Re: [PATCH] sysfs: Optionally count subdirectories to support buggy applications

From: ebiederm@xmission.com (Eric W. Biederman)
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jslaby@suse.cz>, Alan Cox <alan@lxorguk.ukuu.org.uk>,
	LKML <linux-kernel@vger.kernel.org>,
	Al Viro <viro@zeniv.linux.org.uk>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Maciej Rutecki <maciej.rutecki@gmail.com>
Subject: Re: [PATCH] sysfs:  Optionally count subdirectories to support buggy applications
Date: Wed, 01 Feb 2012 14:44:32 -0800	[thread overview]
Message-ID: <m139auxjfz.fsf@fess.ebiederm.org> (raw)
In-Reply-To: <20120201222441.GB2333@kroah.com> (Greg Kroah-Hartman's message of "Wed, 1 Feb 2012 14:24:41 -0800")

Greg Kroah-Hartman <gregkh@linuxfoundation.org> writes:

> On Wed, Feb 01, 2012 at 02:21:59PM -0800, Eric W. Biederman wrote:
>> 
>> lm_sensors and possibly other applications get confused if all sysfs
>> directories return nlink == 1.  The lm_sensors code that got confused
>> was just wrong and a fixed version of lm_sensors should be released
>> shortly.
>> 
>> There may be other applications that have problems with sysfs return
>> nlink == 1 for directories.  To allow people to continue to use old
>> versions of userspace with new kernels add to sysfs a compile time
>> option to maintain mostly precise directory counts for those people who
>> don't mind the cost.
>> 
>> I have moved where we keep nlink in sysfs_dirent as compared to previous
>> versions of subdirectory counting to a location that packs better.
>> 
>> Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
>> ---
>>  fs/sysfs/Kconfig |   15 +++++++++++++++
>>  fs/sysfs/dir.c   |    8 ++++++++
>>  fs/sysfs/inode.c |    2 ++
>>  fs/sysfs/sysfs.h |   38 ++++++++++++++++++++++++++++++++++++++
>>  4 files changed, 63 insertions(+), 0 deletions(-)
>> 
>> diff --git a/fs/sysfs/Kconfig b/fs/sysfs/Kconfig
>> index 8c41fea..9b403e9 100644
>> --- a/fs/sysfs/Kconfig
>> +++ b/fs/sysfs/Kconfig
>> @@ -21,3 +21,18 @@ config SYSFS
>>  	example, "root=03:01" for /dev/hda1.
>>  
>>  	Designers of embedded systems may wish to say N here to conserve space.
>> +
>> +config SYSFS_COUNT_LINKS
>> +	bool "sysfs count subdirectoires to support buggy applications"
>> +	default n
>
> As we don't want to break things, this should be default y, right?

The new behavior is backwards compatible.  What the new behavior is not
is bug compatible.  So nothing *should* break.

Furthermore the breaking we have seen so far is limited to just
lm_sensors.  That is exactly one program that is not a server failing to
start.  That seems pretty minor in the worst case.

So I really don't expect anyone who ships 3.4 to enable this option.

I have written the option solely so that in case my assessment turns out
to be wrong there is already a tested solution.  I have been through the
pain of not being able to upgrade/test a new kernel because of a
backwards incompatible change and it can be very unpleasant.

> Also, should we list this in the feature_removal list as well so that we
> can get rid of it in a year or so?

Good idea.  I don't know if anyone actually reads feature removal but it
is good to serve notice.  I will cook up a patch for that.

Eric