Re: [PATCH] Add --enable-default-semantic-interposition to GCC configure

[Fangrui Song <i@maskray.me>]

On 2021-06-07, Jakub Jelinek wrote:
>On Mon, Jun 07, 2021 at 12:01:55PM -0600, Jeff Law via Gcc-patches wrote:
>> > This breaks assumptions across the board.  If software packages want
>> > to use -fno-semantic-interposition that is one thing.  But distros
>> > should not be changing the default.  This is just like using
>> > -ffast-math :).
>> Some distros already force immediate binding at link time for security
>> purposes on a distro-wide basis which, IIUC, does the same thing, but
>> without the benefits from a code generation standpoint.
>
>If you are talking about -Wl,-z,now, that is very different, semantic
>interposition then still works just fine.
>If you are talking about the glibc style by hand "protected" visibility,
>bind calls to symbols defined in the same library through internal symbols,
>then that is done only for a couple of packages and is stronger than
>-fno-semantic-interposition.
>
>	Jakub
>

-fno-semantic-interposition can save a PLT entry (and associated
R_*_JUMP_SLOT dynamic relocation) if a default visibility STB_GLOBAL
function is only called in its defining TU, not by other TUs linked into
the shared object.

This is a subset of the PLT-suppressing optimization if a distribution defaults to ld -Bsymbolic-non-weak-functions
(https://maskray.me/blog/2021-05-16-elf-interposition-and-bsymbolic#the-last-alliance-of-elf-and-men)



Binding definitions in the same component can make software securer.

https://twitter.com/CarlosODonell/status/1400879768028028935
"Disable PRELOAD/AUDIT, which is what I'm going to pursue e.g.  system-wide glibc hardening tunable."
If such a thing is deployed, why cannot a passionate distribution default to gcc -fno-semantic-interposition  and ld -Bsymbolic-non-weak-functions
can bring back the lost performance (15+% for my clang; 27% for cpython; ...)



Last, the "assumption" is just GCC's mapping from source code to the ELF binary
format.
https://maskray.me/blog/2021-05-09-fno-semantic-interposition#source-level-implication
We could also argue that C++ odr rule doesn't like us doing semantic interposition.
(I know it's vague https://stackoverflow.com/questions/55632222/odr-violation-when-linking-static-and-dynamic-library )