system call allows a suitably-privileged application
to tweak various kernel parameters. It is a useful feature which, as it
happens, is almost never used. The reason for that is the existence of the
virtual directory hierarchy which exports the same
functionality in a form which is much easier to use. Callers of
have been encouraged to use /proc/sys
for a long time and the addition of new parameters to sysctl()
considered to be against the rules. One year ago, sysctl()
was removed from the 2.6.19-rc
, only to be restored before the final release.
sysctl() is part of the user-space ABI; it is supposed to continue
working forever. That is why the attempt to remove it was ultimately
rolled back. So it may be surprising to some to see a new removal attempt by Eric Biederman. His
latest patch adds a new deprecation warning and an entry in the feature
removal schedule putting the end of sysctl() in September, 2010.
After adding checking to register_sysctl_table and finding a whole
new set of bugs. Missed by countless code reviews and testers I
have finally lost patience with the binary sysctl interface.
The binary sysctl interface has been sort of deprecated for years
and finding a user space program that uses the syscall is more
difficult then finding a needle in a haystack. Problems continue
to crop up, with the in kernel implementation. So since supporting
something that no one uses is silly, deprecate sys_sysctl with a
sufficient grace period and notice that the handful of user space
applications that care can be fixed or replaced.
Eric's claim is that this interface is so little-used that it is visibly
rotting. There is sufficiently little common code between the
sysctl() and /proc/sys implementations that it is easy
for the two to diverge. In the long term, he says, the kernel community
will do a better job of not breaking applications by getting rid of
sysctl() in favor of the interface which is actually used and
The new patch has, predictably, drawn opposition from developers who do not
want to see the user-space ABI broken in this way. Alan Cox has also suggested that the deprecation warning
approach will not be successful in getting the few remaining users to
switch to /proc/sys:
The whole "whine a bit" process simply doesn't work when you are
trying to persuade people to move in a non-hobbyist context. They
don't want to move, the message is simply an annoyance, their
upstream huge package vendor won't change just to deal with it and
they'll class it as a regression from previous releases, an
incompatibility and file bugs until it goes away.
Andrew Morton, instead, is not opposed to
I think it's worth a try. It might take two, three or five years,
who knows? If it turns out to be impractical then we we can just
change our minds later, no big loss.
While there is little disagreement with the policy that the user-space ABI
should never break, it does seem that there is room for discussion on how
that goal might best be met. Unused code has always had a tendency to
break accidentally, and sysctl() looks to be very close to being
entirely unused. One could, presumably, address this problem with some
sort of regression test suite - something the kernel could use more of in
general. But the maintenance of interfaces which of almost entirely
historical interest is not really helpful to Linux users. So, perhaps,
there needs to be a way to remove system calls which have fallen into
disuse for a long-enough period. Should this patch go through, we shall
see whether three years is sufficient warning for such a change or not.
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