uefi-services: Return event in init

[JohnAZoidberg]

As mentioned in #917 If the app built with uefi-services exits before ExitBootServices, UEFI will call UnloadImage on it. Then the callback function is not in memory anymore and when ExitBootServices runs, invalid memory is accessed.

QEMU will crash like this when the Linux EFISTUB calls ExitBootServices:

!!!! X64 Exception Type - 06(#UD - Invalid Opcode)  CPU Apic ID - 00000000 !!!!
RIP  - 000000003E13F018, CS  - 0000000000000038, RFLAGS - 0000000000210246
RAX  - 0000000000000000, RCX - 000000003E191E98, RDX - 0000000000000000
RBX  - 0000000000000004, RSP - 000000003FF0FD08, RBP - 000000003FF28740
RSI  - 000000003E167518, RDI - 000000003FF6A5E0
R8   - 000000003FF27140, R9  - 000000003F4807A0, R10 - 0000000000000001
R11  - 00000000000000E1, R12 - 000000003E191EE0, R13 - 0000000000000100
R14  - 000000003FF28840, R15 - 000000003E191E98
DS   - 0000000000000030, ES  - 0000000000000030, FS  - 0000000000000030
GS   - 0000000000000030, SS  - 0000000000000030
CR0  - 0000000080010033, CR2 - 0000000000000000, CR3 - 000000003FC01000
CR4  - 0000000000000668, CR8 - 0000000000000000
DR0  - 0000000000000000, DR1 - 0000000000000000, DR2 - 0000000000000000
DR3  - 0000000000000000, DR6 - 00000000FFFF0FF0, DR7 - 0000000000000400
GDTR - 000000003F9DE000 0000000000000047, LDTR - 0000000000000000
IDTR - 000000003F471018 0000000000000FFF,   TR - 0000000000000000
FXSAVE_STATE - 000000003FF0F960
!!!! Find image based on IP(0x3E13F018) (No PDB)  (ImageBase=000000003DF1E000, EntryPoint=000000003DF1F000) !!!!

To avoid the crash, the user should close the event before the app exits.

fn main(image_handle: Handle, mut system_table: SystemTable<Boot>) -> Status {
    let event: Option<Event> = uefi_services::init(&mut system_table).unwrap();
    let boot_services = system_table.boot_services();

    println!("Hello World");

    if let Some(event) = event {
        // Will creash when Linux boots if we don't call this
        boot_services.close_event(event).unwrap();
    }
    Status::SUCCESS
}

It would be nice to have this happen automatically by a Drop implementation but this already lets users do it on their own, if they need.

Checklist

• Sensible git history (for example, squash "typo" or "fix" commits). See the Rewriting History guide for help.
• Update the changelog (if necessary)

[nicholasbishop]

Taking another look at this, I think we're still not quite ready to implement a fix from #943. So I will go ahead and merge your fix, as it's a sensible and minimal update that allows applications to solve the issue. Thanks for the PR, sorry it took a while :)