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    Subject[PATCH 3/4] x86/unwind: include __schedule() in stack traces
    Date
    In the following commit:

    0100301bfdf5 ("sched/x86: Rewrite the switch_to() code")

    ... the layout of the 'inactive_task_frame' struct was designed to have
    a frame pointer header embedded in it, so that the unwinder could use
    the 'bp' and 'ret_addr' fields to report __schedule() on the stack (or
    ret_from_fork() for newly forked tasks which haven't actually run yet).

    Finish the job by changing get_frame_pointer() to return a pointer to
    inactive_task_frame's 'bp' field rather than 'bp' itself. This allows
    the unwinder to start one frame higher on the stack, so that it properly
    reports __schedule().

    Reported-by: Miroslav Benes <mbenes@suse.cz>
    Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
    ---
    arch/x86/include/asm/stacktrace.h | 5 +----
    arch/x86/include/asm/switch_to.h | 10 +++++++++-
    2 files changed, 10 insertions(+), 5 deletions(-)

    diff --git a/arch/x86/include/asm/stacktrace.h b/arch/x86/include/asm/stacktrace.h
    index 20ce3db..2e41c50 100644
    --- a/arch/x86/include/asm/stacktrace.h
    +++ b/arch/x86/include/asm/stacktrace.h
    @@ -52,16 +52,13 @@ static inline bool on_stack(struct stack_info *info, void *addr, size_t len)
    static inline unsigned long *
    get_frame_pointer(struct task_struct *task, struct pt_regs *regs)
    {
    - struct inactive_task_frame *frame;
    -
    if (regs)
    return (unsigned long *)regs->bp;

    if (task == current)
    return __builtin_frame_address(0);

    - frame = (struct inactive_task_frame *)task->thread.sp;
    - return (unsigned long *)READ_ONCE_NOCHECK(frame->bp);
    + return &((struct inactive_task_frame *)task->thread.sp)->bp;
    }
    #else
    static inline unsigned long *
    diff --git a/arch/x86/include/asm/switch_to.h b/arch/x86/include/asm/switch_to.h
    index 5cb436a..fcc5cd3 100644
    --- a/arch/x86/include/asm/switch_to.h
    +++ b/arch/x86/include/asm/switch_to.h
    @@ -36,7 +36,10 @@ static inline void prepare_switch_to(struct task_struct *prev,

    asmlinkage void ret_from_fork(void);

    -/* data that is pointed to by thread.sp */
    +/*
    + * This is the structure pointed to by thread.sp for an inactive task. The
    + * order of the fields must match the code in __switch_to_asm().
    + */
    struct inactive_task_frame {
    #ifdef CONFIG_X86_64
    unsigned long r15;
    @@ -48,6 +51,11 @@ struct inactive_task_frame {
    unsigned long di;
    #endif
    unsigned long bx;
    +
    + /*
    + * These two fields must be together. They form a stack frame header,
    + * needed by get_frame_pointer().
    + */
    unsigned long bp;
    unsigned long ret_addr;
    };
    --
    2.7.4
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