Re: Inherited patch question

Holger,

Thanks for the explanation.  I'm happy to remove the [2] and use scalar
context.

Craig

On Wed, Mar 29, 2017 at 3:06 PM, Holger Parplies <[email protected]> wrote:

> Hi,
>
> Richard Shaw wrote on 2017-03-29 13:38:16 -0500 [[BackupPC-devel]
> Inherited patch question]:
> > [...]
> > $ cat rpmbuild/BackupPC/SOURCES/BackupPC-4.0.0-fix-shadow-access.patch
> > [...]
> > -           && $> != (my $uid = (getpwnam($Conf{BackupPCUser}))[2]) ) {
> > +           && $> != (my $uid = (getpwnam($Conf{BackupPCUser}))) ) {
> > [...]
> > What's the effect of removing the [2] from these?
>
> well, in theory (and practise, at least on my local system here) getpwnam
> returns something like 'split /:/, $passwd_line' in list context and the
> uid in scalar context. The third element (index [2]) of the split would
> also be the uid, which explains why the two lines can be equivalent, even
> though they seem very different.
>
> >From the *name* of the patch, I would guess that there might be a
> potential
> problem on systems with shadow passwords in some cases, though I can't see
> one here on my system. I could *imagine* though, that there might be
> systems
> that differ.
>
> A closer look reveals the following:
>
>         % perl -e 'my @p = getpwnam "foo"; print ">", (join ",", @p),
> "<\n";'
>         foo,x,1234,1234,,,Holger Parplies,/home/foo,/bin/tcsh
>         # perl -e 'my @p = getpwnam "foo"; print ">", (join ",", @p),
> "<\n";'
>         foo,<my-hashed-password>,1234,1234,,,Holger
> Parplies,/home/foo,/bin/tcsh
>
> (no, my user name is not "foo" and my uid is not 1234 ;-), so my Perl (or
> rather getpwnam(3)) merges in the shadow password, privilege permitting.
> Although I can't find any hint in the documentation, I could imagine that
> the attempt to do so could trigger unwanted behaviour (e.g. an audit log or
> even termination of the process) under some security systems, depending on
> how the determination of "privilege permitting" might be implemented.
>
> In any case, I would *hope* that the scalar context case would be slightly
> more efficient, because the unneeded information in the additional array
> elements not corresponding to /etc/passwd fields ($quota, $comment,
> $expire)
> does not need to be retrieved.
>
> For an explanation of the getpwnam function look at 'perldoc -f getpwuid'
> (strangely, 'perldoc -f getpwnam' is not very helpful, at least on some
> systems ;-).
>
> Regards,
> Holger
>
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