[PATCH v8 3/6] module: use relative references for __ksymtab entries
From: Ard Biesheuvel <hidden>
Date: 2018-03-11 14:12:23
Also in:
linuxppc-dev, lkml
On 11 March 2018 at 12:38, Ard Biesheuvel [off-list ref] wrote:
An ordinary arm64 defconfig build has ~64 KB worth of __ksymtab entries, each consisting of two 64-bit fields containing absolute references, to the symbol itself and to a char array containing its name, respectively. When we build the same configuration with KASLR enabled, we end up with an additional ~192 KB of relocations in the .init section, i.e., one 24 byte entry for each absolute reference, which all need to be processed at boot time. Given how the struct kernel_symbol that describes each entry is completely local to module.c (except for the references emitted by EXPORT_SYMBOL() itself), we can easily modify it to contain two 32-bit relative references instead. This reduces the size of the __ksymtab section by 50% for all 64-bit architectures, and gets rid of the runtime relocations entirely for architectures implementing KASLR, either via standard PIE linking (arm64) or using custom host tools (x86). Note that the binary search involving __ksymtab contents relies on each section being sorted by symbol name. This is implemented based on the input section names, not the names in the ksymtab entries, so this patch does not interfere with that. Given that the use of place-relative relocations requires support both in the toolchain and in the module loader, we cannot enable this feature for all architectures. So make it dependent on whether CONFIG_HAVE_ARCH_PREL32_RELOCATIONS is defined. Cc: Arnd Bergmann <arnd@arndb.de> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Ingo Molnar <mingo@kernel.org> Cc: Kees Cook <redacted> Cc: Thomas Garnier <redacted> Cc: Nicolas Pitre <redacted> Acked-by: Jessica Yu <jeyu@kernel.org> Signed-off-by: Ard Biesheuvel <redacted> --- arch/x86/include/asm/Kbuild | 1 + arch/x86/include/asm/export.h | 5 --- include/asm-generic/export.h | 12 ++++- include/linux/compiler.h | 19 ++++++++ include/linux/export.h | 46 +++++++++++++++----- kernel/module.c | 32 +++++++++++--- 6 files changed, 91 insertions(+), 24 deletions(-)
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quoted hunk ↗ jump to hunk
diff --git a/include/linux/compiler.h b/include/linux/compiler.h index ab4711c63601..0a9328ea9dbd 100644 --- a/include/linux/compiler.h +++ b/include/linux/compiler.h@@ -280,6 +280,25 @@ unsigned long read_word_at_a_time(const void *addr) #endif /* __KERNEL__ */ +/* + * Force the compiler to emit 'sym' as a symbol, so that we can reference + * it from inline assembler. Necessary in case 'sym' could be inlined + * otherwise, or eliminated entirely due to lack of references that are + * visible to the compiler. + */ +#define __ADDRESSABLE(sym) \ + static void * const __attribute__((section(".discard"), used)) \ + __PASTE(__addressable_##sym, __LINE__) = (void *)&sym; +
kernelci.org tells me that I need to drop the 'const' here, or we may end up with .discard sections with conflicting attributes (r/o vs r/w) in some cases (CONFIG_DEBUG_FORCE_WEAK_PER_CPU=y)