Date: Wed, 21 Nov 2018 09:25:50 -0500 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Cc: Gernot Reisinger <Gernot.Reisinger@...ino.at> Subject: Re: Question regarding dynamic loader On Wed, Nov 21, 2018 at 02:55:19PM +0100, Gernot Reisinger wrote: > Hi, > I recently stumbled upon an issue with preloading a shared object into a Go > application (see related Go ticket https://github.com/golang/go/issues/28909 > ). > > In short - Go comes with an internal linker which will not link crt code to > the application. The entry point will directly execute Go standard library > code. As musl libc calls shared object constructors in crt code, the shared I don't think this assessment of what musl does is correct. It calls the (initially loaded) shared object constructor via __libc_start_main. If the program is not entered via __libc_start_main, libc is not usable. Necessary initialization will have been bypassed. This has little to do with whether the crt code was linked, except that *crt1.o is normally responsible for calling __libc_start_main. If the linking process bypasses crt1, it needs to ensure that __libc_start_main ends up getting called in some other way. As far as I know this is also true for glibc, so I'm not sure why it differs. > objects constructors subsequently will never be invoked. Things will work > on glibc systems / processes. it It seems to be a subtle - but in this case > wide reaching - behavioral difference to glibc. > > I wonder if calling constructor functions from crt code is an intended musl > libc behavior. My personal - non expert - gut feeling considers glibc > behavior "more correct". Is there a chance that musl will change this > behavior? The musl behavior here is intentional. For FDPIC targets, it's impossible to run *any* application code, in the main application or shared libraries, before the main application's crt1 has executed, because there are (essentially -- the equivalent of) self-relocations performed at that stage that the dynamic linker can't see. If any ctors were invoked directly by the dynamic linker before passing control the the main application's entry point, they would run without these relocations in the main application having been performed, possibly resulting in runaway-wrong execution. I believe Go is doing some bad hacks here with regard to its C FFI, but it's likely fixable in some reasonable way. We should get more eyes looking at it. Rich
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