Date: Wed, 17 Apr 2024 13:53:25 +0200
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Hi, Reference: > From: "Poul-Henning Kamp" <phk@phk.freebsd.dk> > Date: Mon, 15 Apr 2024 19:55:22 +0000 "Poul-Henning Kamp" wrote: > -------- > Warner Losh writes: > > > Maybe start there to understand what "LTO" the security thing is doing and > > why it's either wrong or violates an assumption in crunchgen that can be > > fixed. > > Crunch binaries were invented 30 years ago, to make FreeBSD > installation program fit on a single floppy disk. > > Note that the goal was saving disk-space rather than RAM. > > The "architecture" of crunchgen is to take a lot of programs, rename > their main() and link them all together with a new main() which > dispatches to the right program's main() based on argv[0] > > Statistically you save half a disk-allocation unit for each program > which was nothing to sneeze at, but the real disk-space dividend > comes from linking the resulting combi-program static. > > Because it is linked static, only those .o files which are referenced > gets pulled in from the libraries, libm::j0.o only gets pulled in > if you Bessel functions, which, countrary to rumours, sysinstall > did not. > > (The goal of shared libraries is saving RAM: Everybody gets the > complete library, but only one copy of it's code ever gets loaded.) > > But the real trick is actually not crunchgen, which was originally just > a shell script, but rather crunchide(1). > > Crunchide(1) does unnatural acts to an objectfile's symboltabel, > to get around the fact that all the programs have a function called > "main" and that they litter the global symbol namespace with their > private inter-file references. > > To make a crunched binary, the .o files for the individual programs > are first "pre-linked" without libraries so that internal interfile > references are resolved. > > Then crunchide changes all global symbols, except "main" to be local > symbols, so that they become unavailable for symbol resolution in > the final run of the linker. The "main" symbol is also renamed > to a per-program name, something like "cp_main" for cp(1) etc. > > And then all the prelinked .o files, one per program, gets linked > together with the "dispatch main" and this time with libraries. > > I see no reason why crunchgen cannot be done with Link Time > Optimization, but somebody has to write the new crunchide(1), and > I suspect it will have a tougher row to hoe, because pre-linking > cannot be used to take care of the inter-program symbols. > > As I understand it LTO can also link with "normal libraries" > so one option might be to only LTO the final linking step of > the crunch process, treating all the programs as "normal libraries", > but still getting LTO advantage internally in the libraries. > > Poul-Henning Interesting, Nice if some of that were added to man crunchide. Cheers, -- Julian Stacey. Gmail & Googlemail Fail http://berklix.org/jhs/mail/#bad Brits abroad reclaim http://StolenVotes.UK http://www.gov.uk/register-to-vote Arm Ukraine defence. Contraception reduces global warming & resource wars.
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