Re: [PATCH] Revert {send,sendm,recv,recvm}msg conformance changes

[Carlos O'Donell <carlos at redhat dot com>]

On 06/09/2016 05:09 PM, Adhemerval Zanella wrote:
> After some discussion in libc-alpha about this POSIX compliance fix, I see
> that GLIBC should indeed revert back to previous definition of msghdr and
> cmsghdr and implementation of sendmsg, recvmsg, sendmmsg, recvmmsg due some
> reasons:
> 
>  * The possible issue where the syscalls wrapper add the compatibility
>    layer is quite limited in scope and range.  And kernel current
>    also add some limits to the values on the internal msghdr and
>    cmsghdr fields:
> 
>      - msghdr::msg_iovlen larger than UIO_MAXIOV (1024) returns
>        EMSGSIZE.
>      - msghdr::msg_controllen larger than INT_MAX returns ENOBUFS.
> 
>  * There is a small performance hit for recvmsg/sendmsg/recmmsg which
>    is neglectable, but it is a big hit for sendmmsg since now instead
>    of calling the syscall for the packed structure, GLIBC is calling
>    multiple sendmsg.  This defeat the very existence of the syscall.
> 
>  * It currently breaks libsanitizer build on GCC [1] (I fixed on compiler-rt).
>    However the fix is incomplete because it does add any runtime check
>    since libsanitizer currently does not have any facility to intercept
>    symbols with multiple version [2].
> 
>    This, along with incorret dlsym/dlvsym return for versioned symbol due
>    another bug [3], makes hard to interpose versioned symbols.
> 
>    Also, current approach of fixing GCC PR#71445 leads to half-baked
>    solutions without versioned symbol interposing.
> 
> This patch basically reverts commits 2f0dc39029ae08, 222c2d7f4357d66,
> af7f7c7ec8dea1.  I decided to not revert abf29edd4a3918 (Adjust
> kernel-features.h defaults for recvmsg and sendmsg) mainly because it
> does not really address the POSIX compliance original issue and also
> adds some cleanups.
> 
> Tested on x86, i386, s390, s390x, aarch64, and powerpc64le.
> 
> [1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71445
> [2] https://github.com/google/sanitizers/issues/628
> [3] https://sourceware.org/bugzilla/show_bug.cgi?id=14932

Thank you for all the work you put into these patches.

While we ended up reverting them, I am wont to say "experience is knowing
what not to do" and it is hard won.

While I am in favour of these patches, the subsequent problems make them
difficult to justify until such problems are fixed.

Several of the problems are not specific to your changes either, they are
general architectural problems with the tooling that will need to be fixed
at some point as we version more and more of the implementation.

-- 
Cheers,
Carlos.