On powerpc, a const variable may end up in the .sdata section, which is writable. This means authors cannot depend on the "const" marking to mean "read-only", as is required for sane Linux kernel memory protection security. Thread here: https://lkml.org/lkml/2017/10/2/488 At the very least, there should be a way to request never putting a const variable into a writable section.
Actually it is just undefined what happens when a write to a const variable. So a trap or the write happened for a const both are valid thing. Now const really should be put in the read only section if possible. Putting it in the sdata section is valid thing to do but I doubt some people want it there. Powerpc should have an option which disabled this optimization for const variables. Right now there is a way to disable all of sdata by -G0 option which is a good workaround.
To access this as sdata is faster and smaller than as rodata (one instruction instead of two). You can use -G0 as Andrew says, or -mno-sdata (or -msdata=none).
To clarify, using -mno-sdata means all things are removed from sdata, not just const, yes? I'd like to be able to leave writable stuff there, to avoid any additional performance penalty.
That what it means yes. You can use it as a workaround. There is no option yet to put only writable data in sdata.
RFC patch is at https://gcc.gnu.org/ml/gcc-patches/2018-02/msg01533.html ; so far it has been tested with Linux and with EABI.
Author: segher Date: Wed Mar 7 20:27:11 2018 New Revision: 258340 URL: https://gcc.gnu.org/viewcvs?rev=258340&root=gcc&view=rev Log: rs6000: -mreadonly-in-sdata (PR82411) This adds a new option -mreadonly-in-sdata (on by default) that controls whether readonly data can be put in sdata. (For EABI this does nothing, readonly data is put in sdata2 as usual). PR target/82411 * config/rs6000/rs6000.c (rs6000_elf_in_small_data_p): Don't put readonly data in sdata, if that is disabled. * config/rs6000/sysv4.opt (mreadonly-in-sdata): New option. * doc/invoke.texi (RS/6000 and PowerPC Options): Document -mreadonly-in-sdata option. gcc/testsuite/ PR target/82411 * gcc.target/powerpc/ppc-sdata-2.c: Skip if -mno-readonly-in-sdata. Modified: trunk/gcc/ChangeLog trunk/gcc/config/rs6000/rs6000.c trunk/gcc/config/rs6000/sysv4.opt trunk/gcc/doc/invoke.texi trunk/gcc/testsuite/ChangeLog trunk/gcc/testsuite/gcc.target/powerpc/ppc-sdata-2.c
Done on trunk; backports pending.
Author: segher Date: Tue Mar 27 23:13:02 2018 New Revision: 258907 URL: https://gcc.gnu.org/viewcvs?rev=258907&root=gcc&view=rev Log: rs6000: -mreadonly-in-sdata (PR82411) This adds a new option -mreadonly-in-sdata (on by default) that controls whether readonly data can be put in sdata. (For EABI this does nothing, readonly data is put in sdata2 as usual). Backport from mainline 2018-03-08 Segher Boessenkool <segher@kernel.crashing.org> PR target/82411 * config/rs6000/rs6000.c (rs6000_elf_in_small_data_p): Don't put readonly data in sdata, if that is disabled. * config/rs6000/sysv4.opt (mreadonly-in-sdata): New option. * doc/invoke.texi (RS/6000 and PowerPC Options): Document -mreadonly-in-sdata option. Backport from mainline 2018-03-08 Segher Boessenkool <segher@kernel.crashing.org> PR target/82411 * gcc.target/powerpc/ppc-sdata-2.c: Skip if -mno-readonly-in-sdata. Modified: branches/gcc-7-branch/gcc/ChangeLog branches/gcc-7-branch/gcc/config/rs6000/rs6000.c branches/gcc-7-branch/gcc/config/rs6000/sysv4.opt branches/gcc-7-branch/gcc/doc/invoke.texi branches/gcc-7-branch/gcc/testsuite/ChangeLog branches/gcc-7-branch/gcc/testsuite/gcc.target/powerpc/ppc-sdata-2.c
Author: segher Date: Tue Mar 27 23:28:25 2018 New Revision: 258909 URL: https://gcc.gnu.org/viewcvs?rev=258909&root=gcc&view=rev Log: rs6000: -mreadonly-in-sdata (PR82411) This adds a new option -mreadonly-in-sdata (on by default) that controls whether readonly data can be put in sdata. (For EABI this does nothing, readonly data is put in sdata2 as usual). Backport from mainline 2018-03-08 Segher Boessenkool <segher@kernel.crashing.org> PR target/82411 * config/rs6000/rs6000.c (rs6000_elf_in_small_data_p): Don't put readonly data in sdata, if that is disabled. * config/rs6000/sysv4.opt (mreadonly-in-sdata): New option. * doc/invoke.texi (RS/6000 and PowerPC Options): Document -mreadonly-in-sdata option. gcc/testsuite/ Backport from mainline 2018-03-08 Segher Boessenkool <segher@kernel.crashing.org> PR target/82411 * gcc.target/powerpc/ppc-sdata-2.c: Skip if -mno-readonly-in-sdata. Modified: branches/gcc-6-branch/gcc/ChangeLog branches/gcc-6-branch/gcc/config/rs6000/rs6000.c branches/gcc-6-branch/gcc/config/rs6000/sysv4.opt branches/gcc-6-branch/gcc/doc/invoke.texi branches/gcc-6-branch/gcc/testsuite/ChangeLog branches/gcc-6-branch/gcc/testsuite/gcc.target/powerpc/ppc-sdata-2.c
All done.