Re: [musl] Running musl executables without a preinstalled dynamic linker

[Colin Cross <ccross@android.com>]

On Mon, Sep 26, 2022 at 4:02 PM Rich Felker <dalias@libc.org> wrote:
>
> 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@android.com> wrote:
> > >
> > > On Tue, Aug 23, 2022 at 1:18 AM Szabolcs Nagy <nsz@port70.net> wrote:
> > > >
> > > > * Colin Cross <ccross@android.com> [2022-08-22 17:22:06 -0700]:
> > > > > On Sat, Aug 20, 2022 at 2:43 AM Szabolcs Nagy <nsz@port70.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@android.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@google.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

No, I had not seen that, and it looks to have identical functionality.
I'll experiment with switching to it.