From: Florian Weimer <fweimer@redhat.com>
To: Rich Felker <dalias@libc.org>
Cc: 39236@debbugs.gnu.org, musl@lists.openwall.com
Subject: Re: [musl] coreutils cp mishandles error return from lchmod
Date: Wed, 22 Jan 2020 16:32:45 +0100 [thread overview]
Message-ID: <87zhefik0y.fsf@oldenburg2.str.redhat.com> (raw)
In-Reply-To: <20200122151507.GB30412@brightrain.aerifal.cx> (Rich Felker's message of "Wed, 22 Jan 2020 10:15:07 -0500")
* Rich Felker:
> On Wed, Jan 22, 2020 at 04:08:26PM +0100, Florian Weimer wrote:
>> * Rich Felker:
>>
>> > On Wed, Jan 22, 2020 at 03:34:18PM +0100, Florian Weimer wrote:
>> >> * Rich Felker:
>> >>
>> >> > coreutils should be opting to use the system-provided lchmod, which is
>> >> > safe, and correctly handling error returns (silently treating
>> >> > EOPNOTSUPP as success) rather than as hard errors.
>> >>
>> >> glibc's lchmod always returns ENOSYS (except on Hurd). I don't know how
>> >> lchmod is used in coreutils, but I suspect it is not particularly
>> >> useful.
>> >
>> > When preserving permissions (cp -p, archive extraction, etc.), you
>> > want lchmod to work correctly just for the purpose of *not* following
>> > the link and thereby unwantedly changing the permissions of the link
>> > target. But, fchmodat with AT_SYMLINK_NOFOLLOW works just as well and
>> > is standard, and that's really what coreutils should be using.
>>
>> I think you misread what I wrote: lchmod *always* returns ENOSYS. Even
>> if the file is not a symbolic link. Likewise, fchmodat with
>> AT_SYMLINK_NOFOLLOW *always* returns ENOTSUP.
>
> Yes, I understood that. I was going into why there should be a real
> implementation, but didn't make it clear that that was what I was
> doing.
Ah, yes, there should be a real implementation if we can get full
lchmod/AT_SYMLINK_NOFOLLOW behavior on file systems that support it. If
we can't, I'm not sure if there is a point to it.
>> The reason for this is that the kernel does not provide a suitable
>> system call to implement this, even though some file systems allow a
>> mode change for symbolic links. I think we can do better, although I
>> should note that each time we implement such emulation in userspace, it
>> comes back to bite us eventually.
>
> Emulations in userspace that are approximate, have race conditions,
> etc. are bad. Ones that are rigorous are good, though.
Is there a reason for the S_ISLNK check in the musl implementation?
With current kernels, chmod on the proc pseudo-file will not traverse
the symbolic link, but I have yet to check if this has always been the
case.
Thanks,
Florian
next prev parent reply other threads:[~2020-01-22 15:33 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-22 14:15 Rich Felker
2020-01-22 14:34 ` Florian Weimer
2020-01-22 14:42 ` Rich Felker
2020-01-22 15:08 ` Florian Weimer
2020-01-22 15:15 ` Rich Felker
2020-01-22 15:32 ` Florian Weimer [this message]
2020-01-22 16:07 ` Rich Felker
2020-01-22 16:19 ` Florian Weimer
2020-01-22 17:15 ` Rich Felker
2020-01-22 20:48 ` Florian Weimer
2020-01-22 20:56 ` Rich Felker
2020-01-22 21:05 ` Florian Weimer
2020-01-22 21:55 ` bug#39236: " Paul Eggert
2020-01-22 22:05 ` Rich Felker
2020-02-08 0:37 ` Paul Eggert
2020-02-12 11:50 ` Florian Weimer
2020-02-12 13:05 ` Rich Felker
2020-02-12 19:07 ` Rich Felker
2020-02-12 19:13 ` Florian Weimer
2020-02-12 19:59 ` A. Wilcox
2020-02-12 20:56 ` Rich Felker
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