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



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