Re: vim vs nano on editing system files

On 2013-08-08, Markus Osterhoff wrote:
> * Dahong Tang [130808 04:59]:
> > > $ FILE=/tmp/testfile
> > > $ sudo touch $FILE
> > > $ sudo chmod 400 $FILE
> > Thanks Sven. Your explanation makes sense if I used the command vim
> > $FILE, but I used sudo vim $FILE. Why does vim even bother to set the
> > file to readonly when I used sudo? That is the part that I don't
> > understand.
>
> Because you told it so: If a file has no (w)rite permission (as in 400,
> which is r--), vim would open it as read-only, so a
> :w
> fails. But now that you are root (sudo), no-one prevents you from
> "changing" the flags, writing, and "changing flags back". I quoted this,
> since I assume that the Linux kernel just allows root to write files
> that she/he has no obvious write permissions for.
>
> Short: if vim sees a file without w-permission, it opens it as readonly;
> but root may overwrite it nevertheless.

Not just root but any user who is the owner of the file. You can
repeat Sven's demonstration as a normal user.

touch testfile
chmod 400 testfile
vim testfile

:w

will fail but

:w!

will succeed in writing the file. See ":help :w" where is says that
"[t]his may change the permission and ownership of the file...."

You can see how Vim does this on Unix by looking at this section of
the buf_write() function in fileio.c:


#if defined(UNIX) && !defined(ARCHIE)
/* When using ":w!" and the file was read-only: make it writable */
if (forceit && perm >= 0 && !(perm & 0200) && st_old.st_uid == getuid()
&& vim_strchr(p_cpo, CPO_FWRITE) == NULL)
{
perm |= 0200;
(void)mch_setperm(fname, perm);
made_writable = TRUE;
}