Re: [PATCH] powerpc/xmon: really enable xmon when a breakpoint is set

On Tue, 22 May 2018 12:53:53 +0530
Vaibhav Jain [off-list ref] wrote:

Thanks for the patch Michal,

Michal Suchanek [off-list ref] writes:

quoted

When single-stepping kernel code from xmon without a debug hook
enabled the kernel crashes. This can happen when kernel starts with
xmon on crash disabled but xmon is entered using sysrq.

Commit e1368d0c9edb ("powerpc/xmon: Setup debugger hooks when first
break-point is set") adds force_enable_xmon function that prints
"xmon: Enabling debugger hooks" but does not enable them.  

Debugger hooks are enabled just befores debugger() is entered from
sysrq_handle_xmon(). Thats why force_enable_xmon() simply sets sets
'xmon_on=1' and exits.

The problem you are seeing is probably due to sysrq_handle_xmon()
clearing the debugger hooks on return from debugger() as xmon_on was
never set for the 's' xmon command.

Indeed, setting xmon_on is sufficient. Will resend the patch.

Thanks

Michal

quoted

Add the call to xmon_init to install the debugger hooks in
force_enable_xmon and also call force_enable_xmon when
single-stepping in xmon.  

Only calling force_enable_xmon() from do_step() should be suffice as
on exit from the debugger() the value of xmon_on is checked and if
required the debugger hooks are kept instead of getting cleared.

quoted

 arch/powerpc/xmon/xmon.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c
index a0842f1ff72c..504bd1c3d8b0 100644
--- a/arch/powerpc/xmon/xmon.c
+++ b/arch/powerpc/xmon/xmon.c  

quoted

@@ -1275,6 +1279,7 @@ static inline void force_enable_xmon(void)
 	if (!xmon_on) {
 		printf("xmon: Enabling debugger hooks\n");
 		xmon_on = 1;
+		xmon_init(1);
 	}
 }  

As mentioned above call to force_enable_xmon() is usually done in
context of sysrq_handle_xmon() which sets the debugger hooks as soon
as its entered. So I think that this hunk is not really needed.