Accessing large static global behaves incorrectly

[zyedidia]

I tried this code:

static VALS: [u8; 1024 * 1024 * 1024 * 4] = [0u8; 1024 * 1024 * 1024 * 4];

fn main() {
    for i in 0..2048 {
        println!("{}", VALS[i]);
        if VALS[i] != 0 {
            println!("NON-ZERO: {}", VALS[i]);
            return
        }
    }
}

I expected to see this happen: it should print 2048 zeroes, since the array is statically initialized to zero.

Instead, this happened: it prints random values of memory. In debug mode, it also exits at the first non-zero value and prints "NON-ZERO". In release mode, it never prints "NON-ZERO", but the values that it displays are in fact non-zero (I believe this is cause it is optimizing out the if statement).

It appears that the linker is not including the data for the VALS array in the binary, but a symbol is still included, which causes the loads to access other variables stored at the location of the symbol.

Meta

rustc --version --verbose:

rustc 1.78.0-nightly (878c8a2a6 2024-02-29)
binary: rustc
commit-hash: 878c8a2a62d49ca5c454547ad67290a1df746cb5
commit-date: 2024-02-29
host: x86_64-unknown-linux-gnu
release: 1.78.0-nightly
LLVM version: 18.1.0

[bjorn3]

In the playground trying to get the llvm ir or output assembly results in a crash. Maybe there is an overflow at the llvm ir level?

[clubby789]

Emitting asm/llvm works fine for me on CLI, I think it's just a playground issue

[erer1243]

@rustbot claim

[erer1243]

This issue is caused by a known problem in LLVM, which is the use of unsigned instead of size_t in this group of functions: https://llvm.org/doxygen/group__LLVMCCoreValueConstantComposite.html. On x86_64, unsigned is 32 bits, giving us an overflow at a length 2^32 array. This explains the inclusion of a symbol but no data - the linker is "correctly" including a zero-sized array.

This was encountered by Zig at ziglang/zig#1424, and one of their folks created an updated version of LLVMConstArray which was accepted into LLVM 17. This was also acknowledged in rustc https://github.com/rust-lang/rust/blame/89b78304e82dc5114e3b2faa0fbec747a28a2b37/compiler/rustc_codegen_llvm/src/llvm/ffi.rs#L951.

Unfortunately this doesn't help because the problem call is actually to LLVMConstStringInContext, made here

rust/compiler/rustc_codegen_llvm/src/common.rs

Line 332 in 89b7830

llvm::LLVMConstStringInContext(llcx, ptr, bytes.len() as c_uint, True)

The best fix would be to update all of these functions in LLVM (why didn't the first PR just update them all? :P), then eventually integrate them here.

In the meantime, we can add correct implementations of these functions to rustc_llvm/llvm-wrapper/RustWrapper.cpp. This is not a complete fix because introducing huge statics requires a larger memory model when linking (https://www.smcm.iqfr.csic.es/docs/intel/compiler_f/main_for/bldaps_for/lin/bldaps_spec_memmods.htm). After doing this fix, the given program won't link on my end for this reason. I don't know how easy it is to change the memory model that rustc invokes.

Alternatively we can just add overflow checks to all of the callsites and disallow 2^32-length constant arrays & strings. GCC & Clang also choke on const char VALS[1024l * 1024l * 1024l * 4l] = {0}; on my machine, fwiw.

If the goal is just to have a huge static block of zeros at runtime, you can also just make VALS mutable. This moves it into .bss, sidestepping the entire issue. Also, the reason the playground doesn't like this code is because the const evaluator must generate and store those 2^32 zeros in memory.

[zyedidia]

Thanks for the info. I actually initially ran into this problem while trying to make a large mutable array, but I used the immutable example in the report because it didn't require unsafe to access (I forgot this would cause it not to go in the BSS). The following example still breaks, even though it should be possible to put the data in the BSS (I think).

static mut VALS: [u8; 1024 * 1024 * 1024 * 4] = [0u8; 1024 * 1024 * 1024 * 4];

fn main() {
    unsafe {
        for i in 0..2048 {
            println!("{}", VALS[i]);
            if VALS[i] != 0 {
                println!("NON-ZERO: {}", VALS[i]);
                return
            }
        }
    }
}