[PATCH v6 11/14] gdb: Fix issue with breakpoints defined before the GRUB image is loaded

[Glenn Washburn]

On some platforms, notably x86, software breakpoints set in GDB before
the GRUB image is loaded will be cleared when the image is loaded. This
is because the breakpoints work by overwriting the memory of the break-
point location with a special instruction which when hit will cause the
debugger to stop execution. Just before execution is resumed by the
debugger, the original instruction bytes are put back. When a breakpoint
is set before the GRUB image is loaded, the special debugger instruction
will be written to memory and when the GRUB image is loaded by the
firmware, which has no knowledge of the debugger, the debugger instruction
is overwritten. To the GDB user, GDB will show the breakpoint as set, but
it will never be hit. Furthermore, GDB now becomes confused, such that
even deleting and re-setting the breakpoint after the GRUB image is loaded
will not allow for a working breakpoint.

To work around this, in run_on_start, first a watchpoint is set on _start,
which will be triggered when the firmware starts loading the GRUB image.
When the _start watchpoint is hit, the current breakpoints are saved to a
file and then deleted by GDB before they can be overwritten by the firmware
and confuse GDB. Then a temporary software breakpoint is set on _start,
which will get triggered when the firmware hands off to GRUB to execute. In
that breakpoint load the previously saved and deleted breakpoints now that
there is no worry of them getting overwritten by the firmware.

Note that watchpoints are generally types of hardware breakpoints on x86, so
its deleted as soon as it gets triggered so that a minimal set of hardware
breakpoints are used, allowing more for the user.

Signed-off-by: Glenn Washburn <development@efficientek.com>
---
 grub-core/gdb_grub.in | 33 +++++++++++++++++++++++++++------
 1 file changed, 27 insertions(+), 6 deletions(-)

diff --git a/grub-core/gdb_grub.in b/grub-core/gdb_grub.in
index 281dfb5927..8e89bbf368 100644
--- a/grub-core/gdb_grub.in
+++ b/grub-core/gdb_grub.in
@@ -59,14 +59,35 @@ end
 define run_on_start
        # TODO: Add check to see if _start symbol is defined, if not, then
        # the symbols have not yet been loaded and this command will not work.
-       thbreak _start
+       watch *_start
+       set $break_efi_start_bpnum = $bpnum
        commands
                silent
-
-               runtime_load_module
-
-               if $is_user_command("onstart")
-                       onstart
+               delete $break_efi_start_bpnum
+
+               # Save the breakpoints here before the GRUB image is loaded
+               # into memory, then delete them. Later they will be reloaded
+               # once the GRUB image has been loaded. This avoids the issue
+               # where the loading of the GRUB image overwrites the software
+               # breakpoints, thus confusing GDB and effectively clearing
+               # those breakpoints.
+               save breakpoints .early-breakpoints.gdb
+               delete breakpoints
+
+               tbreak _start
+               commands
+                       silent
+
+                       # Reload the breakpoints now that the GRUB image has
+                       # finished being loaded into memory.
+                       source .early-breakpoints.gdb
+
+                       runtime_load_module
+
+                       if $is_user_command("onstart")
+                               onstart
+                       end
+                       continue
                end
                continue
        end
-- 
2.34.1