Commit 71810db2 authored by Ard Biesheuvel's avatar Ard Biesheuvel Committed by Linus Torvalds
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modversions: treat symbol CRCs as 32 bit quantities 



The modversion symbol CRCs are emitted as ELF symbols, which allows us
to easily populate the kcrctab sections by relying on the linker to
associate each kcrctab slot with the correct value.

This has a couple of downsides:

 - Given that the CRCs are treated as memory addresses, we waste 4 bytes
   for each CRC on 64 bit architectures,

 - On architectures that support runtime relocation, a R_<arch>_RELATIVE
   relocation entry is emitted for each CRC value, which identifies it
   as a quantity that requires fixing up based on the actual runtime
   load offset of the kernel. This results in corrupted CRCs unless we
   explicitly undo the fixup (and this is currently being handled in the
   core module code)

 - Such runtime relocation entries take up 24 bytes of __init space
   each, resulting in a x8 overhead in [uncompressed] kernel size for
   CRCs.

Switching to explicit 32 bit values on 64 bit architectures fixes most
of these issues, given that 32 bit values are not treated as quantities
that require fixing up based on the actual runtime load offset.  Note
that on some ELF64 architectures [such as PPC64], these 32-bit values
are still emitted as [absolute] runtime relocatable quantities, even if
the value resolves to a build time constant.  Since relative relocations
are always resolved at build time, this patch enables MODULE_REL_CRCS on
powerpc when CONFIG_RELOCATABLE=y, which turns the absolute CRC
references into relative references into .rodata where the actual CRC
value is stored.

So redefine all CRC fields and variables as u32, and redefine the
__CRC_SYMBOL() macro for 64 bit builds to emit the CRC reference using
inline assembler (which is necessary since 64-bit C code cannot use
32-bit types to hold memory addresses, even if they are ultimately
resolved using values that do not exceed 0xffffffff).  To avoid
potential problems with legacy 32-bit architectures using legacy
toolchains, the equivalent C definition of the kcrctab entry is retained
for 32-bit architectures.

Note that this mostly reverts commit d4703aef ("module: handle ppc64
relocating kcrctabs when CONFIG_RELOCATABLE=y")

Acked-by: default avatarRusty Russell <rusty@rustcorp.com.au>
Signed-off-by: default avatarArd Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 56067812

arch/powerpc/Kconfig

+1 −0
Original line number Diff line number Diff line
@@ -484,6 +484,7 @@ config RELOCATABLE 
	bool "Build a relocatable kernel"

	depends on (PPC64 && !COMPILE_TEST) || (FLATMEM && (44x || FSL_BOOKE))

	select NONSTATIC_KERNEL

	select MODULE_REL_CRCS if MODVERSIONS

	help

	  This builds a kernel image that is capable of running at the

	  location the kernel is loaded at. For ppc32, there is no any

arch/powerpc/include/asm/module.h

+0 −4
Original line number Diff line number Diff line
@@ -90,9 +90,5 @@ static inline int module_finalize_ftrace(struct module *mod, const Elf_Shdr *sec 
}

#endif



#if defined(CONFIG_MODVERSIONS) && defined(CONFIG_PPC64)

#define ARCH_RELOCATES_KCRCTAB

#define reloc_start PHYSICAL_START

#endif

#endif /* __KERNEL__ */

#endif	/* _ASM_POWERPC_MODULE_H */

arch/powerpc/kernel/module_64.c

+0 −8
Original line number Diff line number Diff line
@@ -286,14 +286,6 @@ static void dedotify_versions(struct modversion_info *vers, 
	for (end = (void *)vers + size; vers < end; vers++)

		if (vers->name[0] == '.') {

			memmove(vers->name, vers->name+1, strlen(vers->name));

#ifdef ARCH_RELOCATES_KCRCTAB

			/* The TOC symbol has no CRC computed. To avoid CRC

			 * check failing, we must force it to the expected

			 * value (see CRC check in module.c).

			 */

			if (!strcmp(vers->name, "TOC."))

				vers->crc = -(unsigned long)reloc_start;

#endif

		}

}

include/asm-generic/export.h

+6 −5
Original line number Diff line number Diff line
@@ -9,18 +9,15 @@ 
#ifndef KSYM_ALIGN

#define KSYM_ALIGN 8

#endif

#ifndef KCRC_ALIGN

#define KCRC_ALIGN 8

#endif

#else

#define __put .long

#ifndef KSYM_ALIGN

#define KSYM_ALIGN 4

#endif

#endif

#ifndef KCRC_ALIGN

#define KCRC_ALIGN 4

#endif

#endif



#ifdef CONFIG_HAVE_UNDERSCORE_SYMBOL_PREFIX

#define KSYM(name) _##name
@@ -52,7 +49,11 @@ KSYM(__kstrtab_\name): 
	.section ___kcrctab\sec+\name,"a"

	.balign KCRC_ALIGN

KSYM(__kcrctab_\name):

	__put KSYM(__crc_\name)

#if defined(CONFIG_MODULE_REL_CRCS)

	.long KSYM(__crc_\name) - .

#else

	.long KSYM(__crc_\name)

#endif

	.weak KSYM(__crc_\name)

	.previous

#endif

include/linux/export.h

+14 −0
Original line number Diff line number Diff line
@@ -43,6 +43,13 @@ extern struct module __this_module; 
#ifdef CONFIG_MODVERSIONS

/* Mark the CRC weak since genksyms apparently decides not to

 * generate a checksums for some symbols */

#if defined(CONFIG_MODULE_REL_CRCS)

#define __CRC_SYMBOL(sym, sec)						\

	asm("	.section \"___kcrctab" sec "+" #sym "\", \"a\"	\n"	\

	    "	.weak	" VMLINUX_SYMBOL_STR(__crc_##sym) "	\n"	\

	    "	.long	" VMLINUX_SYMBOL_STR(__crc_##sym) " - .	\n"	\

	    "	.previous					\n");

#elif !defined(CONFIG_64BIT)

#define __CRC_SYMBOL(sym, sec)						\

	extern __visible void *__crc_##sym __attribute__((weak));	\

	static const unsigned long __kcrctab_##sym			\
@@ -50,6 +57,13 @@ extern struct module __this_module; 
	__attribute__((section("___kcrctab" sec "+" #sym), used))	\

	= (unsigned long) &__crc_##sym;

#else

#define __CRC_SYMBOL(sym, sec)						\

	asm("	.section \"___kcrctab" sec "+" #sym "\", \"a\"	\n"	\

	    "	.weak	" VMLINUX_SYMBOL_STR(__crc_##sym) "	\n"	\

	    "	.long	" VMLINUX_SYMBOL_STR(__crc_##sym) "	\n"	\

	    "	.previous					\n");

#endif

#else

#define __CRC_SYMBOL(sym, sec)

#endif

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