Re: eh_frame confusion
Michael Ellerman wrote: "Naveen N. Rao" writes: Rasmus Villemoes wrote: I'm building a ppc32 kernel, and noticed that after upgrading from gcc-7 to gcc-8 all object files now end up having .eh_frame section. For vmlinux, that's not a problem, because they all get discarded in arch/powerpc/kernel/vmlinux.lds.S . However, they stick around in modules, which doesn't seem to be useful - given that everything worked just fine with gcc-7, and I don't see anything in the module loader that handles .eh_frame. The reason I care is that my target has a rather tight rootfs budget, and the .eh_frame section seem to occupy 10-30% of the file size (obviously very depending on the particular module). Comparing the .foo.o.cmd files, I don't see change in options that might explain this (there's a bunch of new -Wno-*, and the -mspe=no spelling is apparently no longer supported in gcc-8). Both before and after, there's -fno-dwarf2-cfi-asm about which gcc's documentation says '-fno-dwarf2-cfi-asm' Emit DWARF unwind info as compiler generated '.eh_frame' section instead of using GAS '.cfi_*' directives. Looking into where that comes from got me even more confused, because both arm and unicore32 say # Never generate .eh_frame KBUILD_CFLAGS += $(call cc-option,-fno-dwarf2-cfi-asm) while the ppc32 case at hand says # FIXME: the module load should be taught about the additional relocs # generated by this. # revert to pre-gcc-4.4 behaviour of .eh_frame Michael opened a task to look into this recently and I had spent some time last week on this. The original commit/discussion adding -fno-dwarf2-cfi-asm refers to R_PPC64_REL32 relocations not being handled by our module loader: http://lkml.kernel.org/r/20090224065112.ga6...@bombadil.infradead.org I opened that issue purely based on noticing the wart in the Makefile, not because I'd actually tested it. However, that is now handled thanks to commit 9f751b82b491d: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=9f751b82b491d Haha, written by me, what an idiot. So the Makefile hack can presumably be dropped, because the module loader can handle the relocations. And then maybe we also want to turn off the unwind tables, but that would be a separate patch. I did a test build and a simple module loaded fine, so I think -fno-dwarf2-cfi-asm is not required anymore, unless Michael has seen some breakages with it. Michael? No, as I said above it was just reading the Makefile. Ok, thanks for clarifying. To test, I did 'allmodconfig' builds across three environments: - gcc (Ubuntu 9.2.1-9ubuntu2) 9.2.1 20191008 -- ppc64le - gcc (SUSE Linux) 7.5.0 -- ppc64le - gcc (GCC) 8.2.1 20181215 (Red Hat 8.2.1-6) -- ppc64 (BE) Then, used the below command to list all relocations in the modules: $ find . -name '*.ko' | xargs -n 1 readelf -Wr | grep -v "Relocation " | grep -v "Offset " | cut -d' ' -f4 | sort | uniq R_PPC64_ADDR32 R_PPC64_ADDR64 R_PPC64_ENTRY R_PPC64_REL24 R_PPC64_REL32 R_PPC64_REL64 R_PPC64_TOC R_PPC64_TOC16_HA R_PPC64_TOC16_LO R_PPC64_TOC16_LO_DS All three environments show up similar set of relocations, all of which we handle in the module loader today. If Rasmus/Christophe can confirm that this is true for ppc32 as well, then we should be fine. - Naveen
Re: eh_frame confusion
"Naveen N. Rao" writes: > Rasmus Villemoes wrote: >> I'm building a ppc32 kernel, and noticed that after upgrading from gcc-7 >> to gcc-8 all object files now end up having .eh_frame section. For >> vmlinux, that's not a problem, because they all get discarded in >> arch/powerpc/kernel/vmlinux.lds.S . However, they stick around in >> modules, which doesn't seem to be useful - given that everything worked >> just fine with gcc-7, and I don't see anything in the module loader that >> handles .eh_frame. >> >> The reason I care is that my target has a rather tight rootfs budget, >> and the .eh_frame section seem to occupy 10-30% of the file size >> (obviously very depending on the particular module). >> >> Comparing the .foo.o.cmd files, I don't see change in options that might >> explain this (there's a bunch of new -Wno-*, and the -mspe=no spelling >> is apparently no longer supported in gcc-8). Both before and after, there's >> >> -fno-dwarf2-cfi-asm >> >> about which gcc's documentation says >> >> '-fno-dwarf2-cfi-asm' >> Emit DWARF unwind info as compiler generated '.eh_frame' section >> instead of using GAS '.cfi_*' directives. >> >> Looking into where that comes from got me even more confused, because >> both arm and unicore32 say >> >> # Never generate .eh_frame >> KBUILD_CFLAGS += $(call cc-option,-fno-dwarf2-cfi-asm) >> >> while the ppc32 case at hand says >> >> # FIXME: the module load should be taught about the additional relocs >> # generated by this. >> # revert to pre-gcc-4.4 behaviour of .eh_frame > > Michael opened a task to look into this recently and I had spent some > time last week on this. The original commit/discussion adding > -fno-dwarf2-cfi-asm refers to R_PPC64_REL32 relocations not being > handled by our module loader: > http://lkml.kernel.org/r/20090224065112.ga6...@bombadil.infradead.org I opened that issue purely based on noticing the wart in the Makefile, not because I'd actually tested it. > However, that is now handled thanks to commit 9f751b82b491d: > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=9f751b82b491d Haha, written by me, what an idiot. So the Makefile hack can presumably be dropped, because the module loader can handle the relocations. And then maybe we also want to turn off the unwind tables, but that would be a separate patch. > I did a test build and a simple module loaded fine, so I think > -fno-dwarf2-cfi-asm is not required anymore, unless Michael has seen > some breakages with it. Michael? No, as I said above it was just reading the Makefile. cheers
Re: eh_frame confusion
On 02/03/2020 18.32, Naveen N. Rao wrote: > Naveen N. Rao wrote: >> Michael opened a task to look into this recently and I had spent some >> time last week on this. The original commit/discussion adding >> -fno-dwarf2-cfi-asm refers to R_PPC64_REL32 relocations not being >> handled by our module loader: >> http://lkml.kernel.org/r/20090224065112.ga6...@bombadil.infradead.org >> >> However, that is now handled thanks to commit 9f751b82b491d: >> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=9f751b82b491d >> >> >> I did a test build and a simple module loaded fine, so I think >> -fno-dwarf2-cfi-asm is not required anymore, unless Michael has seen >> some breakages with it. Michael? >> >>> >>> but prior to gcc-8, .eh_frame didn't seem to get generated anyway. >>> >>> Can .eh_frame sections be discarded for modules (on ppc32 at least), or >>> is there some magic that makes them necessary when building with gcc-8? >> >> As Segher points out, it looks like we need to add >> -fno-asynchronous-unwind-tables. Most other architectures seem to use >> that too. Yes. Thanks, Segher, that explains that part. > Can you check if the below patch works? I am yet to test this in more > detail, but would be good to know the implications for ppc32. I'll see if that produces a bootable kernel, but I think I'd prefer a more piecemeal approach. One patch to add -fno-asynchronous-unwind-tables (given that other arches do it unconditionally I don't think cc-option is needed), with a commit log saying something like "no-op for gcc < 8, prevents .eh_frame sections that are discarded anyway for vmlinux and waste disk space for modules". Then another patch can get rid of -fno-dwarf2-cfi-asm if that's no longer required. Rasmus
Re: eh_frame confusion
Segher Boessenkool wrote: On Mon, Mar 02, 2020 at 11:56:05AM +0100, Rasmus Villemoes wrote: I'm building a ppc32 kernel, and noticed that after upgrading from gcc-7 to gcc-8 all object files now end up having .eh_frame section. Since GCC 8, we enable -fasynchronous-unwind-tables by default for PowerPC. See https://gcc.gnu.org/r259298 . For vmlinux, that's not a problem, because they all get discarded in arch/powerpc/kernel/vmlinux.lds.S . However, they stick around in modules, which doesn't seem to be useful - given that everything worked just fine with gcc-7, and I don't see anything in the module loader that handles .eh_frame. It is useful for debugging. Not many people debug the kernel like this, of course. I'm trying to understand if we need that. Other architectures seems to pass -fasynchronous-unwind-tables only for the vdso, but disable it for the kernel build. I suppose we can do the same. If using -fno-asynchronous-unwind-tables, would crash/perf have problems? - Naveen
Re: eh_frame confusion
On Mon, Mar 02, 2020 at 11:56:05AM +0100, Rasmus Villemoes wrote: > I'm building a ppc32 kernel, and noticed that after upgrading from gcc-7 > to gcc-8 all object files now end up having .eh_frame section. Since GCC 8, we enable -fasynchronous-unwind-tables by default for PowerPC. See https://gcc.gnu.org/r259298 . > For > vmlinux, that's not a problem, because they all get discarded in > arch/powerpc/kernel/vmlinux.lds.S . However, they stick around in > modules, which doesn't seem to be useful - given that everything worked > just fine with gcc-7, and I don't see anything in the module loader that > handles .eh_frame. It is useful for debugging. Not many people debug the kernel like this, of course. Segher
