Technically unsafe behavior in safe rust using #![no_main]

[CensoredUsername]

Using #![no_main], and a #[no_mange] pub static main array the linker will assume that the main array is actually the main function and execute whatever is in there. Outside of a warning for nonstandard naming this compiles fine on stable and nightly on both debug and release mode.

Example

This example will print the text Main is usually a function\n on x64 linux and probably crash violently on anything else.

It's rather hard to trigger this behavior without trying, but it should probably not compile without errors. Maybe add a check for #![no_main] crates that whatever's declared as #[no_mangle] main is actually a function with the correct signature.

Meta

The issue exists on rustc 1.25.0-nightly, rustc 1.23.0 and anything in between.

[retep998]

#[no_mangle] is dangerous and unsafe in general. Combining it with #![no_main] is not a novel source of bad behavior.

#47449
#28179

[CensoredUsername]

Whoops, missed those while searching (only searched for no_main).

It should be noted though that the proposed solution in the other issues (errors on duplicate symbols) would not work in this case as #![no_main] avoids emitting the main symbol in general.

[MarkDDR]

Why does the compiler put the array of (effectively) bytes in .text instead of .data? That should prevent at least this particular instance from occurring.

[CensoredUsername]

The correct place for it to be would be .rodata . But it used to be quite normal to map constants into the .text section. Mapping it into .data would be less efficient as .data has to be copied into the process memory space while .text can just be directly mapped into the process space.

[L-as]

I don't think this should result in a compilation error, since it might be useful occasionally. However I do think there should be a lint against it, or perhaps some kind of unsafe {} required?

[steveklabnik]

Triage: no change

[jonas-schievink]

This is basically a duplicate of #28179, so closing in favor of that