Re: [PATCH] Add renameat2 function [BZ #17662]

[Florian Weimer <fweimer at redhat dot com>]

On 07/02/2018 07:38 PM, Paul Eggert wrote:
> Florian Weimer wrote:
> > Without kernel support, a non-zero argument returns EINVAL, not ENOSYS.
> > This mirrors what the kernel does for invalid renameat2 flags.
>
> The Gnulib renameat2 function 
> <https://www.gnu.org/software/gnulib/MODULES.html#module=renameat2> has 
> different semantics with non-zero flags. On GNU/Linux if 
> flags==RENAME_NOREPLACE and the Linux syscall fails due to 
> EINVAL/ENOSYS/ENOTSUP, Gnulib renameat2 falls back on fstatatting the 
> destination, failing if fstatat succeeds, and using ordinary renameat 
> otherwise. Of course this implementation has a race condition, but 
> Gnulib-using applications like GNU 'mv' prefer this implementation since 
> if the kernel doesn't support RENAME_NOREPLACE they'd just fall back on 
> fstatat themselves anyway, if renameat2 didn't do that for them.

Surely that's a gnulib bug because the main reason for the 
RENAME_NOREPLACE variant renameat2 was to avoid exactly that race (or 
the other race where the file exists under both the old and new path).

The gnulib function should simply be called something else, not 
renameat2.  The present situation is unfortunate, but I don't think it 
would be an improvement if glibc copies the buggy gnulib behavior.

Thanks,
Florian