[Regression] Editting stale symlink removes symlink and creates file symlink points to

[Pavel Roskin]

Hello!

> Max Derzhak <max /at/linux.zp.ua> reported new regression in mcedit.  If 
> you type F4 on stale symlink and press Esc-Esc or F10 after it (no 
> editting at all) mcedit creates empty file symlink points to and removes 
> original symlink.

The problem is not related to VFS, so I'll talk in terms on libc
functions.

stat() fails, the file is opened with O_CREAT, creating the link target.  
If the file has not been modified, it's removed.  open() resolves
symlinks, unlink() doesn't, hence the result.

I think that editing stale symlinks is insane.  If stat() fails, then we
expect to create a new file.  That's what O_EXCL is for.  I'm applying a
patch that uses O_EXCL whenever stat() fails.  Editing stale symlinks is
now impossible.  I think it's reasonable.

I realize that O_EXCL may not be implemented in every VFS, but it's a
separate problem.  Actually, direntry.c knows this flag, so only
filesystems with its own open() need to be changed.

-- 
Regards,
Pavel Roskin