linux-kernel - Re: [PATCH net] mpls: fix af

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Date:   Tue, 11 Jun 2019 19:39:51 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Matteo Croce <mcroce@...hat.com>
Cc:     David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Linux Next Mailing List <linux-next@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Ahern <dsahern@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH net] mpls: fix af_mpls dependencies

On 6/11/19 5:08 PM, Matteo Croce wrote:
> On Wed, Jun 12, 2019 at 1:07 AM Randy Dunlap <rdunlap@...radead.org> wrote:
>>
>> On 6/9/19 7:57 PM, David Miller wrote:
>>> From: Matteo Croce <mcroce@...hat.com>
>>> Date: Sat,  8 Jun 2019 14:50:19 +0200
>>>
>>>> MPLS routing code relies on sysctl to work, so let it select PROC_SYSCTL.
>>>>
>>>> Reported-by: Randy Dunlap <rdunlap@...radead.org>
>>>> Suggested-by: David Ahern <dsahern@...il.com>
>>>> Signed-off-by: Matteo Croce <mcroce@...hat.com>
>>>
>>> Applied, thanks.
>>>
>>
>> This patch causes build errors when
>> # CONFIG_PROC_FS is not set
>> because PROC_SYSCTL depends on PROC_FS.  The build errors are not
>> in fs/proc/ but in other places in the kernel that never expect to see
>> PROC_FS not set but PROC_SYSCTL=y.
>>
> 
> Hi,
> 
> Maybe I'm missing something, if PROC_SYSCTL depends on PROC_FS, how is
> possible to have PROC_FS not set but PROC_SYSCTL=y?

When MPLS=y and MPLS_ROUTING=[y|m], MPLS_ROUTING selects PROC_SYSCTL.
That enables PROC_SYSCTL, whether PROC_FS is set/enabled or not.

There is a warning about this in Documentation/kbuild/kconfig-language.rst:

  Note:
	select should be used with care. select will force
	a symbol to a value without visiting the dependencies.
	By abusing select you are able to select a symbol FOO even
	if FOO depends on BAR that is not set.
	In general use select only for non-visible symbols
	(no prompts anywhere) and for symbols with no dependencies.
	That will limit the usefulness but on the other hand avoid
	the illegal configurations all over.


> I tried it by manually editing .config. but make oldconfig warns:
> 
> WARNING: unmet direct dependencies detected for PROC_SYSCTL
>   Depends on [n]: PROC_FS [=n]
>   Selected by [m]:
>   - MPLS_ROUTING [=m] && NET [=y] && MPLS [=y] && (NET_IP_TUNNEL [=n]
> || NET_IP_TUNNEL [=n]=n)

Yes, I get this also.

> *
> * Restart config...
> *
> *
> * Configure standard kernel features (expert users)
> *
> Configure standard kernel features (expert users) (EXPERT) [Y/?] y
>   Multiple users, groups and capabilities support (MULTIUSER) [Y/n/?] y
>   sgetmask/ssetmask syscalls support (SGETMASK_SYSCALL) [N/y/?] n
>   Sysfs syscall support (SYSFS_SYSCALL) [N/y/?] n
>   Sysctl syscall support (SYSCTL_SYSCALL) [N/y/?] (NEW)

So I still say that MPLS_ROUTING should depend on PROC_SYSCTL,
not select it.

-- 
~Randy