• Ard Biesheuvel's avatar
    module: use relative references for __ksymtab entries · 7290d580
    Ard Biesheuvel authored
    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.
    
    Link: http://lkml.kernel.org/r/20180704083651.24360-4-ard.biesheuvel@linaro.orgSigned-off-by: default avatarArd Biesheuvel <ard.biesheuvel@linaro.org>
    Acked-by: default avatarJessica Yu <jeyu@kernel.org>
    Acked-by: default avatarMichael Ellerman <mpe@ellerman.id.au>
    Reviewed-by: default avatarWill Deacon <will.deacon@arm.com>
    Acked-by: default avatarIngo Molnar <mingo@kernel.org>
    Cc: Arnd Bergmann <arnd@arndb.de>
    Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
    Cc: Bjorn Helgaas <bhelgaas@google.com>
    Cc: Catalin Marinas <catalin.marinas@arm.com>
    Cc: James Morris <james.morris@microsoft.com>
    Cc: James Morris <jmorris@namei.org>
    Cc: Josh Poimboeuf <jpoimboe@redhat.com>
    Cc: Kees Cook <keescook@chromium.org>
    Cc: Nicolas Pitre <nico@linaro.org>
    Cc: Paul Mackerras <paulus@samba.org>
    Cc: Petr Mladek <pmladek@suse.com>
    Cc: Russell King <linux@armlinux.org.uk>
    Cc: "Serge E. Hallyn" <serge@hallyn.com>
    Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
    Cc: Steven Rostedt <rostedt@goodmis.org>
    Cc: Thomas Garnier <thgarnie@google.com>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    7290d580
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