Commit b8347c219649 ("x86/debug: Handle warnings before the notifier chain, to fix KGDB crash") changed the ordering of fixups, and did not take into account the case of x86 processing non-WARN() and non-BUG() exceptions. This would lead to output of a false BUG line with no other information. In the case of a refcount exception, it would be immediately followed by the refcount WARN(), producing very strange double-"cut here": lkdtm: attempting bad refcount_inc() overflow ------------[ cut here ]------------ Kernel BUG at 0000000065f29de5 [verbose debug info unavailable] ------------[ cut here ]------------ refcount_t overflow at lkdtm_REFCOUNT_INC_OVERFLOW+0x6b/0x90 in cat[3065], uid/euid: 0/0 WARNING: CPU: 0 PID: 3065 at kernel/panic.c:657 refcount_error_report+0x9a/0xa4 ... In the prior ordering, exceptions were searched first: do_trap_no_signal(struct task_struct *tsk, int trapnr, char *str, ... if (fixup_exception(regs, trapnr)) return 0; - if (fixup_bug(regs, trapnr)) - return 0; - As a result, fixup_bugs()'s is_valid_bugaddr() didn't take into account needing to search the exception list first, since that had already happened. So, instead of searching the exception list twice (once in is_valid_bugaddr() and then again in fixup_exception()), just add a simple sanity check to report_bug() that will immediately bail out if a BUG() (or WARN()) entry is not found.
Unified · Split
| @@ -150,6 +150,8 @@ enum bug_trap_type report_bug(unsigned long bugaddr, struct pt_regs *regs) · 149 unmodified lines | |||
| 150 | return BUG_TRAP_TYPE_NONE; | 150 | return BUG_TRAP_TYPE_NONE; |
| 151 | 151 | ||
| 152 | bug = find_bug(bugaddr); | 152 | bug = find_bug(bugaddr); |
| 153 | if (!bug) | ||
| 154 | return BUG_TRAP_TYPE_NONE; | ||
| 153 | 155 | ||
| 154 | file = NULL; | 156 | file = NULL; |
| 155 | line = 0; | 157 | line = 0; |