It looks like that when scp is using the SFTP protocol (the default now), the following use case doesn't work anymore. Reproducer: $ echo bar > /tmp/foo $ cat /tmp/foo bar $ scp /tmp/foo localhost:/tmp/foo foo 0% 0 0.0KB/s --:-- ETA Then, data in /tmp/foo is lost: $ cat /tmp/foo $ It looks like sftp opens the destination file with O_TRUNC which is different than with the legacy SCP, which works as expected: $ echo bar > /tmp/foo $ scp -O /tmp/foo localhost:/tmp/foo foo 100% 4 2.6KB/s 00:00 $ cat /tmp/foo bar
A couple of notes: - that might look like a weird usecase (Don't do this!), but this can happen with clustershell copy mode (clush --copy) where it's very easy to send a file to all configured machines with the same path, and if the source machine is included in that list we will run something like this scp command (that's the reason Stephane found that out -- I just think it's better to explain where we come from) - regardless of the localhost case, in case a transfer is interrupted in the middle both the old scp protocol and sftp will leave the target file clobbered. O_TRUNC is objectively better than leaving part of the original content behind here, but a potential better alternative would be to do like rsync and write to a temporary file (rsync uses `.<originalfilename>.<randomchars>`) That file can be removed safely if the connection is interrupted, and the original file would be left untouched. (perhaps it'd be better to ask this not to openssh portable but to the bugs@openbsd list?) Thank you!
This was fixed in OpenSSH 9.1 commit 56a0697fe079ff3e1ba30a2d5c26b5e45f7b71f8 Author: djm@openbsd.org <djm@openbsd.org> Date: Fri May 13 06:31:50 2022 +0000 upstream: arrange for scp, when in sftp mode, to not ftruncate(3) files early previous behavious of unconditionally truncating the destination file would cause "scp ~/foo localhost:" and "scp localhost:foo ~/" to delete all the contents of their destination. spotted by solene@ sthen@, also bz3431; ok dtucker@ OpenBSD-Commit-ID: ca39fdd39e0ec1466b9666f15cbcfddea6aaa179
OpenSSH 9.3 has been released. Close resolved bugs