TEXTREL in i686 since 1.41.0

[Cogitri]

Hello,

I'm in the process of upgrading Rust to 1.41.0 on Alpine Linux and it appears that a TEXTREL has been introduced in 1.41.0:

TEXTREL /usr/lib/librustc_macros-4cbed8a36ab240e8.so

The full build log is located here if it's of interest: https://gitlab.alpinelinux.org/Cogitri/aports/-/jobs/44396

[jonas-schievink]

This apparently breaks Android builds, since that forbids text relocations

https://users.rust-lang.org/t/cant-link-rust-code-on-i686-linux-android-with-rust-1-41/37807

[awakecoding]

+1 I am one of the unfortunate winner of a release-blocking CI build failure for Android now that we've upgraded to 1.41.0. We've disabled android-x86 builds for now :(

[brainlock]

I added a minimal repro to my repository here: https://github.com/brainlock/rust-1.41-android-linker-bug-repro, in the hope that it can help debugging this (and because I was curious to dig into this myself :-) ).

You can clone the repo, then

docker build -t repro .

docker run --rm -it repro bash

In the container you can run ./repro-min.sh.

I bisected it using rustup and nightly releases, 2012-12-09 is good, 2019-12-10 is bad:

root@dea5c1fdc0d5:/usr/src/repro# rustup default nightly-2019-12-09
info: using existing install for 'nightly-2019-12-09-x86_64-unknown-linux-gnu'
info: default toolchain set to 'nightly-2019-12-09-x86_64-unknown-linux-gnu'

  nightly-2019-12-09-x86_64-unknown-linux-gnu unchanged - rustc 1.41.0-nightly (59947fcae 2019-12-08)

root@dea5c1fdc0d5:/usr/src/repro# ./repro-min.sh
   Compiling repro v0.1.0 (/usr/src/repro)
    Finished release [optimized] target(s) in 0.20s
root@dea5c1fdc0d5:/usr/src/repro# rustup default nightly-2019-12-10
info: using existing install for 'nightly-2019-12-10-x86_64-unknown-linux-gnu'
info: default toolchain set to 'nightly-2019-12-10-x86_64-unknown-linux-gnu'

  nightly-2019-12-10-x86_64-unknown-linux-gnu unchanged - rustc 1.41.0-nightly (76a252ea9 2019-12-09)

root@dea5c1fdc0d5:/usr/src/repro# ./repro-min.sh
   Compiling repro v0.1.0 (/usr/src/repro)
    Finished release [optimized] target(s) in 0.20s
ld: target/i686-unknown-linux-gnu/release/librepro.a(std-c3eaafedbae89cb4.std.52ue7rpt-cgu.0.rcgu.o): warning: relocation against `__rust_probestack' in read-only section `.text._ZN3std3sys4unix2fs4copy17h783e034bf468da7dE'
ld: warning: creating a DT_TEXTREL in a shared object
root@dea5c1fdc0d5:/usr/src/repro#

When using nightly-2019-12-10:

root@28bb5be323b6:/usr/src/repro# rustc --version
rustc 1.41.0-nightly (76a252ea9 2019-12-09)

[Mark-Simulacrum]

Conveniently, no rollups!

76a252e Auto merge of #67110 - tmandry:bump-compiler-builtins, r=alexcrichton
7de9402 Auto merge of #67096 - mark-i-m:fix-rustc-guide-2, r=ehuss
3ff17e7 Auto merge of #67016 - lqd:placeholder_loans, r=matthewjasper
dbbe4f1 Auto merge of #67004 - estebank:issue-66958, r=eddyb
2b0e6d2 Auto merge of #67003 - cjgillot:corrida, r=Mark-Simulacrum
14195e1 Auto merge of #66984 - GuillaumeGomez:move-clean-types, r=kinnison

I would personally suspect #67110.

Can you try checking this via https://crates.io/crates/rustup-toolchain-install-master/?

Specifically, comparing 76a252e and 7de9402?

[brainlock]

While rustup-toolchain-install-master compiles ( 😅 ): I see that https://github.com/rust-lang/rust/pull/67110 upgrades compiler-builtins. I had a look at the commits in compiler-builtins which touch __rust_probestack, and I think we can likely narrow it down to rust-lang/compiler-builtins@2566aa6.

At this point I'm too much out of my depth, this needs people more knowledgeable than me 😉 . I hope I could help debugging this, and will continue watching the issue as I'm learning a great deal just by investigating this.

Besides the actual fix, there are two additional things I'd love to understand:

1. why doesn't scanelf -qT std-c3eaafedbae89cb4.std.52ue7rpt-cgu.0.rcgu.o detect this? It looks like the obj file is not marked in any way has having a TEXTREL, but maybe the inline asm from that commit defines __rust_probestack in such a way that relocating it needs touching the .text section (wild guess)

2. how would we go at testing this? Where in the rust project would I add a test for this? And given that scanelf can't detect this, maybe a good way to "integration test" this would be to build a minimal rust static lib, and try to link it into a shared object with --warn-shared-textrel --fatal-warnings like I'm doing in the repro code. Do I understand correctly that "all code produced by rustc should be relocatable" is a guaranteed property (thus worth testing), or am I completely mistaken?

[brainlock]

Meanwhile, I can confirm that we got the right commit (76a252e):

albe@myo ~/code/repro [master] % docker build -t repro .
[...]
albe@myo ~/code/repro [master] % docker run --rm -it repro bash
root@c74a4c84b096:/usr/src/repro# rustup default 76a252ea9e7be93a61ffdf33b3533e24a9cf459d
info: default toolchain set to '76a252ea9e7be93a61ffdf33b3533e24a9cf459d'
root@c74a4c84b096:/usr/src/repro# rustc --version
rustc 1.41.0-nightly (76a252ea9 2019-12-09)
root@c74a4c84b096:/usr/src/repro# ./repro-min.sh
ld: target/i686-unknown-linux-gnu/release/librepro.a(std-c3eaafedbae89cb4.std.52ue7rpt-cgu.0.rcgu.o): warning: relocation against `__rust_probestack' in read-only section `.text._ZN3std3sys4unix2fs4copy17h783e034bf468da7dE'
ld: warning: creating a DT_TEXTREL in a shared object
root@c74a4c84b096:/usr/src/repro#
root@c74a4c84b096:/usr/src/repro#
root@c74a4c84b096:/usr/src/repro# rustup default 7de9402b77ded0d8ec9e1c554521b2121449ef2b
info: default toolchain set to '7de9402b77ded0d8ec9e1c554521b2121449ef2b'
root@c74a4c84b096:/usr/src/repro# rustc --version
rustc 1.41.0-nightly (7de9402b7 2019-12-09)
root@c74a4c84b096:/usr/src/repro# ./repro-min.sh
root@c74a4c84b096:/usr/src/repro# echo $?
0
root@c74a4c84b096:/usr/src/repro#

[pnkfelix]

triage: P-high. Assigning to self. Leaving nominated because I'd like to discuss this at today's meeting.