Re: [PATCH v9 0/4] MR6855: server: Fix renaming file to a hardlink of itself.

[Gabriel Ivăncescu (＠insn)]

On Sat Mar 15 12:38:32 2025 +0000, Jinoh Kang wrote:
> If multiple wineservers (and thus wineprefixes) were to attempt to
> rename the same hardlink, they may end up allocating the *same*[^1]
> tempname and race against each other.
> This fixes the race without increasing the number of syscalls (untested):
>         if (!is_dir)
>         {
>             /* to avoid TOCTTOU, probe existence of tmpname and link at
> the same time */
>             if (linkat( dirfd, srcname, dirfd, tmpname )) continue;
>             if (unlinkat( dirfd, srcname ))
>             {
>                 file_set_error();
>                 unlinkat( dirfd, tmpname );
>                 goto ret;
>             }
>             break;
>         }
>         /* directories can't have hardlinks, so just rename it to
> tmpname */
>         else if (fstatat( dirfd, tmpname, &st, 0 ) 
>  &&  /* tmpname doesn't exist */
>                  !renameat( dirfd, srcname, dirfd, tmpname ))
>             break;
>
> (And remove the renameat srcname,tmpname below)
> [^1]: Each wineserver starts with `tmp_value = 0`. Each wineserver has
> the same `current_time` at the same time if clock sampling aligns.

Good point. I've added it along with checking for error codes properly (we only want to keep going if it actually exists; if it fails for other reasons, there's no point retrying since it will likely keep failing).