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    Subject[RFC PATCH 0/4] tracing/probes: uaccess: Add support user-space access
    Date
    Hi,

    Here is an RFC series of probe-event to support user-space access
    methods, which we discussed in previous thread.

    https://lkml.kernel.org/r/20190220171019.5e81a4946b56982f324f7c45@kernel.org

    So in this thread, it is clear that current probe_kernel_read()
    and strncpy_from_unsafe() are not enough to access user-space
    variables from kprobe events on some arch. On such arch, user
    address space and kernel address space can overlap so we have
    to change the memory segment to user-mode before copying.
    But probe_kernel_read() is designed to access primarily kernel
    memory, it may fail to get, or get unexpected value on such
    arch. So we need to expand kprobe fetcharg to support new options
    for such case.

    For user-space access extension, this series adds 2 features,
    "ustring" type and user-space dereference syntax. "ustring" is
    used for recording a null-terminated string in user-space from
    kprobe events.

    "ustring" type is easy, it is able to use instead of "string"
    type, so if you want to record a user-space string via
    "__user char *", you can use ustring type instead of string.
    For example,

    echo 'p do_sys_open path=+0($arg2):ustring' >> kprobe_events

    will record the path string from user-space.

    The user-space dereference syntax is also simple. Thi just
    adds 'u' prefix before an offset value.

    +|-u<OFFSET>(<FETCHARG>)

    e.g. +u8(%ax), +u0(+0(%si))

    This is more generic. If you want to refer the variable in user-
    space from its address or access a field in data structure in
    user-space, you need to use this.

    For example, if you probe do_sched_setscheduler(pid, policy,
    param) and record param->sched_priority, you can add new
    probe as below;

    p do_sched_setscheduler priority=+u0($arg3)

    Actually, with this feature, "ustring" type is not absolutely
    necessary, because these are same meanings.

    +0($arg2):ustring == +u0($arg2):string

    Perhups, we may be better removing "ustring" and just introducing
    this user-space dereference syntax...

    Note that kprobe event provides these methods, but it doesn't
    change it from kernel to user automatically because we do not
    know whether the given address is in userspace or kernel on
    some arch.
    Moreover, from perf-probe, at this moment it is not able to
    switch. Since __user is not for compiler but checker, we have
    no clue which data structure is in user-space, in debuginfo.

    BTW, according to Linus's comment, I implemented probe_user_read()
    and strncpy_from_unsafe_user() APIs. And since those use
    "access_ok()" inside it, if CONFIG_DEBUG_ATOMIC_SLEEP=y on x86,
    it will get a warn message at once. It should be solved before
    merging this series.

    Thank you,

    ---

    Masami Hiramatsu (4):
    uaccess: Make sure kernel_uaccess_faults_ok is updated before pagefault
    uaccess: Add non-pagefault user-space read functions
    tracing/probe: Add ustring type for user-space string
    tracing/probe: Support user-space dereference


    Documentation/trace/kprobetrace.rst | 13 ++-
    Documentation/trace/uprobetracer.rst | 9 +-
    fs/namespace.c | 2
    include/linux/uaccess.h | 13 +++
    kernel/trace/trace.c | 7 +-
    kernel/trace/trace_kprobe.c | 65 ++++++++++++++++
    kernel/trace/trace_probe.c | 39 ++++++++--
    kernel/trace/trace_probe.h | 3 +
    kernel/trace/trace_probe_tmpl.h | 36 +++++++--
    kernel/trace/trace_uprobe.c | 19 +++++
    mm/maccess.c | 138 ++++++++++++++++++++++++++++++----
    11 files changed, 302 insertions(+), 42 deletions(-)

    --
    Masami Hiramatsu (Linaro) <mhiramat@kernel.org>

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