Date: Wed, 20 May 2020 21:02:08 +0000 (UTC) From: Kyle Evans <kevans@FreeBSD.org> To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: svn commit: r361295 - head/stand/common Message-ID: <202005202102.04KL28th082692@repo.freebsd.org>
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Author: kevans Date: Wed May 20 21:02:08 2020 New Revision: 361295 URL: https://svnweb.freebsd.org/changeset/base/361295 Log: 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 "$Interpreter:" marker started getting strip from all loaders, with exception to userboot since it used bootprog_interp to determine what flavor of userboot it was. At some point, it had been brought to my attention that this was no longer working and I had worked up some potential solutions to use the variable that involved printing it out. My vague recollection is that this was rejected, and I forgot to explore the alternatives; I cannot find records of this discussion anymore. Fast forward to present day, Andrew reported that it was non-functional and offered (effectively) this patch (sans comment) to stop the compiler from optimizing it out by assigning it to a volatile variable. This removes concerns about user-facing change while retaining the interpreter marker. Furthermore, it could certainly be uglier. Reported and tested by: Andrew Gierth <andrew_tao173.riddles.org.uk> MFC after: 3 days Modified: head/stand/common/interp.c Modified: head/stand/common/interp.c ============================================================================== --- head/stand/common/interp.c Wed May 20 20:58:48 2020 (r361294) +++ head/stand/common/interp.c Wed May 20 21:02:08 2020 (r361295) @@ -45,8 +45,17 @@ __FBSDID("$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");
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