Hi, In a separate mail thread, bluhm@ mentioned that panic(9) does not cleanly handle multiple CPUs entering it simultaneously:
https://marc.info/?l=openbsd-tech&m=161908805925325&w=2 I'm unsure which part of panic(9) is causing the problem he mentions, but one obvious issue I see is that panicstr is not set atomically, so two CPUs entering panic(9) simultaneously may clobber panicbuf. If we set panicstr atomically only one CPU will write panicbuf. Thoughts? Index: kern/subr_prf.c =================================================================== RCS file: /cvs/src/sys/kern/subr_prf.c,v retrieving revision 1.102 diff -u -p -r1.102 subr_prf.c --- kern/subr_prf.c 28 Nov 2020 17:53:05 -0000 1.102 +++ kern/subr_prf.c 13 May 2021 00:04:28 -0000 @@ -97,7 +97,7 @@ struct mutex kprintf_mutex = */ extern int log_open; /* subr_log: is /dev/klog open? */ -const char *panicstr; /* arg to first call to panic (used as a flag +volatile const char *panicstr; /* arg to first call to panic (used as a flag to indicate that panic has already been called). */ const char *faultstr; /* page fault string */ #ifdef DDB @@ -195,12 +195,10 @@ panic(const char *fmt, ...) bootopt = RB_AUTOBOOT | RB_DUMP; va_start(ap, fmt); - if (panicstr) + if (atomic_cas_ptr(&panicstr, NULL, panicbuf) != NULL) bootopt |= RB_NOSYNC; - else { + else vsnprintf(panicbuf, sizeof panicbuf, fmt, ap); - panicstr = panicbuf; - } va_end(ap); printf("panic: "); Index: sys/systm.h =================================================================== RCS file: /cvs/src/sys/sys/systm.h,v retrieving revision 1.153 diff -u -p -r1.153 systm.h --- sys/systm.h 28 Apr 2021 09:42:04 -0000 1.153 +++ sys/systm.h 13 May 2021 00:04:28 -0000 @@ -71,7 +71,7 @@ * patched by a stalking hacker. */ extern int securelevel; /* system security level */ -extern const char *panicstr; /* panic message */ +extern volatile const char *panicstr; /* panic message */ extern const char *faultstr; /* fault message */ extern const char version[]; /* system version */ extern const char copyright[]; /* system copyright */
