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Date: Sat, 23 Mar 2013 22:33:38 -0400
From: Rich Felker <dalias@...ifal.cx>
To: musl@...ts.openwall.com
Subject: Re: Difficulty emulating F_DUPFD_CLOEXEC
On Sat, Mar 23, 2013 at 10:27:14PM -0400, Zvi Gilboa wrote:
> On 03/23/2013 10:17 PM, Rich Felker wrote:
> >On Sat, Mar 23, 2013 at 10:10:10PM -0400, Zvi Gilboa wrote:
> >>>This uglifies fcntl.c a bit more, but I think it works. Does the above
> >>>reasoning make sense? Any other ideas?
> >>In the hope that this matches the project's spirit... how about
> >>running these tests during the build, and have a script (or a simple
> >>test program) #define whether the target architecture supports
> >>F_DUPFD_CLOEXEC or not? Potentially, this test could be added at
> >>the very end of alltypes.h.sh
> >It's not a matter of the architecture. It's a matter of the kernel
> >version. F_DUPFD_CLOEXEC was not available until late in the 2.6
> >series, and musl aims to "mostly work" even with early 2.6, and to
> >"partly work" (at least for single-threaded programs that don't use
> >modern features) even on 2.4. For dynamic linking, it could make sense
> >to have a slimmer version of libc.so that only supports up-to-date
> >kernels, but for static linking, it's really frustrating to have
> >binaries that break on older kernels, even if it is the broken
> >kernel's fault.
> >
> >If the lack of these features were just breaking _apps_ that use them,
> >it would be one thing, but several of the very-new atomic
> >close-on-exec interfaces needed internally in musl for some core
> >functionality -- things like dns lookups, popen, system, etc. Thus
> >failure to emulate them when the kernel doesn't have working versions
> >could badly break even "simple" apps that would otherwise be expected
> >to work even on old kernels.
>
> .... thanks, that makes perfect sense. As a second-best try: would
> it makes sense to run the long feature test just once, during
> startup (and save its result to some global variable), instead of
> inside fcntl.c?
The long test only runs when the fcntl syscall returns -EINVAL. So it
should never happen at all on a modern kernel with correct usage. The
tradeoff for saving the result of the test is that, by doing so, you
would get slightly improved performance on old kernels that lack
F_DUPFD_CLOEXEC, or when making a stupid fcntl call that would return
EINVAL even on a new kernel, but you would add more global data.
One byte of data should not really matter, but in a sense it does
because you have to worry that the linking order might put it in the
middle of an otherwise-unused page and thus increase the memory usage
of the whole process by a page. This is actually an issue we need to
address -- libc.so's memory usage is a lot higher than it should be
right now, because the global vars that get touched by EVERY process
at startup time are spread out across several pages. Ordering things
so that they all end up in the same page would fix it, but I'm still
looking for the cleanest way to do that..
Anyway, I think it's best not to save the results of fallback tests
like this. There's also an issue of making sure the saved result is
properly synchronized, which I haven't even touched on yet..
Rich
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