On Tue, Aug 09, 2016 at 11:04:07PM -0400, Mark Salter wrote: > On Tue, 2016-08-09 at 19:09 -0700, Luis R. Rodriguez wrote: > > On Aug 9, 2016 6:50 PM, "Mark Salter" <msal...@redhat.com> wrote: > > > > > > On Tue, 2016-08-09 at 20:40 +0200, Luis R. Rodriguez wrote: > > > > On Tue, Aug 09, 2016 at 01:04:00PM -0400, Mark Salter wrote: > > > > > > > > > > On Tue, 2016-08-09 at 06:37 -0700, Guenter Roeck wrote: > > > > > > > > > > > > On 08/09/2016 01:11 AM, Luis R. Rodriguez wrote: > > > > > > > > > > > > > > > > > > > > > Mark, Aurelien, > > > > > > > > > > > > > > I've run into a linker (ld) issue caused by the linker table work > > > > > > > I've > > > > > > > been working on [0]. I looked into this and for the life of me, I > > > > > > > cannot comprehend what the problem is, so was hoping you folks > > > > > > > might > > > > > > > be able to chime in. > > > > > > > > > > > > > For reference, the error is > > > > > > > > > > > > c6x-elf-ld: drivers/built-in.o: SB-relative relocation but > > > > > > __c6xabi_DSBT_BASE not defined > > > > > > c6x-elf-ld: drivers/built-in.o: SB-relative relocation but > > > > > > __c6xabi_DSBT_BASE not defined > > > > > DSBT is a reference to the no-MMU userspace ABI used by c6x. The > > > > > kernel shouldn't > > > > > be referencing DSBT base. The -mno-dsbt gcc flag should prevent it. > > > > I see -mno-dsbt on arch/c6x/Makefile already -- however at link time > > > > this is > > > > an issue if linker tables are used it seems. Do you have any other > > > > recommendation? > > > > > > > > I will note that it would seem that even i386 and x86-64 > > > > compiler/binutils seem > > > > to have relocation issues on older compiler/binutils, for instance: > > > > > > I see the problem with gcc 6 as well. > > > > > > So there appears to be some toolchain issues at play here. We build the > > > kernel with two > > > c6x-specific options: -mno-dsbt and -msdata=none. I already mentioned > > > dsbt. The sdata > > > option may be one of: > > > > > > -msdata=default > > > Put small global and static data in the .neardata section, which is > > > pointed to by > > > register B14. Put small uninitialized global and static data in the > > > .bss section, > > > which is adjacent to the .neardata section. Put small read-only data > > > into the > > > .rodata section. The corresponding sections used for large pieces of > > > data are > > > .fardata, .far and .const. > > > > > > -msdata=all > > > Put all data, not just small objects, into the sections reserved for > > > small data, > > > and use addressing relative to the B14 register to access them. > > > > > > -msdata=none > > > Make no use of the sections reserved for small data, and use absolute > > > addresses > > > to access all data. Put all initialized global and static data in the > > > .fardata > > > section, and all uninitialized data in the .far section. Put all > > > constant data > > > into the .const section. > > > > > > > > > Both small data and DSBT make use of base register + 15-bit offset to > > > access data > > > and thus the SB-relative reloc in the above error message. > > > > > > I think that gcc sees the .rodata section from DEFINE_LINKTABLE_RO() for > > > builtin_fw > > > and thinks it needs an SB-relative reloc. When the linker sees that > > > reloc, it thinks > > > it needs the dsbt base register and thus the error. Interestingly, weak > > > data is > > > never put in the small data section so if gcc sees that data is weak, it > > > doesn't > > > check the section name to see if it is a small data section. So > > > SB-relative only > > > gets used for builtin_fw__end, but not the weak builtin_fw even though > > > they both > > > are in the .rodata section. > > > > > > I suspect gcc should avoid being fooled by .rodata if -msdata=none is > > > used. > > > Regardless, I think this could all be avoided if the RO tables used .const > > > instead of .rodata for c6x. > > Thanks for the thorough analysis, would you be OK for c6x to use .const for > > all read only linker tables or section ranges ? > > I had not added #ifndef around the core-sections.h main ELF definitons but > > could add one as its needed. In this case perhals that is needed and fine > > by you > > for SECTION_RODATA. > > We can also override any of the core section setter helpers for archs but > > in this case based on what you say it seems this is needed. Unless of > > course just > > -msdata=none is fine and that's not yet used and you prefer that. > > Luis > > We're already using -msdata=none for kernel builds. From the gcc docs, one > would think > all const data goes into .const with -msdata=none, but the kernel forces a > lot of weak > const kallsyms data ,rodata so c6x vmlinux.lds still needs to have a .rodata > section. I > think we need to use .const for the c6x read-only linker tables and keep > .rodata for > RO_DATA_SECTION in vmlinux.lds.h.
OK thanks I've found a clean solution minimal solution to this as follows. This now builds fine. Is this a fine work around for now ? diff --git a/arch/c6x/include/asm/Kbuild b/arch/c6x/include/asm/Kbuild index c62f0fac6226..c54f7cc1f63e 100644 --- a/arch/c6x/include/asm/Kbuild +++ b/arch/c6x/include/asm/Kbuild @@ -64,5 +64,4 @@ generic-y += word-at-a-time.h generic-y += xor.h generic-y += section-core.h generic-y += ranges.h -generic-y += tables.h generic-y += kprobes.h diff --git a/arch/c6x/include/asm/tables.h b/arch/c6x/include/asm/tables.h new file mode 100644 index 000000000000..7a9e31575f44 --- /dev/null +++ b/arch/c6x/include/asm/tables.h @@ -0,0 +1,26 @@ +#ifndef _ASM_C6X_ASM_TABLES_H +#define _ASM_C6X_ASM_TABLES_H +/* + * Copyright (C) 2016 Luis R. Rodriguez <mcg...@kernel.org> + * + * This program is free software; you can redistribute it and/or modify it + * under the terms of copyleft-next (version 0.3.1 or later) as published + * at http://copyleft-next.org/. + */ + +/* + * The c6x toolchain has a bug present even on gcc-6 when non-weak attributes + * are used and send them to .rodata even though waek attributes are put in + * .const, this forces the linker to believe the address is relative relative + * to the a base + offset and you end up with SB-relative reloc error upon + * linking. Wor around this by by forcing the ending RO non-waek linker + * tables to be weak as well to fix this * for now. + * + * [0] https://lkml.kernel.org/r/1470798247.3551.94.ca...@redhat.com + */ + +#define SECTION_TBL_RO .const + +#include <asm-generic/tables.h> + +#endif /* _ASM_C6X_ASM_TABLES_H */ diff --git a/include/asm-generic/tables.h b/include/asm-generic/tables.h index f9c169ef06b4..50b62616075c 100644 --- a/include/asm-generic/tables.h +++ b/include/asm-generic/tables.h @@ -17,6 +17,11 @@ #define SECTION_TBL_ALL(section) \ SECTION_CORE_ALL(section,tbl) +/* Some toolchains are buggy, let them override */ +#ifndef SECTION_TBL_RO +#define SECTION_TBL_RO SECTION_RODATA +#endif + #ifndef set_section_tbl # define set_section_tbl(section, name, level, flags) \ set_section_core(section, tbl, name, level, flags) diff --git a/include/linux/tables.h b/include/linux/tables.h index 639d0144871d..a39ab03751bc 100644 --- a/include/linux/tables.h +++ b/include/linux/tables.h @@ -404,13 +404,17 @@ * @name: linker table name * @level: order level * - * Declares a linker table which only requires read-only access. + * Declares a linker table which only requires read-only access. Contrary + * to LINKTABLE_RO_WEAK() which uses SECTION_RODATA this helper uses the + * section SECTION_TBL_RO here due to possible toolchains bug on some + * architectures, for instance the c6x architicture stuffs non-weak data + * into different sections other than the one intended. */ #define LINKTABLE_RO(name, level) \ const __typeof__(VMLINUX_SYMBOL(name)[0]) \ __attribute__((used, \ __aligned__(LINUX_SECTION_ALIGNMENT(name)),\ - section(SECTION_TBL(SECTION_RODATA, \ + section(SECTION_TBL(SECTION_TBL_RO, \ name, level)))) /**
