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FileManager: avoid a TOCTOU issue in computing CWD #1035

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Nov 7, 2024
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FileManager: avoid a TOCTOU issue in computing CWD #1035

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19 changes: 14 additions & 5 deletions Sources/FoundationEssentials/FileManager/FileManager+Directories.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -466,13 +466,22 @@ extension _FileManagerImpl {

var currentDirectoryPath: String? {
#if os(Windows)
let dwLength: DWORD = GetCurrentDirectoryW(0, nil)
return withUnsafeTemporaryAllocation(of: WCHAR.self, capacity: Int(dwLength)) {
if GetCurrentDirectoryW(dwLength, $0.baseAddress) == dwLength - 1 {
return String(decodingCString: $0.baseAddress!, as: UTF16.self)
var dwLength: DWORD = GetCurrentDirectoryW(0, nil)
guard dwLength > 0 else { return nil }

for _ in 0 ... 8 {
if let szCurrentDirectory = withUnsafeTemporaryAllocation(of: WCHAR.self, capacity: Int(dwLength), {
let dwResult: DWORD = GetCurrentDirectoryW(dwLength, $0.baseAddress)
if dwResult == dwLength - 1 {
return String(decodingCString: $0.baseAddress!, as: UTF16.self)
}
dwLength = dwResult
return nil
}) {
return szCurrentDirectory
}
return nil
}
return nil
#else
withUnsafeTemporaryAllocation(of: CChar.self, capacity: FileManager.MAX_PATH_SIZE) { buffer in
guard getcwd(buffer.baseAddress!, FileManager.MAX_PATH_SIZE) != nil else {
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One quick question - on non-Windows we just allocate a buffer of MAX_PATH_SIZE (1024 bytes IIRC) and assume that the path will fit (returning nil if for some reason it doesn't). Just confirming - I assume there's no equivalent for Windows that we should use either instead of requesting the buffer size or using in the case of the size check failing?

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Yeah, there is no real equivalent for Windows. The majority of the API surface on Windows is designed to assume that you will query the required sizes. In the case of the file system, the only place there is a small buffer size guarantee is the Win32 API surface and in Foundation we will bypass that layer entirely when working with paths to ensure that long paths work.

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