From 2583c3374d3fabb6c6b9e855265ebfcc37b6609c Mon Sep 17 00:00:00 2001 From: Kyle Evans Date: Wed, 20 May 2020 21:02:08 +0000 Subject: [PATCH] loader: fix userboot's ability to detect a guest's interpreter Some time after r338418, I believe with -Os/-Oz -ffunction-sections -fdata-sections, the bootprog_interp variable that held our "$Interpreter:" marker started getting strip from all loaders, with exception to userboot since it used bootprog_interp to determine what flavor of userboot it was. At some point, it had been brought to my attention that this was no longer working and I had worked up some potential solutions to use the variable that involved printing it out. My vague recollection is that this was rejected, and I forgot to explore the alternatives; I cannot find records of this discussion anymore. Fast forward to present day, Andrew reported that it was non-functional and offered (effectively) this patch (sans comment) to stop the compiler from optimizing it out by assigning it to a volatile variable. This removes concerns about user-facing change while retaining the interpreter marker. Furthermore, it could certainly be uglier. Reported and tested by: Andrew Gierth MFC after: 3 days --- stand/common/interp.c | 11 ++++++++++- 1 file changed, 10 insertions(+), 1 deletion(-) diff --git a/stand/common/interp.c b/stand/common/interp.c index 8941101f6479..a0a6bd486614 100644 --- a/stand/common/interp.c +++ b/stand/common/interp.c @@ -45,8 +45,17 @@ __FBSDID("$FreeBSD$"); void interact(void) { - static char input[256]; /* big enough? */ + static char input[256]; /* big enough? */ + const char * volatile interp_identifier; + /* + * Because interp_identifier is volatile, it cannot be optimized out by + * the compiler as it's considered an externally observable event. This + * prevents the compiler from optimizing out our carefully placed + * $Interpreter:4th string that userboot may use to determine that + * we need to switch interpreters. + */ + interp_identifier = bootprog_interp; interp_init(); printf("\n");