mv: fail closed when recreating a cross-device directory destination - #13627
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The EXDEV directory fallback removes the destination, then recreates it with create_dir_all. create_dir_all treats an existing symlink-to-directory as success, so anything planted at the path in the gap redirects the entire moved tree through the link, outside the destination. Measured with a tight symlink() loop (which only succeeds while the path is absent, i.e. exactly in that gap), 30 rounds of a cross-device `mv -T srcdir dest`: uutils before 30/30 rounds wrote the source files inside the off-tree victim uutils after 0/30 GNU 9.10 0/30 Use create_dir instead, at the top-level destination and for each subdirectory, so a path that reappeared is refused with EEXIST rather than followed. Normal cross-device moves are unaffected: dest-absent, into-existing-dir, and -T onto an existing empty dir all still behave as before and match GNU.
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The EXDEV directory fallback removes the destination, then recreates it with create_dir_all. create_dir_all treats an existing symlink-to-directory as success, so anything planted at the path in the gap redirects the entire moved tree through the link, outside the destination.