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Ayush Ranjan
191f10948f
renameat2(2) support was added in 61f761343f ("Implement renameat2."),
but only for the gofer filesystem; tmpfs and overlayfs still return
EINVAL for RENAME_EXCHANGE.
Implement RENAME_EXCHANGE in tmpfs by swapping the two dentries between
their parent directories. When the exchanged files differ in type, the
parents' link counts (for "..") and the exchanged directory's reference
on its parent are transferred.
Implement RENAME_EXCHANGE in overlayfs by copying up the destination
file (and its descendants, if it is a directory) in addition to the
source, and then exchanging both files on the upper layer. Unlike a
regular rename, no whiteout is created at the source location, since
both locations remain occupied. Exchanged directories are marked opaque
at their new locations to prevent merging with lower layer directories
there.
RENAME_EXCHANGE permits the two files to differ in type and does not
require a directory being exchanged to be empty, but exchanging a file
with an ancestor directory must fail with EINVAL; see Linux's
fs/namei.c:__start_renaming(). The gofer filesystem previously required
both files to have the same type; lift that restriction, and update
parent link counts when files of different types are exchanged across
directories.
The RENAME_EXCHANGE syscall tests no longer accept EINVAL; they are
skipped where the flag remains unsupported, i.e. on FUSE and on gVisor
kernels other than runsc. New tests cover mixed-type exchanges and
exchanges with ancestor directories.
Fixes #7895