[Bug]: rust-lld linker output silently zeroed on VirtualBox shared folder (vboxsf) due to MAP_SHARED writes not persisting

[Hlipa]

Version

7.2.16

Host OS Type

Linux

Host OS name + version

Ubuntu 24.04 LTS

Host Architecture

x86

Guest OS Type

Linux

Guest Architecture

ARM

Guest OS name + version

Ubuntu 26.04 LTS

Component

Storage and Virtual Disks

What happened?

A Rust/Cargo build placed on a VirtualBox shared folder can fail with a misleading generated-file error:

error: couldn't read .../serde_core-.../out/private.rs:
No such file or directory (os error 2)

The missing private.rs is a downstream symptom. The confirmed failure is lower in the stack: rust-lld creates linker output through a writable, shared memory mapping. On the tested vboxsf mount, the relevant system calls all return success, but the data written through MAP_SHARED is not persisted. The resulting executable has the expected length but contains zeros and has no allocated data blocks.

Moving Cargo's target directory to a container-local filesystem such as /tmp avoids the problem.

How can we reproduce this?

Following script works fine in local FS, but it fails when file is created on vboxsf:

#!/usr/bin/env python3
import mmap
import os
import sys
if len(sys.argv) != 2:
    raise SystemExit(f"usage: {sys.argv[0]} OUTPUT_FILE")
path = sys.argv[1]
size = 4096
expected = b"VISIBLE!"
fd = os.open(path, os.O_RDWR | os.O_CREAT | os.O_TRUNC, 0o644)
os.ftruncate(fd, size)
mapping = mmap.mmap(
    fd,
    size,
    flags=mmap.MAP_SHARED,
    prot=mmap.PROT_READ | mmap.PROT_WRITE,
)
mapping[: len(expected)] = expected
mapping.flush()  # msync(MS_SYNC)
os.fsync(fd)
mapping.close()
os.close(fd)
with open(path, "rb") as stream:
    actual = stream.read(len(expected))
print(f"written: {expected!r}")
print(f"read:    {actual!r}")
assert actual == expected, "MAP_SHARED writes were lost"

Did you upload all of your necessary log files, screenshots, etc.?

• Yes, I've uploaded all pertinent files to this issue.