On Thu, Nov 17, 2022 at 1:46 PM Fangrui Song <mask...@google.com> wrote: > > On Thu, Nov 17, 2022 at 1:37 PM Andrew Pinski <pins...@gmail.com> wrote: > > > > On Thu, Nov 17, 2022 at 1:21 PM maskray--- via Gcc-patches > > <gcc-patches@gcc.gnu.org> wrote: > > > > > > > +.. option:: -mdirect-extern-access, -mno-direct-extern-access > > > > + > > > > + Use direct accesses for external data symbols. It avoids a GOT > > > > indirection > > > > + on all external data symbols with :option:`-fpie` or > > > > :option:`-fPIE`. This is > > > > + useful for executables linked with :option:`-static` or > > > > :option:`-static-pie`. > > > > + With :option:`-fpic` or :option:`-fPIC`, it only affects accesses to > > > > protected > > > > + data symbols. It has no effect on non-position independent code. > > > > The default > > > > + is :option:`-mno-direct-extern-access`. > > > > + > > > > + .. warning:: > > > > + > > > > + Use :option:`-mdirect-extern-access` either in shared libraries or > > > > in > > > > + executables, but not in both. Protected symbols used both in a > > > > shared > > > > + library and executable may cause linker errors or fail to work > > > > correctly. > > > > > > I think current GCC and Clang's behavior is: > > > > > > * -mdirect-extern-access is the default for -fno-pic. This is to enable > > > optimizations for -static programs but may introduce copy relocations. > > > * -mno-direct-extern-access is the default for -fpie and -fpic. This uses > > > some GOT-generating relocations which can be optimized out (lld, see > > > https://maskray.me/blog/2021-08-29-all-about-global-offset-table) but the > > > instruction is nevertheless slightly longer. > > > > > > (-mdirect-extern-access for -fpic probably doesn't make sense.) > > > > > > The option I introduced to Clang is -fdirect-access-external-data > > > (see > > > https://maskray.me/blog/2021-01-09-copy-relocations-canonical-plt-entries-and-protected). > > > If -mdirect-extern-access gets more popular, I can add a Clang alias. > > > But I am opposed to forcing a GNU property for > > > -mdirect-extern-access/-mno-direct-extern-access. > > > > > > FWIW I used > > > https://gist.github.com/MaskRay/c03a90922003df666551589f1629df22 to test > > > my Clang changes related to -fno-semantic-interposition > > > on various visibility attributes x non-weak/weak x nopic/pie/pic x > > > dllimport/not x ... > > > > > > The x86_64 discussion about this is here > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98112 . > > I think clang changing the ABI is just broken and should think twice > > before we do it for GCC. > > > > And there is a lot of visibility protected issues filed in GCC bug > > databases specifically about copy relocs too. > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56527 > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=37611 > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=19520 > > https://sourceware.org/bugzilla/show_bug.cgi?id=28875 > > https://sourceware.org/bugzilla/show_bug.cgi?id=28877 > > I also suspect clang's behavior is still broken too. > > > > Thanks, > > Andrew > > Well, I don't think Clang changed ABI regarding -fno-pic/-fpie/-fpic. > As I did archaeology on > https://maskray.me/blog/2021-01-09-copy-relocations-canonical-plt-entries-and-protected > "Reflection on protected data symbols and copy relocations" > GCC 5 x86-64 made a change and GCC aarch64 accidentally picked up the change.
You missed: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65248 (or rather r5-7961-ga5eef8e9b02474 ) was the change to fix protected . > > """ > On the GCC side, in -fpic mode, using GOT-generating relocations when > accessing a protected variable subverts the point using the protected > visibility. The unneeded pessimization is the foremost complaint. The > pessimization applies to all ports with #define TARGET_BINDS_LOCAL_P > default_binds_local_p_2. aarch64 moved to default_binds_local_p_2 > accidentally by > https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=cbddf64c0243816b45e6680754a251c603245dbc. This was NOT by accident. In fact you just looked into the commit and NOT the actually email which submitted the patch: https://gcc.gnu.org/legacy-ml/gcc-patches/2015-04/msg01432.html https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65780 "As s390/arm/aarch64 seems to work fine (generate a COPY relocation and thus define symbol locally) in non-PIE executables, this patch changes those to a function that has been added for that behavior." Thanks, Andrew Pinski > > For GCC<5 (and all versions of Clang), direct accesses to protected > variables are produced in -fpic code. Mixing such object files can > still silently break copy relocations on protected data symbols. > Therefore, GNU ld made the controversial change > https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=commit;h=ca3fe95e469b9daec153caa2c90665f5daaec2b5 > to error in -shared mode. > """ > > > > > > > > On 2022-11-17, Ramana Radhakrishnan wrote: > > > >On Thu, Nov 17, 2022 at 5:30 PM Richard Sandiford via Gcc-patches > > > ><gcc-patches@gcc.gnu.org> wrote: > > > >> > > > >> Wilco Dijkstra <wilco.dijks...@arm.com> writes: > > > >> > Hi Richard, > > > >> > > > > >> >> Can you go into more detail about: > > > >> >> > > > >> >> Use :option:`-mdirect-extern-access` either in shared libraries > > > >> >> or in > > > >> >> executables, but not in both. Protected symbols used both in a > > > >> >> shared > > > >> >> library and executable may cause linker errors or fail to work > > > >> >> correctly > > > >> >> > > > >> >> If this is LLVM's default for PIC (and by assumption shared > > > >> >> libraries), > > > >> >> is it then invalid to use -mdirect-extern-access for any PIEs that > > > >> >> are linked against those shared libraries and use protected symbols > > > >> >> from those libraries? How would a user know that one of the shared > > > >> >> libraries they're linking against was built in this way? > > > >> > > > > >> > Yes, the usage model is that you'd either use it for static PIE or > > > >> > only on > > > >> > data that is not shared. If you get it wrong them you'll get the copy > > > >> > relocation error. > > > >> > > > >> Thanks. I think I'm still missing something though. If, for the > > > >> non-executable case, people should only use the feature on data that > > > >> is not shared, why do we need to relax the binds-local condition for > > > >> protected symbols on -fPIC? Oughtn't the symbol to be hidden rather > > > >> than protected if the data isn't shared? > > > >> > > > >> I can understand the reasoning for the PIE changes but I'm still > > > >> struggling with the PIC-but-not-PIE bits. > > > > > > > >I think I'm with Richard S on hidden vs protected on first reading. I > > > >can see why this works out of the box and can even be default for > > > >static-pie. > > > > > > > >Any reason why this is not on by default - it's early enough in the > > > >stage3 cycle and we can always flip the defaults if there are more > > > >problems found. > > > > > > > >You probably need a rebase for the documentation bits,. > > > > > > > >regards > > > >Ramana > > > > > > > > > > > >Ramana > > > > > > > > > + is :option:`-mno-direct-extern-access`. > > > > -- > 宋方睿
