linux-kernel - [PATCH v2] xmon: Setup xmon debugger hooks when first break-point is set

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Date:   Sun,  4 Mar 2018 23:00:25 +0530
From:   Vaibhav Jain <vaibhav@...ux.vnet.ibm.com>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Balbir Singh <bsingharora@...il.com>
Cc:     Vaibhav Jain <vaibhav@...ux.vnet.ibm.com>,
        Frederic Barrat <fbarrat@...ux.vnet.ibm.com>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Douglas Miller <dougmill@...ux.vnet.ibm.com>
Subject: [PATCH v2] xmon: Setup xmon debugger hooks when first break-point is set

Presently sysrq key for xmon('x') is registered during kernel init
irrespective of the value of kernel param 'xmon'. Thus xmon is enabled
even if 'xmon=off' is passed on the kernel command line. However this
doesn't enable the kernel debugger hooks needed for instruction or data
breakpoints. Thus when a break-point is hit with xmon=off a kernel oops
of the form below is reported:

Oops: Exception in kernel mode, sig: 5 [#1]
< snip >
Trace/breakpoint trap

To fix this the patch checks and enables debugger hooks when an
instruction or data break-point is set via xmon console.

Signed-off-by: Vaibhav Jain <vaibhav@...ux.vnet.ibm.com>
---
Change-log:

v2 -> Moved the code that was force enabling xmon to a separate inline
      function (Balbir)

      The change related to clearing all breakpoints when xmon is
      disabled via debugfs will be respinned as a separate
      patch. (Mpe)
---
 arch/powerpc/xmon/xmon.c | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c
index 82e1a3ee6e0f..e9c7ec8f15f8 100644
--- a/arch/powerpc/xmon/xmon.c
+++ b/arch/powerpc/xmon/xmon.c
@@ -1273,6 +1273,18 @@ static long check_bp_loc(unsigned long addr)
 	return 1;
 }
 
+/* Force enable xmon if not already enabled */
+static inline void force_enable_xmon(void)
+{
+	static const char warnxmon[] = "xmon: Enabling debugger hooks\n";
+
+	/* Enable xmon hooks if needed */
+	if (!xmon_on) {
+		printf(warnxmon);
+		xmon_on = 1;
+	}
+}
+
 static char *breakpoint_help_string =
     "Breakpoint command usage:\n"
     "b                show breakpoints\n"
@@ -1315,6 +1327,8 @@ bpt_cmds(void)
 			dabr.address &= ~HW_BRK_TYPE_DABR;
 			dabr.enabled = mode | BP_DABR;
 		}
+
+		force_enable_xmon();
 		break;
 
 	case 'i':	/* bi - hardware instr breakpoint */
@@ -1335,6 +1349,7 @@ bpt_cmds(void)
 		if (bp != NULL) {
 			bp->enabled |= BP_CIABR;
 			iabr = bp;
+			force_enable_xmon();
 		}
 		break;
 #endif
@@ -1399,8 +1414,10 @@ bpt_cmds(void)
 		if (!check_bp_loc(a))
 			break;
 		bp = new_breakpoint(a);
-		if (bp != NULL)
+		if (bp != NULL) {
 			bp->enabled |= BP_TRAP;
+			force_enable_xmon();
+		}
 		break;
 	}
 }
-- 
2.14.3