regression: UB with extern fn() -> ! and LTO #39253
Description
STR
$ cargo new --lib ub && cd $_
$ edit src/lib.rs && cat $_
#![feature(lang_items)]
#![no_std]
use core::ptr;
#[no_mangle]
pub unsafe fn _start() {
extern "C" {
fn main() -> !;
}
// Just to have "something" before `main`
ptr::read_volatile(0x0 as *const usize);
main();
}
// stubs
#[lang = "eh_personality"]
extern "C" fn eh_personality() {}
#[lang = "panic_fmt"]
extern "C" fn panic_fmt() {}$ edit examples/app.rs && cat $_
#![no_std]
#![no_main]
extern crate ub;
#[no_mangle]
pub fn main() -> ! {
loop {}
}Compiling with LTO produces:
$ cargo rustc --release --example app -- -C lto -C link-args=-nostartfiles
$ objdump -Cd target/release/examples/app
Disassembly of section .text:
00000000000002c0 <_start>:
2c0: 48 8b 04 25 00 00 00 mov 0x0,%rax
2c7: 00
Note that the infinite loop is missing. This program will execute stuff that's not in the .text section.
Whereas compiling without LTO produces:
$ cargo rustc --release --example app -- -C link-args=-nostartfiles
$ objdump -Cd target/release/examples/app
Disassembly of section .text:
00000000000002c0 <main>:
2c0: eb fe jmp 2c0 <main>
2c2: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1)
2c9: 00 00 00
2cc: 0f 1f 40 00 nopl 0x0(%rax)
00000000000002d0 <_start>:
2d0: 50 push %rax
2d1: 48 8b 04 25 00 00 00 mov 0x0,%rax
2d8: 00
2d9: e8 e2 ff ff ff callq 2c0 <main>
Both main and the infinite loop are preserved.
Meta
$ rustc -V
rustc 1.16.0-nightly (a52da95ce 2017-01-20)
This is a regression because nightly-2016-08-01 (I haven't bisected) preserved the infinite loop in presence of LTO.
$ cargo +nightly-2016-08-01 rustc --release --example app -- -C lto -C link-args=-nostartfiles
$ objdump -Cd target/release/examples/app
Disassembly of section .text:
00000000000002c0 <_start>:
2c0: 48 8b 04 25 00 00 00 mov 0x0,%rax
2c7: 00
2c8: 0f 1f 84 00 00 00 00 nopl 0x0(%rax,%rax,1)
2cf: 00
2d0: eb fe jmp 2d0 <_start+0x10>
Looking at the emitted object files, before they are sent to the linker, reveals more information:
$ cargo rustc --release --example app -- -C lto --emit=obj -C link-arg=-nostartfiles
$ objdump -Cd ./target/release/examples/app-e892a9c04992a722.o
Disassembly of section .text.main:
0000000000000000 <main>:
0: eb fe jmp 0 <main>
Disassembly of section .text._start:
0000000000000000 <_start>:
0: 48 8b 04 25 00 00 00 mov 0x0,%rax
7: 00
Note that today _start is simply not calling main at all.
$ cargo +nightly-2016-08-01 rustc --release --example app -- -C lto -C link-args=-nostartfiles --emit=obj
$ objdump -Cd ./target/release/examples/app.o
Disassembly of section .text.main:
0000000000000000 <main>:
0: eb fe jmp 0 <main>
Disassembly of section .text._start:
0000000000000000 <_start>:
0: 48 8b 04 25 00 00 00 mov 0x0,%rax
7: 00
8: 0f 1f 84 00 00 00 00 nopl 0x0(%rax,%rax,1)
f: 00
10: eb fe jmp 10 <_start+0x10>