[PATCH] Make sure /dev/pts/ptmx is world-writable

[PATCH] Make sure /dev/pts/ptmx is world-writable

[Ludovic Courtès]

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Hello,

While running Python 3’s test suite, we noticed that on some systems
/dev/pts/ptmx is created with permissions 0 (that’s the case with my
Nixpkgs-originating 3.0.43 kernel, but someone with a Debian-originating
3.10-3 reported not having this problem.)

There’s still the problem that people without
CONFIG_DEVPTS_MULTIPLE_INSTANCES=y are screwed (as noted in build.cc),
but I don’t see how we could work around it.

Thoughts?

Ludo’.


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diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index e846995..53a92c1 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -2079,6 +2079,10 @@ void DerivationGoal::initChild()
                 if (mount("none", (chrootRootDir + "/dev/pts").c_str(), "devpts", 0, "newinstance,mode=0620") == -1)
                     throw SysError("mounting /dev/pts");
                 createSymlink("/dev/pts/ptmx", chrootRootDir + "/dev/ptmx");
+
+		/* Make sure /dev/pts/ptmx is world-writable.  With some
+		   Linux versions, it is created with permissions 0.  */
+		chmod_(chrootRootDir + "/dev/pts/ptmx", 0666);
             }
 
             /* Do the chroot().  Below we do a chdir() to the

Re: [PATCH] Make sure /dev/pts/ptmx is world-writable

[Lluís Batlle i Rossell]

On Wed, Apr 02, 2014 at 11:41:11PM +0200, Ludovic Courtès wrote:
> Hello,
> 
> While running Python 3’s test suite, we noticed that on some systems
> /dev/pts/ptmx is created with permissions 0 (that’s the case with my
> Nixpkgs-originating 3.0.43 kernel, but someone with a Debian-originating
> 3.10-3 reported not having this problem.)
> 
> There’s still the problem that people without
> CONFIG_DEVPTS_MULTIPLE_INSTANCES=y are screwed (as noted in build.cc),
> but I don’t see how we could work around it.
> 
> Thoughts?
> 
> Ludo’.

Usually it's handled with the devpts special mountable device. The
"ptmxmode" variable handles that. Don't you want to mount that in the
chroot?
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Re: [PATCH] Make sure /dev/pts/ptmx is world-writable

[Ludovic Courtès]

Hello!

Lluís Batlle i Rossell <viric@viric.name> skribis:

> On Wed, Apr 02, 2014 at 11:41:11PM +0200, Ludovic Courtès wrote:
>> Hello,
>> 
>> While running Python 3’s test suite, we noticed that on some systems
>> /dev/pts/ptmx is created with permissions 0 (that’s the case with my
>> Nixpkgs-originating 3.0.43 kernel, but someone with a Debian-originating
>> 3.10-3 reported not having this problem.)

Actually the doc confirms that:

  https://www.kernel.org/doc/Documentation/filesystems/devpts.txt

>> There’s still the problem that people without
>> CONFIG_DEVPTS_MULTIPLE_INSTANCES=y are screwed (as noted in build.cc),
>> but I don’t see how we could work around it.
>> 
>> Thoughts?
>> 
>> Ludo’.
>
> Usually it's handled with the devpts special mountable device. The
> "ptmxmode" variable handles that. Don't you want to mount that in the
> chroot?

We do want to mount /dev/pts, yes.  What’s the ‘ptmxmode variable’?

Ludo’.
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Re: [PATCH] Make sure /dev/pts/ptmx is world-writable

[Ludovic Courtès]

Lluís Batlle i Rossell <viric@viric.name> skribis:

> Usually it's handled with the devpts special mountable device. The
> "ptmxmode" variable handles that. Don't you want to mount that in the
> chroot?

Ah sorry, I see the ptmxmode mount option:

  https://www.kernel.org/doc/Documentation/filesystems/devpts.txt

Well it seems both chmod and that mount option would work, but the
ptmxmode option is marked as “new” in the doc above, so maybe it’s safer
to use chmod?

Ludo’.
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Re: [PATCH] Make sure /dev/pts/ptmx is world-writable

[Lluís Batlle i Rossell]

On Thu, Apr 03, 2014 at 09:31:11AM +0200, Ludovic Courtès wrote:
> Lluís Batlle i Rossell <viric@viric.name> skribis:
> 
> > Usually it's handled with the devpts special mountable device. The
> > "ptmxmode" variable handles that. Don't you want to mount that in the
> > chroot?
> 
> Ah sorry, I see the ptmxmode mount option:
> 
>   https://www.kernel.org/doc/Documentation/filesystems/devpts.txt
> 
> Well it seems both chmod and that mount option would work, but the
> ptmxmode option is marked as “new” in the doc above, so maybe it’s safer
> to use chmod?

I think that ptmxmode is there since 2.6 kernels, at least. :)

There is a reason for having the devpts fs instead of static nodes, but I don't remember it.
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