Re: c6x linker issue on linux-next-20160808 + some linker table work

["Luis R. Rodriguez" <mcgrof@kernel.org>]

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@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 <mcgrof@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.camel@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))))
 
 /**