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    Subject[tip:perf/core] uprobes: Teach handle_swbp() to rely on "is_swbp" rather than uprobes_srcu
    Commit-ID:  56bb4cf6475d702d2fb00fc641aa6441097c0330
    Gitweb: http://git.kernel.org/tip/56bb4cf6475d702d2fb00fc641aa6441097c0330
    Author: Oleg Nesterov <oleg@redhat.com>
    AuthorDate: Tue, 29 May 2012 21:29:47 +0200
    Committer: Ingo Molnar <mingo@kernel.org>
    CommitDate: Wed, 6 Jun 2012 17:22:03 +0200

    uprobes: Teach handle_swbp() to rely on "is_swbp" rather than uprobes_srcu

    Currently handle_swbp() assumes that it can't race with
    unregister, so it roughly does:

    if (find_uprobe(vaddr))
    process_uprobe();
    else
    send_sig(SIGTRAP);

    This relies on the not-really-working uprobes_srcu code we are
    going to remove, see the next patch.

    With this patch we rely on the result of
    is_swbp_at_addr(bp_vaddr) if find_uprobe() fails.

    If is_swbp == 1, then we hit the normal int3, we should send
    SIGTRAP.

    If is_swbp == 0, we raced with uprobe_unregister(), we simply
    restart this insn again.

    The "difficult" case is is_swbp == -EFAULT, when we can't read
    this memory. In this case I think we should restart too, and
    this is more correct compared to the current code which sends
    SIGTRAP.

    Ignoring ENOMEM/etc from get_user_pages(), this can only happen
    if another thread unmaps this memory before find_active_uprobe()
    takes mmap_sem. It would be better to pretend it was unmapped
    before this insn was executed, restart, and get SIGSEGV.

    Signed-off-by: Oleg Nesterov <oleg@redhat.com>
    Acked-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
    Cc: Ananth N Mavinakayanahalli <ananth@in.ibm.com>
    Cc: Anton Arapov <anton@redhat.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Link: http://lkml.kernel.org/r/20120529192947.GF8057@redhat.com
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    ---
    kernel/events/uprobes.c | 18 +++++++++++++++---
    1 files changed, 15 insertions(+), 3 deletions(-)

    diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
    index a2ed82b..1f02e3b 100644
    --- a/kernel/events/uprobes.c
    +++ b/kernel/events/uprobes.c
    @@ -1530,14 +1530,26 @@ static void handle_swbp(struct pt_regs *regs)
    struct uprobe_task *utask;
    struct uprobe *uprobe;
    unsigned long bp_vaddr;
    - int is_swbp;
    + int uninitialized_var(is_swbp);

    bp_vaddr = uprobe_get_swbp_addr(regs);
    uprobe = find_active_uprobe(bp_vaddr, &is_swbp);

    if (!uprobe) {
    - /* No matching uprobe; signal SIGTRAP. */
    - send_sig(SIGTRAP, current, 0);
    + if (is_swbp > 0) {
    + /* No matching uprobe; signal SIGTRAP. */
    + send_sig(SIGTRAP, current, 0);
    + } else {
    + /*
    + * Either we raced with uprobe_unregister() or we can't
    + * access this memory. The latter is only possible if
    + * another thread plays with our ->mm. In both cases
    + * we can simply restart. If this vma was unmapped we
    + * can pretend this insn was not executed yet and get
    + * the (correct) SIGSEGV after restart.
    + */
    + instruction_pointer_set(regs, bp_vaddr);
    + }
    return;
    }


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