Fix apphost/bundle creation failure on chmod-hostile filesystems

[elinor-fung]

Summary

dotnet build and single-file dotnet publish fail inside rootless podman devcontainers because File.SetUnixFileMode (chmod 755) returns EPERM on bind-mounted Windows folders. Under rootless podman's default user-namespace mapping, the bind-mounted files appear root-owned to the non-root container user (e.g. vscode, uid 1000), so a non-owner cannot chmod them — even though they are writable. This surfaces as:

error MSB4018: The "CreateAppHost" task failed unexpectedly.
System.UnauthorizedAccessException: Access to the path '.../obj/Debug/net10.0/apphost' is denied.
---> System.IO.IOException: Operation not permitted
at System.IO.File.SetUnixFileMode(String path, UnixFileMode mode)
at Microsoft.NET.HostModel.AppHost.HostWriter.CreateAppHost(...)

Fixes#129040.

cc @dotnet/appmodel @AaronRobinsonMSFT

Change

Set the executable permissions when the file is created (via the open syscall, using FileStreamOptions.UnixCreateMode) instead of relying on a separate chmod, which is not permitted on such filesystems. The post-write chmod is now only attempted when the file is actually missing the required permissions, so it is:

• skipped on chmod-hostile filesystems where the file was already created executable (fixing the bug), and
• still applied where chmod is supported, enforcing exact 0755 (e.g. under a restrictive umask, where UnixCreateMode alone would be masked).

Both HostWriter (apphost) and Bundler (single-file bundle) are affected by the same bug and now share two helpers in HostModelUtils:

• CreateFileStreamForHost — creates the output stream, requesting 0755 at creation time on Unix.
• SetPermissionsForHost — best-effort enforcement that only chmods when needed.

In HostWriter, SetPermissionsForHost runs inside the existing try/catch so that if the result is genuinely non-executable, the broken apphost is deleted and the error is surfaced rather than silently shipped.

The net472-only libcchmod P/Invoke shim (Utils/UnixUtils.cs) is removed — it was the only consumer of File.SetUnixFileMode on net472, and net472 only runs on Windows in the modern SDK, where Unix permissions do not apply.

Testing

• Added ExecutableImageOverwritesExistingNonExecutableFile (Unix-only) verifying that creating an apphost over a pre-existing non-executable file still yields -rwxr-xr-x.
• Existing ExecutableImage test continues to cover the executable-permission invariant.
• Validated end-to-end under real rootless podman (non-root user, root-owned bind mount, .NET 10 SDK): both dotnet build and Bundler.GenerateBundle fail without this change and succeed with it. The conditional-chmod fallback was also confirmed on a normal Linux filesystem.

Note

This pull request was authored with the assistance of GitHub Copilot.

[Copilot]

Pull request overview

This PR updates Microsoft.NET.HostModel’s apphost and single-file bundle output writing logic to better handle Unix permission setting on filesystems where a separate chmod step can fail (e.g., EPERM on certain bind mounts). It centralizes “create executable at file creation time” and “enforce permissions when needed” behavior into HostModelUtils, and adds a Unix-only regression test for apphost overwrite behavior.

Changes:

• Add HostModelUtils.CreateFileStreamForHost (uses FileStreamOptions.UnixCreateMode on Unix) and HostModelUtils.SetPermissionsForHost (conditional permission enforcement).
• Update HostWriter and Bundler to use the shared helpers instead of unconditional post-write SetUnixFileMode.
• Add a Unix-only test ensuring apphost overwrites a pre-existing non-executable destination with executable permissions; remove the net472-only Unix chmod shim.

Show a summary per file

File	Description
src/installer/tests/Microsoft.NET.HostModel.Tests/AppHost/CreateAppHost.cs	Adds Unix-only regression coverage for overwriting an existing non-executable apphost destination.
src/installer/managed/Microsoft.NET.HostModel/Utils/UnixUtils.cs	Removes the net472-only Unix chmod shim (no longer needed with #if NET gating).
src/installer/managed/Microsoft.NET.HostModel/HostModelUtils.cs	Introduces shared helpers for “create with exec perms” and “conditional chmod enforcement” on Unix.
src/installer/managed/Microsoft.NET.HostModel/Bundle/Bundler.cs	Switches bundle output creation to the new helpers and centralizes permission enforcement.
src/installer/managed/Microsoft.NET.HostModel/AppHost/HostWriter.cs	Switches apphost destination stream creation to the new helper and uses shared permission enforcement.

Copilot's findings

• Files reviewed: 5/5 changed files
• Comments generated: 2

[Copilot]

Copilot's findings

• Files reviewed: 5/5 changed files
• Comments generated: 2