The filesystem hosting ~/QubesIncoming/srcVM/ needs to support O_TMPFILE
too, in addition to the kernel. If it doesn't, take the use_tmpfile = 0
fallback.
POSIX requires that a read(2) which can be proved to occur after a
write() has returned returns the new data.
We want here only that other processes in the same VM will see the
file either fully written, or not see it at all. So ensuring that
linkat(2) is called after write is completed should be enough.
FixesQubesOS/qubes-issues#1257
When file opened with O_TMPFILE but use_tmpfile==0, the file will not be
linked to the directory (the code at the end of process_one_file_reg).
Additionally it is waste of time trying using O_TMPFILE when it's
already known it shouldn't be.
Also use_tmpfile==0 can mean we don't have access to /proc
(set_procfs_fd wasn't called), so even if linking the file to its
directory would be attempted, it would fail. This is the case for
dom0-updates copy.
Otherwise source domain can modify (append) the file while the user
already is accessing it. While incoming files should be treated as
untrusted, this problem could allow file modification after the user
makes some sanity checks.
By passing an empty file with a declared negative size,
a hostile VM can decrease the total bytes counter, while
not have do supply a huge amount of data, thus disabing
the byte size check, and potentially filling the target
filesystem.
Also do not rely on unpack being called just once if we don't
have to and initialize counts.
Since we don't know directory size before populating with files,
we just accumulate the size on the second pass, but do not actually
check for the limit being reached. If there's any file after that,
that'll trip the check.
