Date: Mon, 26 Sep 2022 19:02:42 -0400 From: Rich Felker <dalias@...c.org> To: Colin Cross <ccross@...roid.com> Cc: musl@...ts.openwall.com, Ryan Prichard <rprichard@...gle.com> Subject: Re: Running musl executables without a preinstalled dynamic linker On Mon, Sep 26, 2022 at 03:42:01PM -0700, Colin Cross wrote: > On Mon, Sep 26, 2022 at 3:38 PM Colin Cross <ccross@...roid.com> wrote: > > > > On Tue, Aug 23, 2022 at 1:18 AM Szabolcs Nagy <nsz@...t70.net> wrote: > > > > > > * Colin Cross <ccross@...roid.com> [2022-08-22 17:22:06 -0700]: > > > > On Sat, Aug 20, 2022 at 2:43 AM Szabolcs Nagy <nsz@...t70.net> wrote: > > > > > i would not use Scrt1.o though, the same toolchain should be > > > > > usable for normal linking and relinterp linking, just use a > > > > > different name like Xcrt1.o. > > > > > > > > Is there some way to get gcc/clang to use Xcrt1.o without using > > > > -nostdlib and passing all the crtbegin/end objects manually? > > > > > > this requires compiler changes (new cmdline flag) but then i think > > > the code is upstreamable. > > > > I've used relinterp.o for now, and selected instead of Scrt1.o in > > musl-gcc.specs and ld.musl-clang. > > > > > > > > > > i would make Xcrt1.o self-contained and size optimized: it only > > > > > runs at start up, this is a different requirement from the -O3 > > > > > build of normal string functions. and then there is no dependency > > > > > on libc internals (which may have various instrumentations that > > > > > does not work in Xcrt1.o). > > > > > > > > Doesn't this same logic apply to most of the code in dynlink.c? My > > > > main worry with a self contained implementation is that it requires > > > > reimplementations of various string functions that are easy to get > > > > wrong. The current prototype reuses the C versions of musl's string > > > > functions, but implements its own syscall wrappers to avoid > > > > interactions with musl internals like errno. > > > > > > dynlink is in libc.so so it can use code from there. > > > > > > but moving libc code into the executable has different constraints. > > > so you will have to make random decisions that string functions are > > > in but errno is out, wrt which libc internal makes sense in the exe. > > > > > > i would just keep a separate implementation (or at least compile > > > the code separately). string functions are easy to implement if > > > you dont try to optimize them imo. then you have full control over > > > what is going on in the exe entry code. > > > > I left the reimplementations of string functions and syscalls as > > suggested. Patch attached. > From 0df460188b95f79272003bd0e5c12bceb2a3c25f Mon Sep 17 00:00:00 2001 > From: Colin Cross <ccross@...roid.com> > Date: Thu, 22 Sep 2022 19:14:01 -0700 > Subject: [PATCH] Add entry point to find dynamic loader relative to the > executable > > Distributing binaries built against musl to systems that don't already > have musl is problematic due to the hardcoded absolute path to the > dynamic loader (e.g. /lib/ld-musl-$ARCH.so.1) in the PT_INTERP header. > This patch adds a feature to avoid the problem by leaving out PT_INTERP > and replacing Scrt1.o with an entry point that can find the dynamic > loader using DT_RUNPATH or LD_LIBRARY_PATH. > > The entry point is in crt/relinterp.c. It uses auxval to get the > program headers and find the load address of the binary, then > searches LD_LIBRARY_PATH or DT_RUNPATH for the dynamic loader. > Once found, it mmaps the loader similar to the way the kernel > does when PT_INTERP is set. The musl loader uses PT_INTERP to set > the path to the loader in the shared library info exported to the > debugger, so relinterp creates a copy of the program headers > with the PT_INTERP entry added pointing to the found location of > the dynamic loader. It updates AT_BASE to point to the address > of the dynamic loader, then jumps to the loaders entry point. > > The dynamic loader then loads shared libraries and handles > relocations before jumping to the executable's entry point, which is > the entry point in relinterp.c again. Relinterp detects that > relocations have been performed and calls __libc_start_main, the > same way Scrt1.o would have. > > Since relinterp runs before relocations have been performed it has > to avoid referecing any libc functions. That means reimplementing > the few syscalls and string functions that it uses, and avoiding > implicit calls to memcpy and memset that may be inserted by the > compiler. > > Enabling relinterp is handled in the spec file for gcc and in > the linker script for clang via a -relinterp argument. > > Normally gdb and lldb look for a symbol named "_dl_debug_state" in > the interpreter to get notified when the dynamic loader has modified > the list of shared libraries. When using relinterp the debugger is > not aware of the interpreter (at process launch PT_INTERP is unset > and auxv AT_BASE is 0) so it doesn't know where to look for the symbol. > > They fall back to looking in the executable, so we can provide a symbol > in relinterp.c for it to find. The dynamic loader is then modified > to also find the symbol in the exectuable and to call it from its own > _dl_debug_state function. > > The same tests in libc_test pass with or without LDFLAGS += -relinterp > with both musl-gcc and musl-clang. > > Ryan Prichard (rprichard@...gle.com) authored the original prototype > of relinterp. Have you looked at https://www.openwall.com/lists/musl/2020/03/29/9 where this has already been done? It's not upstream but my understanding is that the author has been using it successfully for a long time, and it's been through some review and as I recall was at least close to acceptable for upstream. Rich
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