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

From: Eric W. Biederman
Date: Wed Feb 01 2012 - 17:41:56 EST

Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxx>
>> ---
>> 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.
>> +
>> + 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.


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