RFC: renameat2 emulation

[Shiv <coderaccords%gmail.com@localhost>]

As part of my GSoC project, me and my mentor have
decided to extend support for renameat2 in the compat_linux.
We have decided to enable support for 2 newer flags, namely
RENAME_NOREPLACE and RENAME_EXCHANGE. There are
2 particular questions, on which we would like to gather communities
insights:

1. To prevent TOCTOU attack and provide atomicity, most of the
renameat code has been reimplemented in the compat layer. Is
this an acceptable approach or are there some better alternatives?

2. RENAME_EXCHANGE requires atomic swap of filenames and the
filetypes need not be the same. Can any of the existing VFS operations
be used to do so? And if this should be done in VFS layer or any specific
support is expected from the file system to implement this?


Shivraj