GNU logs - #39236, boring messages

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Subject: bug#39236: [musl] coreutils cp mishandles error return from lchmod
Resent-From: Florian Weimer <fweimer@HIDDEN>
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To: Rich Felker <dalias@HIDDEN>
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From: Florian Weimer <fweimer@HIDDEN>
References: <20200122141557.GA8157@HIDDEN>
Date: Wed, 22 Jan 2020 15:34:18 +0100
In-Reply-To: <20200122141557.GA8157@HIDDEN> (Rich Felker's
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* 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.

Thanks,
Florian

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Subject: bug#39236: Acknowledgement ([musl] coreutils cp mishandles error
 return from lchmod)
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Subject: bug#39236: [musl] coreutils cp mishandles error return from lchmod
Resent-From: Florian Weimer <fweimer@HIDDEN>
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To: Rich Felker <dalias@HIDDEN>
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From: Florian Weimer <fweimer@HIDDEN>
References: <20200122141557.GA8157@HIDDEN>
 <87ftg7k1at.fsf@HIDDEN>
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Date: Wed, 22 Jan 2020 16:08:26 +0100
In-Reply-To: <20200122144243.GZ30412@HIDDEN> (Rich Felker's
 message of "Wed, 22 Jan 2020 09:42:43 -0500")
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* Rich Felker:

> On Wed, Jan 22, 2020 at 03:34:18PM +0100, Florian Weimer wrote:
>> * Rich Felker:
>>=20
>> > 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.
>>=20
>> 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.

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.

Thanks,
Florian

Message sent to bug-coreutils@HIDDEN:

Subject: bug#39236: [musl] coreutils cp mishandles error return from lchmod
Resent-From: Florian Weimer <fweimer@HIDDEN>
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From: Florian Weimer <fweimer@HIDDEN>
References: <20200122141557.GA8157@HIDDEN>
 <87ftg7k1at.fsf@HIDDEN>
 <20200122144243.GZ30412@HIDDEN>
 <87a76fjzpx.fsf@HIDDEN>
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Date: Wed, 22 Jan 2020 16:32:45 +0100
In-Reply-To: <20200122151507.GB30412@HIDDEN> (Rich Felker's
 message of "Wed, 22 Jan 2020 10:15:07 -0500")
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* Rich Felker:

> On Wed, Jan 22, 2020 at 04:08:26PM +0100, Florian Weimer wrote:
>> * Rich Felker:
>>=20
>> > On Wed, Jan 22, 2020 at 03:34:18PM +0100, Florian Weimer wrote:
>> >> * Rich Felker:
>> >>=20
>> >> > 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.
>> >>=20
>> >> 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.
>>=20
>> 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

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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.

Rich

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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.

> 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.

Rich

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On Wed, Jan 22, 2020 at 04:32:45PM +0100, Florian Weimer wrote:
> * 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 point is to fail when the target is a symlink, rather than
(erroneously and possibly dangerously) applying the chmod to the link
target. Actually supporting link modes is useless. It's the "not
modifying the target" that's important.

> >> 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.

It's explained in the bz you just replied on,
https://sourceware.org/bugzilla/show_bug.cgi?id=14578

The point of the S_ISLNK check is to fail out early with the ENOTSUPP,
which the caller should treat as "success-like", in the non-racing
condition, without the need to open a fd (which may fail with
ENFILE/EMFILE) and without the need for /proc to be mounted.
Otherwise, a different error will be produced when one of those cases
is hit, and the caller will treat it as a real error.

Rich

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* Rich Felker:

> On Wed, Jan 22, 2020 at 04:32:45PM +0100, Florian Weimer wrote:
>> * Rich Felker:
>>=20
>> > On Wed, Jan 22, 2020 at 04:08:26PM +0100, Florian Weimer wrote:
>> >> * Rich Felker:
>> >>=20
>> >> > On Wed, Jan 22, 2020 at 03:34:18PM +0100, Florian Weimer wrote:
>> >> >> * Rich Felker:
>> >> >>=20
>> >> >> > coreutils should be opting to use the system-provided lchmod, wh=
ich is
>> >> >> > safe, and correctly handling error returns (silently treating
>> >> >> > EOPNOTSUPP as success) rather than as hard errors.
>> >> >>=20
>> >> >> glibc's lchmod always returns ENOSYS (except on Hurd).  I don't kn=
ow 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* followi=
ng
>> >> > the link and thereby unwantedly changing the permissions of the lin=
k
>> >> > target. But, fchmodat with AT_SYMLINK_NOFOLLOW works just as well a=
nd
>> >> > is standard, and that's really what coreutils should be using.
>> >>=20
>> >> I think you misread what I wrote: lchmod *always* returns ENOSYS.  Ev=
en
>> >> 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.
>>=20
>> 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 point is to fail when the target is a symlink, rather than
> (erroneously and possibly dangerously) applying the chmod to the link
> target. Actually supporting link modes is useless. It's the "not
> modifying the target" that's important.

The kernel supports it on some file systems, though:

$ ls -l /tmp/x
l---------. 1 fweimer fweimer 6 Jan 22 15:27 /tmp/x -> /tmp/x

Although mode 0 curiously does not prevent readlink calls.

> It's explained in the bz you just replied on,
> https://sourceware.org/bugzilla/show_bug.cgi?id=3D14578
>
> The point of the S_ISLNK check is to fail out early with the ENOTSUPP,
> which the caller should treat as "success-like", in the non-racing
> condition, without the need to open a fd (which may fail with
> ENFILE/EMFILE) and without the need for /proc to be mounted.
> Otherwise, a different error will be produced when one of those cases
> is hit, and the caller will treat it as a real error.

Hmm.  The way I read the musl code, the O_PATH descriptor already
exists.  At this point, you can just chmod the O_PATH descriptor, and
have the kernel report EOPNOTSUPP if the file system does not support
that.

Thanks,
Florian

Last modified: Wed, 22 Jan 2020 16:30:02 UTC

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