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    Subject[PATCH 5.5 13/65] mips: vdso: fix jalr t9 crash in vdso code
    Date
    From: Victor Kamensky <kamensky@cisco.com>

    [ Upstream commit d3f703c4359ff06619b2322b91f69710453e6b6d ]

    Observed that when kernel is built with Yocto mips64-poky-linux-gcc,
    and mips64-poky-linux-gnun32-gcc toolchain, resulting vdso contains
    'jalr t9' instructions in its code and since in vdso case nobody
    sets GOT table code crashes when instruction reached. On other hand
    observed that when kernel is built mips-poky-linux-gcc toolchain, the
    same 'jalr t9' instruction are replaced with PC relative function
    calls using 'bal' instructions.

    The difference boils down to -mrelax-pic-calls and -mexplicit-relocs
    gcc options that gets different default values depending on gcc
    target triplets and corresponding binutils. -mrelax-pic-calls got
    enabled by default only in mips-poky-linux-gcc case. MIPS binutils
    ld relies on R_MIPS_JALR relocation to convert 'jalr t9' into 'bal'
    and such relocation is generated only if -mrelax-pic-calls option
    is on.

    Please note 'jalr t9' conversion to 'bal' can happen only to static
    functions. These static PIC calls use mips local GOT entries that
    are supposed to be filled with start of DSO value by run-time linker
    (missing in VDSO case) and they do not have dynamic relocations.
    Global mips GOT entries must have dynamic relocations and they should
    be prevented by cmd_vdso_check Makefile rule.

    Solution call out -mrelax-pic-calls and -mexplicit-relocs options
    explicitly while compiling MIPS vdso code. That would get correct
    and consistent between different toolchains behaviour.

    Reported-by: Bruce Ashfield <bruce.ashfield@gmail.com>
    Signed-off-by: Victor Kamensky <kamensky@cisco.com>
    Signed-off-by: Paul Burton <paulburton@kernel.org>
    Cc: linux-mips@vger.kernel.org
    Cc: Ralf Baechle <ralf@linux-mips.org>
    Cc: James Hogan <jhogan@kernel.org>
    Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
    Cc: richard.purdie@linuxfoundation.org
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    arch/mips/vdso/Makefile | 1 +
    1 file changed, 1 insertion(+)

    diff --git a/arch/mips/vdso/Makefile b/arch/mips/vdso/Makefile
    index e05938997e696..96afd73c94e8a 100644
    --- a/arch/mips/vdso/Makefile
    +++ b/arch/mips/vdso/Makefile
    @@ -29,6 +29,7 @@ endif
    cflags-vdso := $(ccflags-vdso) \
    $(filter -W%,$(filter-out -Wa$(comma)%,$(KBUILD_CFLAGS))) \
    -O3 -g -fPIC -fno-strict-aliasing -fno-common -fno-builtin -G 0 \
    + -mrelax-pic-calls -mexplicit-relocs \
    -fno-stack-protector -fno-jump-tables -DDISABLE_BRANCH_PROFILING \
    $(call cc-option, -fno-asynchronous-unwind-tables) \
    $(call cc-option, -fno-stack-protector)
    --
    2.20.1


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