Re: sort and -lm

[Pádraig Brady]

On 07/10/2023 22:29, Paul Eggert wrote:
> On 2023-10-07 04:42, Pádraig Brady wrote:
> > The auto linking is globally controlled with the --with-openssl
> > cofigure option, but you could build sort (and md5sum)
> > without that dependency with:
> >
> >     ./configure ac_cv_lib_crypto_MD5=no
>
> Thanks, I was thinking more along the lines that Bruno suggested, which
> to continue to link to libcrypto, but do it with dlopen/dlsym in 'sort'
> only when need_random is true.
>
> It's not clear to me offhand whether this should be done entirely in
> Coreutils, or whether we should add some Gnulib support to make it
> easier to do this sort of lazier linking.

I was wondering if this was worth worrying about at all,
but it is a significant overhead that's worth improving.
To quantify the overhead I compared optimized builds,
with and without the above configure option, giving:

$ time seq 10000 | xargs -I'{}' src/sort /dev/null -k'{}'
real    0m7.009s
user    0m3.462s
sys     0m3.578s

$ time seq 10000 | xargs -I'{}' src/sort-lc /dev/null -k'{}'
real    0m12.950s
user    0m3.754s
sys     0m9.200s


So we should do something. Now dlopening libcrypto on demand
would work, but there may be better solutions.
sort doesn't have to use md5. It could use blake2 routines
already in coreutils to avoid the issue (and get some speed ups).
Alternatively it might use some other hash function.
For example see the other 128 bit functions compared at:
https://github.com/Cyan4973/xxHash

BTW there was mention of static linking as an option in this thread.
That's is an option to provide better speed an isolation for binaries,
however it's best left to the system builders to use this for their builds.
There can be security implications for prompt library updating,
and libcrypto is particularly sensitive in this regard.

> Also, why doesn't coreutils/configure's ---with-linux-crypto option
> cause 'sort' to use Linux kernel crypto? Is this because the syscall
> overhead would be too high and it's significantly faster to do it in
> user space?
>
> I naively thought that --with-linux-crypto would mean we wouldn't need
> to link to libcrypto. Evidently not.....

Well --with-linux-crypto is for specialized setups
and not really for general usage, due to syscall overhead,
differing network syscall class (consider locked down environments),
and generally slower than libcrypto. That was detailed at:
https://git.sv.gnu.org/cgit/gnulib.git/commit/?id=71cc633a0f

cheers,
Pádraig