WASI: path_open regression in Rust 1.51 when using LTO

[PiotrSikora]

Code

I tried this code:

fn main() {
    let _ = std::fs::File::open("/proxy-wasm-sandbox/regression.txt");
}

$ cargo +beta build --target wasm32-wasi --release

I expected to see this happen:

This is executed inside Envoy Proxy with preopen dir /proxy-wasm-sandbox, but hopefully the output is readable:

[host->vm] _start()
[vm->host] wasi_snapshot_preview1.fd_prestat_get(3, 1048568)
[vm<-host] wasi_snapshot_preview1.fd_prestat_get return: 0
[vm->host] wasi_snapshot_preview1.fd_prestat_dir_name(3, 1114128, 19)
[vm<-host] wasi_snapshot_preview1.fd_prestat_dir_name return: 0
[vm->host] wasi_snapshot_preview1.fd_prestat_get(4, 1048568)
[vm<-host] wasi_snapshot_preview1.fd_prestat_get return: 8
[vm->host] wasi_snapshot_preview1.path_open(3, 1, 1114320, 14, 0, 262667966, 262667966, 0, 1048520)
[vm<-host] wasi_snapshot_preview1.path_open return: 0
[vm->host] wasi_snapshot_preview1.fd_close(4)
[vm<-host] wasi_snapshot_preview1.fd_close return: 0
[host<-vm] _start return: void

The memory address and size passed in the call (path_open(.., 1114320, 14, ..)) correctly point to a regression.txt.

Instead, this happened:

[host->vm] _start()
[vm->host] wasi_snapshot_preview1.fd_prestat_get(3, 1048568)
[vm<-host] wasi_snapshot_preview1.fd_prestat_get return: 0
[vm->host] wasi_snapshot_preview1.fd_prestat_dir_name(3, 1058576, 19)
[vm<-host] wasi_snapshot_preview1.fd_prestat_dir_name return: 0
[vm->host] wasi_snapshot_preview1.fd_prestat_get(4, 1048568)
[vm<-host] wasi_snapshot_preview1.fd_prestat_get return: 8
[vm->host] wasi_snapshot_preview1.path_open(3, 1, 1, 0, 0, 262667966, 262667966, 0, 1048520)
[vm<-host] wasi_snapshot_preview1.path_open return: 44
[host<-vm] _start return: void

The memory address and size passed in the call (path_open(.., 1, 0, ..)) are wrong.

Version it worked on

It most recently worked on: Rust 1.50

rustc --version --verbose:

rustc 1.50.0 (cb75ad5db 2021-02-10)
binary: rustc
commit-hash: cb75ad5db02783e8b0222fee363c5f63f7e2cf5b
commit-date: 2021-02-10
host: x86_64-unknown-linux-gnu
release: 1.50.0

Version with regression

rustc --version --verbose:

rustc 1.51.0-beta.3 (b631c914c 2021-02-24)
binary: rustc
commit-hash: b631c914cdb5b74dac8f182ca96fdfd3e03ab319
commit-date: 2021-02-24
host: x86_64-unknown-linux-gnu
release: 1.51.0-beta.3
LLVM version: 11.0.1

@rustbot modify labels: +regression-from-stable-to-beta -regression-untriaged

[PiotrSikora]

Those PRs look like a possible candidates: #79578, WebAssembly/wasi-libc#214.

Perhaps there is a bug in my code, but it works with Rust 1.50, and the preopens are populated correctly even in Rust 1.51 (otherwise it wouldn't reach path_open), so there aren't too many places left to make a mistake.

cc @alexcrichton

[PiotrSikora]

I think I found the culprit.

This happens only in the release builds with LTO enabled, where __wasilibc_find_relpath_alloc gets optimized out:

  (func $undefined:__wasilibc_find_relpath_alloc (type $9)
    (param $0 i32)
    (param $1 i32)
    (param $2 i32)
    (param $3 i32)
    (param $4 i32)
    (result i32)
    unreachable
    )

[PiotrSikora]

Confirmed: if I "use" __wasilibc_find_relpath_alloc, so that it doesn't get optimized out, then everything works fine:

use libc;

extern "C" {
    fn __wasilibc_find_relpath_alloc(
        path: *const libc::c_char,
        abs_prefix: *mut *const libc::c_char,
        relative_path: *mut *const libc::c_char,
        relative_path_len: libc::size_t,
        can_realloc: libc::c_int,
    ) -> libc::c_int;
}

fn main() {
    unsafe {
        let _ = __wasilibc_find_relpath_alloc(
            0 as *const libc::c_char,
            0 as *mut *const libc::c_char,
            0 as *mut *const libc::c_char,
            0,
            0,
        );
    }
    let _ = std::fs::File::open("/proxy-wasm-sandbox/regression.txt");
}

[alexcrichton]

@PiotrSikora can you clarify how you're building your project? Are you using an external libc or the one shipped with libstd?

[PiotrSikora]

$ find . -type f

./src/main.rs
./Cargo.toml

$ cat Cargo.toml

[package]
name = "regress"
version = "0.1.0"
authors = ["Piotr Sikora <piotrsikora@google.com>"]
edition = "2018"

[profile.release]
lto = true
opt-level = 3
panic = "abort"

$ cat src/main.rs

fn main() {
    let _ = std::fs::File::open("/proxy-wasm-sandbox/regression.txt");
}

$ cargo +beta --version && rustc +beta --version && cargo +beta build --target wasm32-wasi --release

cargo 1.51.0-beta (929458982 2021-02-22)
rustc 1.51.0-beta.3 (b631c914c 2021-02-24)
   Compiling regress v0.1.0 (/tmp/regress)
    Finished release [optimized] target(s) in 1.85s

$ wasm-dis target/wasm32-wasi/release/regress.wasm | grep -A2 __wasilibc_find_relpath_alloc | head -3

 (func $undefined:__wasilibc_find_relpath_alloc (param $0 i32) (param $1 i32) (param $2 i32) (param $3 i32) (param $4 i32) (result i32)
  (unreachable)
 )

[LeSeulArtichaut]

Just to clarify, this does not happen with LTO disabled?

[alexcrichton]

Ok I believe that this is fixed in #82804. The undefined bit you see there is normal, that's what's happening with weak function references during LTO because that function isn't actually used.

[PiotrSikora]

Just to clarify, this does not happen with LTO disabled?

Correct.

Ok I believe that this is fixed in #82804. The undefined bit you see there is normal, that's what's happening with weak function references during LTO because that function isn't actually used.

Thanks! I verified that #82804 fixes the issue for me.