From mboxrd@z Thu Jan 1 00:00:00 1970 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on inbox.vuxu.org X-Spam-Level: X-Spam-Status: No, score=0.6 required=5.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,RCVD_IN_MSPIKE_H2, URIBL_BLACK autolearn=no autolearn_force=no version=3.4.4 Received: (qmail 13479 invoked from network); 26 Sep 2022 23:13:09 -0000 Received: from second.openwall.net (193.110.157.125) by inbox.vuxu.org with ESMTPUTF8; 26 Sep 2022 23:13:09 -0000 Received: (qmail 7600 invoked by uid 550); 26 Sep 2022 23:13:06 -0000 Mailing-List: contact musl-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Reply-To: musl@lists.openwall.com Received: (qmail 7562 invoked from network); 26 Sep 2022 23:13:05 -0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=android.com; s=20210112; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:from:to:cc:subject:date; bh=cdVW0eY7LvKD5RoQ2a9Nzlb1wRKdZEvUqmyMXi2SPWo=; b=YonpKJg1Smul2cwq8jvmeDwL3IMpN1JmODN5olEW6RHiHBUjL0NqWfYPAVSDidiN7l 0oUKh6R3gyv1Qjnfci71F8cjUYZ8BkbKfDdBUHu9vx7HXJJxVteKp6QNkckZ9zlukYms tPuRAAy6B+yeh5gT6LpaJuj10u/j6ANMtWSHdJwx5GxFMD+YMnU0g1ejVFts7aaY7kzr ltNDi5OEZ5pveG+N7RJmj/MdBXGvA9hP7IXbnWshg4p7Y2W837RJoGx6gSZD/ZUsI6TM foetFI0Wbn6lTbQx69VYTFHfPHssKxoz9Zfg8vsi5YLf9YXjTRDj60D4XF34npvIHh+K RY6Q== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:x-gm-message-state:from:to:cc:subject:date; bh=cdVW0eY7LvKD5RoQ2a9Nzlb1wRKdZEvUqmyMXi2SPWo=; b=M6o/ASXWNn23BBGllC3KQolB6ubaQhJn9MzIA0cRERRJbluikLp47X+U3bOqtToshh LuOVcgYbKmOjY+JvcHs24cRrxFk+SCA8V+FyH2im8rlLg6pUst/VJ2GcMSXEnTggIY0G GFEU7Kk8z/EsXLaUhXEEioiAzBSdhA5Nl51Obb/97vH0A1VfKxt0EJ4NhAyx+KTaD5nM DP+hcNj/92FUdtC4fhE6DRDU2MqrKrUZ+e3S+yOe8ocM6xBADiD3/hwRopawsKoxj6AI kaSueHegizOdyEtsUk6ZMwGLoZ+PxlPh+Ktk6dSRYoNOD+G9ZbrIoShn1xKC1hI16zy+ xjpg== X-Gm-Message-State: ACrzQf0IN2owdBPtX1L2rgtWAWuVyxtXwsZ0tRyfOZgGyXaWv5GRrTGT Ie7kLtqO+eWlF+ae/vVtLPQbzqo/F+v96PlHm2WqPRAQa10= X-Google-Smtp-Source: AMsMyM6Wlp32618vkVVjz083IGJXbtrna0lZ1ZO8ujDk2Oo23oIcNAZgg2yZ1/O2Bm7o0F51v2uSPn2SDHxpxqnVWnk= X-Received: by 2002:a05:6e02:d4f:b0:2f6:31eb:1c37 with SMTP id h15-20020a056e020d4f00b002f631eb1c37mr11232700ilj.199.1664233973416; Mon, 26 Sep 2022 16:12:53 -0700 (PDT) MIME-Version: 1.0 References: <20220820094308.GK1320090@port70.net> <20220823081802.GM1320090@port70.net> <20220926230241.GF9709@brightrain.aerifal.cx> In-Reply-To: <20220926230241.GF9709@brightrain.aerifal.cx> From: Colin Cross Date: Mon, 26 Sep 2022 16:12:42 -0700 Message-ID: To: Rich Felker Cc: musl@lists.openwall.com, Ryan Prichard Content-Type: text/plain; charset="UTF-8" Subject: Re: [musl] Running musl executables without a preinstalled dynamic linker On Mon, Sep 26, 2022 at 4:02 PM Rich Felker 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 wrote: > > > > > > On Tue, Aug 23, 2022 at 1:18 AM Szabolcs Nagy wrote: > > > > > > > > * Colin Cross [2022-08-22 17:22:06 -0700]: > > > > > On Sat, Aug 20, 2022 at 2:43 AM Szabolcs Nagy 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 > > 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.