi_debug = xyes; then
+  EFI_DEBUG=1
+else
+  EFI_DEBUG=0
+fi
+AC_SUBST([EFI_DEBUG])
+
 AC_ARG_ENABLE([cache-stats],
              AS_HELP_STRING([--enable-cache-stats],
                              [enable disk cache statistics collection]))
diff --git a/grub-core/Makefile.core.def b/grub-core/Makefile.core.def
index d2b1c3e9e44d..b7401c4b49e8 100644
--- a/grub-core/Makefile.core.def
+++ b/grub-core/Makefile.core.def
@@ -206,6 +206,7 @@ kernel = {
 
   efi = disk/efi/efidisk.c;
   efi = kern/efi/efi.c;
+  efi = kern/efi/debug.c;
   efi = kern/efi/init.c;
   efi = kern/efi/mm.c;
   efi = term/efi/console.c;
diff --git a/grub-core/kern/efi/debug.c b/grub-core/kern/efi/debug.c
new file mode 100644
index 000000000000..506ad90dca06
--- /dev/null
+++ b/grub-core/kern/efi/debug.c
@@ -0,0 +1,38 @@
+/*
+ *  GRUB  --  GRand Unified Bootloader
+ *  Copyright (C) 2022  Free Software Foundation, Inc.
+ *
+ *  GRUB 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.
+ *
+ *  GRUB 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 GRUB.  If not, see <http://www.gnu.org/licenses/>.
+ */
+/* debug.c - aides for debugging the EFI application */
+
+#include <grub/efi/debug.h>
+#include <grub/command.h>
+#include <grub/i18n.h>
+
+static grub_err_t
+grub_cmd_gdbinfo (struct grub_command *cmd __attribute__ ((unused)),
+                 int argc __attribute__ ((unused)),
+                 char **args __attribute__ ((unused)))
+{
+  grub_efi_print_gdb_info ();
+  return 0;
+}
+
+void
+grub_efi_register_debug_commands (void)
+{
+  grub_register_command_lockdown ("gdbinfo", grub_cmd_gdbinfo, 0,
+                                 N_("Print infomation useful for GDB 
debugging"));
+}
diff --git a/grub-core/kern/efi/efi.c b/grub-core/kern/efi/efi.c
index cf49d6357e00..17bd06f7e63a 100644
--- a/grub-core/kern/efi/efi.c
+++ b/grub-core/kern/efi/efi.c
@@ -299,7 +299,7 @@ grub_efi_get_variable (const char *var, const 
grub_efi_guid_t *guid,
 /* Search the mods section from the PE32/PE32+ image. This code uses
    a PE32 header, but should work with PE32+ as well.  */
 grub_addr_t
-grub_efi_modules_addr (void)
+grub_efi_section_addr (const char *section_name)
 {
   grub_efi_loaded_image_t *image;
   struct grub_msdos_image_header *header;
@@ -328,7 +328,7 @@ grub_efi_modules_addr (void)
        i < coff_header->num_sections;
        i++, section++)
     {
-      if (grub_strcmp (section->name, "mods") == 0)
+      if (grub_strcmp (section->name, section_name) == 0)
        break;
     }
 
diff --git a/grub-core/kern/efi/init.c b/grub-core/kern/efi/init.c
index b67bc73a1b01..edacaddf064b 100644
--- a/grub-core/kern/efi/init.c
+++ b/grub-core/kern/efi/init.c
@@ -19,6 +19,7 @@
 
 #include <grub/efi/efi.h>
 #include <grub/efi/console.h>
+#include <grub/efi/debug.h>
 #include <grub/efi/disk.h>
 #include <grub/efi/sb.h>
 #include <grub/lockdown.h>
@@ -104,7 +105,7 @@ grub_addr_t grub_modbase;
 void
 grub_efi_init (void)
 {
-  grub_modbase = grub_efi_modules_addr ();
+  grub_modbase = grub_efi_section_addr ("mods");
   /* First of all, initialize the console so that GRUB can display
      messages.  */
   grub_console_init ();
@@ -123,11 +124,15 @@ grub_efi_init (void)
       grub_lockdown ();
       grub_shim_lock_verifier_setup ();
     }
+  else
+    grub_efi_print_gdb_info ();
 
   efi_call_4 (grub_efi_system_table->boot_services->set_watchdog_timer,
              0, 0, 0, NULL);
 
   grub_efidisk_init ();
+
+  grub_efi_register_debug_commands ();
 }
 
 void (*grub_efi_net_config) (grub_efi_handle_t hnd,
diff --git a/include/grub/efi/debug.h b/include/grub/efi/debug.h
new file mode 100644
index 000000000000..25414b78d9c6
--- /dev/null
+++ b/include/grub/efi/debug.h
@@ -0,0 +1,43 @@
+/*
+ *  GRUB  --  GRand Unified Bootloader
+ *  Copyright (C) 2022  Free Software Foundation, Inc.
+ *
+ *  GRUB 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.
+ *
+ *  GRUB 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 GRUB.  If not, see <http://www.gnu.org/licenses/>.
+ */
+/* debug.h - declare variables and functions for EFI debugging support */
+
+#ifndef GRUB_EFI_DEBUG_HEADER
+#define GRUB_EFI_DEBUG_HEADER  1
+
+#include <grub/efi/efi.h>
+#include <grub/misc.h>
+
+
+void grub_efi_register_debug_commands (void);
+
+static inline void
+grub_efi_print_gdb_info (void)
+{
+#if PRINT_GDB_SYM_LOAD_CMD
+  grub_addr_t text;
+
+  text = grub_efi_section_addr (".text");
+  if (!text)
+    return;
+
+  grub_printf ("dynamic_load_symbols %p\n", (void *)text);
+#endif
+}
+
+#endif /* ! GRUB_EFI_DEBUG_HEADER */
diff --git a/include/grub/efi/efi.h b/include/grub/efi/efi.h
index e61272de5330..586ac856b5e1 100644
--- a/include/grub/efi/efi.h
+++ b/include/grub/efi/efi.h
@@ -109,7 +109,7 @@ grub_err_t grub_arch_efi_linux_boot_image(grub_addr_t addr, 
grub_size_t size,
                                            char *args);
 #endif
 
-grub_addr_t grub_efi_modules_addr (void);
+grub_addr_t grub_efi_section_addr (const char *section);
 
 void grub_efi_mm_init (void);
 void grub_efi_mm_fini (void);
-- 
2.34.1