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    Subject[PATCH AUTOSEL for 4.15 087/102] powerpc/modules: Don't try to restore r2 after a sibling call
    Date
    From: Josh Poimboeuf <jpoimboe@redhat.com>

    [ Upstream commit b9eab08d012fa093947b230f9a87257c27fb829b ]

    When attempting to load a livepatch module, I got the following error:

    module_64: patch_module: Expect noop after relocate, got 3c820000

    The error was triggered by the following code in
    unregister_netdevice_queue():

    14c: 00 00 00 48 b 14c <unregister_netdevice_queue+0x14c>
    14c: R_PPC64_REL24 net_set_todo
    150: 00 00 82 3c addis r4,r2,0

    GCC didn't insert a nop after the branch to net_set_todo() because it's
    a sibling call, so it never returns. The nop isn't needed after the
    branch in that case.

    Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
    Acked-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
    Reviewed-and-tested-by: Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>
    Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
    Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
    ---
    arch/powerpc/include/asm/code-patching.h | 1 +
    arch/powerpc/kernel/module_64.c | 12 +++++++++++-
    arch/powerpc/lib/code-patching.c | 5 +++++
    3 files changed, 17 insertions(+), 1 deletion(-)

    diff --git a/arch/powerpc/include/asm/code-patching.h b/arch/powerpc/include/asm/code-patching.h
    index abef812de7f8..2c895e8d07f7 100644
    --- a/arch/powerpc/include/asm/code-patching.h
    +++ b/arch/powerpc/include/asm/code-patching.h
    @@ -33,6 +33,7 @@ int patch_branch(unsigned int *addr, unsigned long target, int flags);
    int patch_instruction(unsigned int *addr, unsigned int instr);

    int instr_is_relative_branch(unsigned int instr);
    +int instr_is_relative_link_branch(unsigned int instr);
    int instr_is_branch_to_addr(const unsigned int *instr, unsigned long addr);
    unsigned long branch_target(const unsigned int *instr);
    unsigned int translate_branch(const unsigned int *dest,
    diff --git a/arch/powerpc/kernel/module_64.c b/arch/powerpc/kernel/module_64.c
    index 759104b99f9f..180c16f04063 100644
    --- a/arch/powerpc/kernel/module_64.c
    +++ b/arch/powerpc/kernel/module_64.c
    @@ -487,7 +487,17 @@ static bool is_early_mcount_callsite(u32 *instruction)
    restore r2. */
    static int restore_r2(u32 *instruction, struct module *me)
    {
    - if (is_early_mcount_callsite(instruction - 1))
    + u32 *prev_insn = instruction - 1;
    +
    + if (is_early_mcount_callsite(prev_insn))
    + return 1;
    +
    + /*
    + * Make sure the branch isn't a sibling call. Sibling calls aren't
    + * "link" branches and they don't return, so they don't need the r2
    + * restore afterwards.
    + */
    + if (!instr_is_relative_link_branch(*prev_insn))
    return 1;

    if (*instruction != PPC_INST_NOP) {
    diff --git a/arch/powerpc/lib/code-patching.c b/arch/powerpc/lib/code-patching.c
    index d469224c4ada..096d4e4d31e6 100644
    --- a/arch/powerpc/lib/code-patching.c
    +++ b/arch/powerpc/lib/code-patching.c
    @@ -302,6 +302,11 @@ int instr_is_relative_branch(unsigned int instr)
    return instr_is_branch_iform(instr) || instr_is_branch_bform(instr);
    }

    +int instr_is_relative_link_branch(unsigned int instr)
    +{
    + return instr_is_relative_branch(instr) && (instr & BRANCH_SET_LINK);
    +}
    +
    static unsigned long branch_iform_target(const unsigned int *instr)
    {
    signed long imm;
    --
    2.14.1
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