rustc-book: Document `-C target-feature=+crt-static` #71586

petrochenkov · 2020-04-26T15:04:36Z

A preliminary for #70740.

rust-highfive · 2020-04-26T15:04:39Z

r? @matthewjasper

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petrochenkov · 2020-04-26T15:04:53Z

cc @alexcrichton

petrochenkov · 2020-04-26T15:19:34Z

src/doc/rustc/src/codegen-options/index.md

+  - Unknown CPU features go straight to LLVM and appear as LLVM warnings.
+  - At configure time `crt-static` predicate is set even if the target doesn't support `crt-static`.
+  - At link time `crt-static` is sometimes ignored
+    if the target doesn't support it and sometimes not.


Also, I strongly suspect that #69519 was a hack rather than a proper solution, but I need some time to prepare a musl setup and verify things. (Proc-macros shouldn't be any different to cdylibs and dylibs with regards to linking behavior.)

Proc-macros shouldn't be any different to cdylibs and dylibs with regards to linking behavior

That was the problem 😉

TL;DR It's a hack.

Musl target is a "weirdo". Because of the legacy decisions it defaults to static linking so it cannot produce dylibs, cdylibs and proc-macros. Now lack of proc-macro support doesn't matter when cross-compiling from the host which supports them to musl.
It becomes difficult when using musl as the host toolchain. Since it defaults to static linking proc-macros don't work. To build them one had to use target-feature=-crt-static and lose ability to statically link binaries.

@mati865
First, musl toolchain actually supports statically linking self-contained .sos, despite the flag in rustc target spec saying otherwise, I tried it (at least with "hello world"-like examples).

Second, that doesn't mean that we want to use it for proc macros (as well as cdylibs and dylibs) at the same time as we are building a statically linked executable.

So, one common RUSTFLAGS="-Ctarget-feature=+crt-static" (or a target spec default) in this case brings collateral damage on the crates which it is not supposed to apply to.

I see multiple possible solutions here some of which could be more principled than what #69519 did.

Automatically enable -Ctarget-feature=-crt-static for proc macro crates or all libraries in Cargo, modifying RUSTFLAGS. Question: how to opt-out?

Introduce a new options -Ctarget-feature=+crt-static-dylib controlling static linking of libraries instead of -Ctarget-feature=-crt-static. It would almost never be used on Linux (not sure about windows-msvc and wasm).

Keep the existing meaning of crt-static, but introduce a new option -C disable-crt-static-for-dylibs or something. Cargo would then use it for proc macro crates or all libraries. Question: how to opt-out?

Ignore +crt-static for dylibs if the target doesn't support it. This is fragile, musl actually supports it despite the current value of the flag of the musl target spec. If the flag is enabled, proc macros will break.

Ignore +crt-static for dylibs always. They are almost never used on Linux (not sure about windows-msvc and wasm). +crt-static-dylib looks strictly better.

Have two crt-static defaults in target specs - one for executables and one for libraries. Solves one half of the problem, but explicit +crt-static in RUSTFLAGS will still cause collateral damage.

I'll turn this into a proper issue.

steveklabnik · 2020-04-26T16:19:24Z

This includes more than just documentation changes, but also implementation changes ,was that intended?

petrochenkov · 2020-04-26T17:16:10Z

@steveklabnik

Based on #71490, only the last commit is new.

ehuss

Thanks for updating the docs, they are a big improvement!

src/doc/rustc/src/codegen-options/index.md

alexcrichton · 2020-04-27T15:12:23Z

I haven't looked too too closely at this, but my main recommendation would be to not try to exhaustively document the state of things without a disclaimer. There's basically always one reason or another that +/-crt-static is still buggy, so I think it'd be worthwhile to place a disclaimer in the docs saying that things can be tweaked over time and if someone is overly surprised it's recommended they file an issue.

retep998 · 2020-04-27T22:12:10Z

src/doc/rustc/src/codegen-options/index.md

+
+- PE-based executables (Windows).  
+The produced executable will contain code from all the user-level libraries, including
+[C Run-Time Libraries (CRT)](https://docs.microsoft.com/en-us/cpp/c-runtime-library/crt-library-features),


What user-level libraries will it statically link to other than the CRT? For windows-msvc crt-static only controls whether the CRT (consisting of the VC Runtime and the UCRT) is statically linked and nothing else is affected. Also it only applies to windows-msvc while windows-gnu does not support crt-static.

@retep998

What user-level libraries will it statically link to other than the CRT?

Rust crates.
And it should do that for native dependencies as well (#71647), but that doesn't make any difference on Windows since there's no libmylib.a/libmylib.so separation.

If after that anything is still linked dynamically, then we get into the *WARNING!* clause below. It's not as dangerous in the Windows case though, since the goal there is not to produce a fully statically linked executable, like in the ELF case.

Also it only applies to windows-msvc while windows-gnu does not support crt-static.

Good catch.
This paragraph needs the same "If the target supports..." condition as the ELF paragraphs above.

To me crt-static on Windows always had the impression that the only thing it controlled (and ever intended to control) was whether it linked to the static or dynamic CRT. I was not aware that it had any effect on whether a rust crate was linked statically or dynamically, as by default it will link rust crates statically unless you specify -Cprefer-dynamic.

petrochenkov · 2020-04-28T16:40:54Z

TODO: Mention that +crt-static overpowers -C prefer-dynamic.

petrochenkov · 2020-04-28T17:59:15Z

This is kind of blocked on #71651 now.

petrochenkov · 2020-05-02T09:00:05Z

TODO: Figure out what gcc/clang/ld/lld do on Linux if libc is linked statically, but something else is linked dynamically.

smaeul · 2020-05-02T20:58:05Z

TODO: Figure out what gcc/clang/ld/lld do on Linux if libc is linked statically, but something else is linked dynamically.

We send both -static and -Bdynamic -lfoo to the linker, causing it to produce a nonfunctional dynamic ELF.

From #55566 (comment) quoting #40113 (comment):

The current problem is that, when base.dynamic_linking = true, base.crt_static_default = true is not enough to enforce that binaries are statically linked by default.

It does not enforce that all linked libraries must also be statically linked, so when gcc sees dynamic libraries, it creates a dynamic executable (i.e. an ELF with DT_NEEDED entries).

But because you told it you were creating a static executable, gcc not pass -dynamic-linker to ld to add the correct PT_INTERP header to the ELF, and therefore ld uses its hardcoded default for the architecture, e.g. /lib/ld64.so.1 on x86_64.

bors · 2020-05-10T12:14:28Z

☔ The latest upstream changes (presumably #71775) made this pull request unmergeable. Please resolve the merge conflicts.

petrochenkov · 2020-05-10T13:28:28Z

Bibliography 🙂:
#11937 - Link using the linker directly
#31322 - How to compile rust in a musl libc based linux system
#34987 - MUSL dynamic linking support
rust-lang/libc#290 - Can't statically link MSVC runtime
rust-lang/rfcs#1684 - Choosing which CRT to link to
rust-lang/rfcs#1721 - Enable customizing the linkage of a platform's C runtime
#36710 - link: failed to static link to c++ library when global variable is used
#37406 - Tracking issue for customizing CRT linkage
#37545 - rustc: Implement #[link(cfg(..))] and crt-static
#38075 - x86_64-musl: allow building dylibs with -crt-static
#39998 - A new compiler flag: "link everything statically or die, dammit!"
#40113 - Support dynamically-linked and/or native musl targets
#40174 - Unable to find crate proc_macro on musl target
https://internals.rust-lang.org/t/refining-cross-platform-crt-static-semantics/5085 - Refining cross-platform crt-static semantics
#50105 - Use the correct crt*.o files when linking musl targets
#50709 - Revert #50105 until regression is fixed
#51063 - Fix building rustc on and for musl hosts
#53968 - static-pie for musl target (remove crt1.o)
#54243 - x86_64-unknown-linux-musl target and main-returning-Result produces a dynamically-linked executable instead of a statically linked one
#55566 - Require static native libraries when linking static executables
#58575 - Musl host toolchain
#59302 - Tracking issue for musl host toolchain
#64547 - mips-linux-musleab is broken again because it uses the wrong crt for static pie
#65447 - Statically link against glibc
#65760 - musl staticlib builds include all of the musl symbols
#69519 - Don't use static crt by default when build proc-macro
#70693 - RFE: x86_64-unknown-linux-musl static-pie target for ASLR
#71586 - rustc-book: Document -C target-feature=+crt-static
#71647 - rustc should prefer statically linking native dependencies with unspecified kind if crt-static is enabled
#71651 - Enabling +crt-static in a blanket way breaks dynamic libraries including proc macros
#71804 - linker: Support -static-pie and -static -shared
rust-lang/libc#1327 - Don't bundle static libc
rust-lang/cargo#7563 - cannot produce proc-macro when --target is set on musl host toolchain
https://gcc.gnu.org/legacy-ml/gcc/2015-06/msg00008.html - Static PIE support in GCC
https://www.openwall.com/lists/musl/2012/12/08/2 - static linking and dlopen
https://www.openwall.com/lists/musl/2016/07/14/1 - Regarding whole-archive linking libc.a into a shared lib

rustc_target: Remove `pre_link_args_crt` To regain some more control over the definition of `+crt-static` (rust-lang#71586). After rust-lang#71769 this target option wasn't used anywhere except for VxWorks, and I suspect that for VxWorks its use may be redundant as well.

petrochenkov · 2020-06-06T10:34:10Z

I'll reopen this once the related issues are addressed.

rust-highfive assigned matthewjasper Apr 26, 2020

rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Apr 26, 2020

petrochenkov mentioned this pull request Apr 26, 2020

Enabling static-pie for musl #70740

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petrochenkov force-pushed the docstatic branch from 5f113d1 to 2405e8c Compare April 26, 2020 15:45

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rustc-book: Document -C target-feature="+crt-static"

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petrochenkov force-pushed the docstatic branch from 2405e8c to a66f41d Compare April 26, 2020 21:03

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petrochenkov mentioned this pull request Apr 28, 2020

rustc should prefer statically linking native dependencies with unspecified kind if crt-static is enabled #71647

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petrochenkov mentioned this pull request Apr 28, 2020

Enabling +crt-static in a blanket way breaks dynamic libraries including proc macros #71651

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petrochenkov added S-blocked Status: Blocked on something else such as an RFC or other implementation work. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Apr 30, 2020

petrochenkov mentioned this pull request May 10, 2020

linker: More systematic handling of CRT objects #71769

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petrochenkov mentioned this pull request May 18, 2020

Distribute libc.a as a separate library instead of bundling it into liblibc.rlib on musl and wasi #72274

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Elinvynia mentioned this pull request May 20, 2020

link_cfg feature gate is associated to closed issue #72059

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petrochenkov mentioned this pull request May 30, 2020

rustc_target: Remove pre_link_args_crt #72782

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petrochenkov mentioned this pull request Jul 17, 2020

rustc always links against non-debug Windows runtime #39016

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rust / rustup ish-app/ish#67

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paddd ish-app/ish#923

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Rust is linking frankenstein binaries when static linking #82912

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rustc-book: Document `-C target-feature=+crt-static` #71586

rustc-book: Document `-C target-feature=+crt-static` #71586

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rustc-book: Document -C target-feature=+crt-static #71586

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