On Wed, 2016-08-10 at 23:30 +0200, Luis R. Rodriguez wrote: > 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" <msalter@xxxxxxxxxx> 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 ? Almost. You also need: diff --git a/include/linux/tables.h b/include/linux/tables.h index a39ab03..3fa8d4d 100644 --- a/include/linux/tables.h +++ b/include/linux/tables.h @@ -325,7 +325,7 @@ __attribute__((used, \ weak, \ __aligned__(LINUX_SECTION_ALIGNMENT(name)),\ - section(SECTION_TBL(SECTION_RODATA, \ + section(SECTION_TBL(SECTION_TBL_RO, \ name, level)))) /** Otherwise, start and end RO table markers end up in different sections. > > 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 <mcgrof@xxxxxxxxxx> > + * > + * 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.camel@xxxxxxxxxx > + */ > + > +#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)))) > > /** -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
