On 2016-04-14 at 09:41 -0700, Cary Coutant wrote: > > > > 1. a "payload" executable built with "-fPIC -pie" (or "-fPIE -pie") > > which uses threads and thread-local storage; > > > > 2. a "loader" executable, which dlopen()s the payload binary and > > calls > > its main(). > > > > Starting the loader results in a segfault at thread creation time. > > Building payload with "-fPIC -shared" results in normally working > > loader, but I need to be able to launch payload both directly and > > via > > the loader. > > > > Is this a bug or I'm doing something wrong? > > When you build a binary with -pie, the linker makes an executable, > not > a shared library, and in general you can't dlopen an executable, even > if it's position-independent. When I build a binary with -pie, the linker produces an ELF file with ET_DYN in header. So it is pretty much explicitly "a shared library", I guess... > > There are a few reasons for this, but the one I suspect is causing > the > segfault is that the thread model used in an executable is different > from the model used in a shared library. In an executable (including > position-independent executables), all the TLS variables are > allocated > at fixed tp-relative offsets, and the code is bound to those offsets. > When you load a shared library, the dynamic loader expects to be able > to allocate the library's TLS variables at dynamic offsets, and it > expects that the code will use the GOT to find these offsets. > > Even when you compile your payload with -fPIC, which generates TLS > code compatible with a shared library, the linker will rewrite the > code where it can to take advantage of the fixed offsets, knowing > that > it's building an executable. Well, I don't have any knowledge of ELF and toolchain's internals, so I won't argue too much, but see above. Either way, is there any way to tell the linker not to do these things and instead produce just a shared library with an entry point? -- Ivan Shapovalov / intelfx /
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