[PATCH 1/2] Allow the 'cut here' oops line to be displayed before calling BUG()

[David Howells <dhowells@redhat.com>]

Allow the 'cut here' oops line to be displayed before calling BUG() as useful
information is often displayed before BUG() is called.  Also use this to limit
recursive oopsing.

Signed-off-by: David Howells <dhowells@redhat.com>
---

 include/linux/bug.h   |    8 ++++++++
 include/linux/sched.h |    9 +++++++++
 lib/bug.c             |   31 +++++++++++++++++++++++++++++--
 3 files changed, 46 insertions(+), 2 deletions(-)


diff --git a/include/linux/bug.h b/include/linux/bug.h
index d276b55..290309f 100644
--- a/include/linux/bug.h
+++ b/include/linux/bug.h
@@ -26,6 +26,8 @@ enum bug_trap_type report_bug(unsigned long bug_addr, struct pt_regs *regs);
 /* These are defined by the architecture */
 int is_valid_bugaddr(unsigned long addr);
 
+extern bool cut_here(void);
+
 #else	/* !CONFIG_GENERIC_BUG */
 
 static inline enum bug_trap_type report_bug(unsigned long bug_addr,
@@ -34,5 +36,11 @@ static inline enum bug_trap_type report_bug(unsigned long bug_addr,
 	return BUG_TRAP_TYPE_BUG;
 }
 
+static inline bool cut_here(void)
+{
+	return true;
+}
+
 #endif	/* CONFIG_GENERIC_BUG */
+
 #endif	/* _LINUX_BUG_H */
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 0f1ea4a..f4b9782 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1199,6 +1199,9 @@ struct task_struct {
 	 */
 	unsigned char fpu_counter;
 	s8 oomkilladj; /* OOM kill score adjustment (bit shift). */
+
+	unsigned char oopsing;		/* oopsing state */
+
 #ifdef CONFIG_BLK_DEV_IO_TRACE
 	unsigned int btrace_seq;
 #endif
@@ -1700,6 +1703,12 @@ extern cputime_t task_gtime(struct task_struct *p);
 #define PF_FREEZER_NOSIG 0x80000000	/* Freezer won't send signals to it */
 
 /*
+ * current->oopsing flags
+ */
+#define OOPS_CUT_HERE	0x01		/* set if the "cut here" message is displayed */
+#define OOPS_REPORTED	0x02		/* recursion limiter */
+
+/*
  * Only the _current_ task can read/write to tsk->flags, but other
  * tasks can access tsk->flags in readonly mode for example
  * with tsk_used_math (like during threaded core dumping).
diff --git a/lib/bug.c b/lib/bug.c
index 300e41a..2d0ed45 100644
--- a/lib/bug.c
+++ b/lib/bug.c
@@ -125,6 +125,31 @@ const struct bug_entry *find_bug(unsigned long bugaddr)
 	return module_find_bug(bugaddr);
 }
 
+/**
+ * cut_here - Begin BUG() output
+ *
+ * Begin the outputting of a BUG() report with a 'cut here' line.  This ensures
+ * we produce at most one 'cut here' line per process.  This allows us to
+ * extend a BUG() report with extra information in front of it.
+ *
+ * We return true if the caller is free to go ahead and report their oops.
+ *
+ * We return false if we've detected a recursive oops - in which case the
+ * caller should take care, and possibly not do anything other than call BUG(),
+ * lest they cause further recursion.  This is particularly true of things that
+ * might go wrong in the exit path.
+ */
+bool cut_here(void)
+{
+	if (!(current->oopsing & OOPS_CUT_HERE)) {
+		current->oopsing |= OOPS_CUT_HERE;
+		printk(KERN_EMERG "------------[ cut here ]------------\n");
+	}
+
+	return !(current->oopsing & OOPS_REPORTED);
+}
+EXPORT_SYMBOL(cut_here);
+
 enum bug_trap_type report_bug(unsigned long bugaddr, struct pt_regs *regs)
 {
 	const struct bug_entry *bug;
@@ -134,9 +159,9 @@ enum bug_trap_type report_bug(unsigned long bugaddr, struct pt_regs *regs)
 	if (!is_valid_bugaddr(bugaddr))
 		return BUG_TRAP_TYPE_NONE;
 
-	bug = find_bug(bugaddr);
+	cut_here();
 
-	printk(KERN_EMERG "------------[ cut here ]------------\n");
+	bug = find_bug(bugaddr);
 
 	file = NULL;
 	line = 0;
@@ -169,6 +194,8 @@ enum bug_trap_type report_bug(unsigned long bugaddr, struct pt_regs *regs)
 		return BUG_TRAP_TYPE_WARN;
 	}
 
+	current->oopsing |= OOPS_REPORTED;
+
 	if (file)
 		printk(KERN_CRIT "kernel BUG at %s:%u!\n",
 		       file, line);