[bfs2021]

Description

CreateFile opens an existing non-regular target (device, pipe) with os.O_WRONLY|os.O_TRUNC. Windows rejects O_TRUNC on the NUL device with ERROR_INVALID_FUNCTION, so -o NUL — and the same path for -stats NUL / -sarif NUL — failed with Error: creating output file: open NUL: Incorrect function. and exit 1, while -o /dev/null works on Linux and macOS.

This drops O_TRUNC for that branch. Truncating a device or pipe is meaningless, and the branch already writes directly instead of going through the temp-file/rename path, so the flag has no purpose there. Regular files are untouched: they still go through the temporary file and replace the target on Close(true).

Related Issue

Fixes #208

Type of Change

• Bug fix (non-breaking change which fixes an issue)

Root cause

if !fi.Mode().IsRegular() {
	f, err := os.OpenFile(path, os.O_WRONLY|os.O_TRUNC, 0)

os.Stat("NUL") reports a non-regular file on Windows, so this branch runs; CreateFile on the NUL device returns ERROR_INVALID_FUNCTION when O_TRUNC is set. Opening without it succeeds, and writes are discarded as the device intends.

Testing

• Reproduced first: CreateFile("NUL") on unpatched code returns open NUL: Incorrect function.
• TestCreateFileWritesNonRegularTargetDirectly no longer skips on Windows and now passes against the NUL device. It still skips when os.DevNull is a regular file.
• go test ./... — all packages pass on Windows/amd64 (go 1.27.3).
• gofmt -l internal/output clean, go vet ./internal/output/ clean.

Checklist

• I have read the CONTRIBUTING document
• My code follows the code style of this project
• I have added tests that prove my fix is effective
• New and existing unit tests pass locally with my changes
• This change maintains or improves the security controls in the tool
• This change follows the educational/legitimate security testing focus of the project