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    Subject[PATCH AUTOSEL 4.19 08/26] x86/build: Treat R_386_PLT32 relocation as R_386_PC32
    Date
    From: Fangrui Song <maskray@google.com>

    [ Upstream commit bb73d07148c405c293e576b40af37737faf23a6a ]

    This is similar to commit

    b21ebf2fb4cd ("x86: Treat R_X86_64_PLT32 as R_X86_64_PC32")

    but for i386. As far as the kernel is concerned, R_386_PLT32 can be
    treated the same as R_386_PC32.

    R_386_PLT32/R_X86_64_PLT32 are PC-relative relocation types which
    can only be used by branches. If the referenced symbol is defined
    externally, a PLT will be used.

    R_386_PC32/R_X86_64_PC32 are PC-relative relocation types which can be
    used by address taking operations and branches. If the referenced symbol
    is defined externally, a copy relocation/canonical PLT entry will be
    created in the executable.

    On x86-64, there is no PIC vs non-PIC PLT distinction and an
    R_X86_64_PLT32 relocation is produced for both `call/jmp foo` and
    `call/jmp foo@PLT` with newer (2018) GNU as/LLVM integrated assembler.
    This avoids canonical PLT entries (st_shndx=0, st_value!=0).

    On i386, there are 2 types of PLTs, PIC and non-PIC. Currently,
    the GCC/GNU as convention is to use R_386_PC32 for non-PIC PLT and
    R_386_PLT32 for PIC PLT. Copy relocations/canonical PLT entries
    are possible ABI issues but GCC/GNU as will likely keep the status
    quo because (1) the ABI is legacy (2) the change will drop a GNU
    ld diagnostic for non-default visibility ifunc in shared objects.

    clang-12 -fno-pic (since [1]) can emit R_386_PLT32 for compiler
    generated function declarations, because preventing canonical PLT
    entries is weighed over the rare ifunc diagnostic.

    Further info for the more interested:

    https://github.com/ClangBuiltLinux/linux/issues/1210
    https://sourceware.org/bugzilla/show_bug.cgi?id=27169
    https://github.com/llvm/llvm-project/commit/a084c0388e2a59b9556f2de0083333232da3f1d6 [1]

    [ bp: Massage commit message. ]

    Reported-by: Arnd Bergmann <arnd@arndb.de>
    Signed-off-by: Fangrui Song <maskray@google.com>
    Signed-off-by: Borislav Petkov <bp@suse.de>
    Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
    Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
    Tested-by: Nick Desaulniers <ndesaulniers@google.com>
    Tested-by: Nathan Chancellor <natechancellor@gmail.com>
    Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
    Link: https://lkml.kernel.org/r/20210127205600.1227437-1-maskray@google.com
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    arch/x86/kernel/module.c | 1 +
    arch/x86/tools/relocs.c | 12 ++++++++----
    2 files changed, 9 insertions(+), 4 deletions(-)

    diff --git a/arch/x86/kernel/module.c b/arch/x86/kernel/module.c
    index 6645f123419c6..9f0be2c7e3466 100644
    --- a/arch/x86/kernel/module.c
    +++ b/arch/x86/kernel/module.c
    @@ -126,6 +126,7 @@ int apply_relocate(Elf32_Shdr *sechdrs,
    *location += sym->st_value;
    break;
    case R_386_PC32:
    + case R_386_PLT32:
    /* Add the value, subtract its position */
    *location += sym->st_value - (uint32_t)location;
    break;
    diff --git a/arch/x86/tools/relocs.c b/arch/x86/tools/relocs.c
    index 3a6c8ebc8032e..aa046d46ff8ff 100644
    --- a/arch/x86/tools/relocs.c
    +++ b/arch/x86/tools/relocs.c
    @@ -841,9 +841,11 @@ static int do_reloc32(struct section *sec, Elf_Rel *rel, Elf_Sym *sym,
    case R_386_PC32:
    case R_386_PC16:
    case R_386_PC8:
    + case R_386_PLT32:
    /*
    - * NONE can be ignored and PC relative relocations don't
    - * need to be adjusted.
    + * NONE can be ignored and PC relative relocations don't need
    + * to be adjusted. Because sym must be defined, R_386_PLT32 can
    + * be treated the same way as R_386_PC32.
    */
    break;

    @@ -884,9 +886,11 @@ static int do_reloc_real(struct section *sec, Elf_Rel *rel, Elf_Sym *sym,
    case R_386_PC32:
    case R_386_PC16:
    case R_386_PC8:
    + case R_386_PLT32:
    /*
    - * NONE can be ignored and PC relative relocations don't
    - * need to be adjusted.
    + * NONE can be ignored and PC relative relocations don't need
    + * to be adjusted. Because sym must be defined, R_386_PLT32 can
    + * be treated the same way as R_386_PC32.
    */
    break;

    --
    2.27.0
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