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    Subject[PATCH 2/4] powerpc/livepatch: relax reliable stack tracer checks for first-frame
    Date
    The bottom-most stack frame (the first to be unwound) may be largely
    uninitialized, for the "Power Architecture 64-Bit ELF V2 ABI" only
    requires its backchain pointer to be set.

    The reliable stack tracer should be careful when verifying this frame:
    skip checks on STACK_FRAME_LR_SAVE and STACK_FRAME_MARKER offsets that
    may contain uninitialized residual data.

    Fixes: df78d3f61480 ("powerpc/livepatch: Implement reliable stack tracing for the consistency model")
    Signed-off-by: Joe Lawrence <joe.lawrence@redhat.com>
    ---
    arch/powerpc/kernel/stacktrace.c | 32 ++++++++++++++++++++++++--------
    1 file changed, 24 insertions(+), 8 deletions(-)

    diff --git a/arch/powerpc/kernel/stacktrace.c b/arch/powerpc/kernel/stacktrace.c
    index e2c50b55138f..06688f4d557b 100644
    --- a/arch/powerpc/kernel/stacktrace.c
    +++ b/arch/powerpc/kernel/stacktrace.c
    @@ -84,6 +84,12 @@ save_stack_trace_regs(struct pt_regs *regs, struct stack_trace *trace)
    EXPORT_SYMBOL_GPL(save_stack_trace_regs);

    #ifdef CONFIG_HAVE_RELIABLE_STACKTRACE
    +/*
    + * This function returns an error if it detects any unreliable features of the
    + * stack. Otherwise it guarantees that the stack trace is reliable.
    + *
    + * If the task is not 'current', the caller *must* ensure the task is inactive.
    + */
    int
    save_stack_trace_tsk_reliable(struct task_struct *tsk,
    struct stack_trace *trace)
    @@ -142,12 +148,6 @@ save_stack_trace_tsk_reliable(struct task_struct *tsk,
    if (sp & 0xF)
    return 1;

    - /* Mark stacktraces with exception frames as unreliable. */
    - if (sp <= stack_end - STACK_INT_FRAME_SIZE &&
    - stack[STACK_FRAME_MARKER] == STACK_FRAME_REGS_MARKER) {
    - return 1;
    - }
    -
    newsp = stack[0];
    /* Stack grows downwards; unwinder may only go up. */
    if (newsp <= sp)
    @@ -158,11 +158,26 @@ save_stack_trace_tsk_reliable(struct task_struct *tsk,
    return 1; /* invalid backlink, too far up. */
    }

    + /*
    + * We can only trust the bottom frame's backlink, the
    + * rest of the frame may be uninitialized, continue to
    + * the next.
    + */
    + if (firstframe) {
    + firstframe = 0;
    + goto next;
    + }
    +
    + /* Mark stacktraces with exception frames as unreliable. */
    + if (sp <= stack_end - STACK_INT_FRAME_SIZE &&
    + stack[STACK_FRAME_MARKER] == STACK_FRAME_REGS_MARKER) {
    + return 1;
    + }
    +
    /* Examine the saved LR: it must point into kernel code. */
    ip = stack[STACK_FRAME_LR_SAVE];
    - if (!firstframe && !__kernel_text_address(ip))
    + if (!__kernel_text_address(ip))
    return 1;
    - firstframe = 0;

    /*
    * FIXME: IMHO these tests do not belong in
    @@ -183,6 +198,7 @@ save_stack_trace_tsk_reliable(struct task_struct *tsk,
    else
    trace->skip--;

    +next:
    if (newsp == stack_end)
    break;

    --
    2.20.1
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