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    Subject[tip:perf/core] uprobes: Change handle_trampoline() to flush the frames invalidated by longjmp()
    Commit-ID:  5eeb50de42fd3251845d03c556db012267c72b3f
    Gitweb: http://git.kernel.org/tip/5eeb50de42fd3251845d03c556db012267c72b3f
    Author: Oleg Nesterov <oleg@redhat.com>
    AuthorDate: Tue, 21 Jul 2015 15:40:21 +0200
    Committer: Ingo Molnar <mingo@kernel.org>
    CommitDate: Fri, 31 Jul 2015 10:38:05 +0200

    uprobes: Change handle_trampoline() to flush the frames invalidated by longjmp()

    Test-case:

    #include <stdio.h>
    #include <setjmp.h>

    jmp_buf jmp;

    void func_2(void)
    {
    longjmp(jmp, 1);
    }

    void func_1(void)
    {
    if (setjmp(jmp))
    return;
    func_2();
    printf("ERR!! I am running on the caller's stack\n");
    }

    int main(void)
    {
    func_1();
    return 0;
    }

    fails if you probe func_1() and func_2() because
    handle_trampoline() assumes that the probed function should must
    return and hit the bp installed be prepare_uretprobe(). But in
    this case func_2() does not return, so when func_1() returns the
    kernel uses the no longer valid return_instance of func_2().

    Change handle_trampoline() to unwind ->return_instances until we
    know that the next chain is alive or NULL, this ensures that the
    current chain is the last we need to report and free.

    Alternatively, every return_instance could use unique
    trampoline_vaddr, in this case we could use it as a key. And
    this could solve the problem with sigaltstack() automatically.

    But this approach needs more changes, and it puts the "hard"
    limit on MAX_URETPROBE_DEPTH. Plus it can not solve another
    problem partially fixed by the next patch.

    Note: this change has no effect on !x86, the arch-agnostic
    version of arch_uretprobe_is_alive() just returns "true".

    TODO: as documented by the previous change, arch_uretprobe_is_alive()
    can be fooled by sigaltstack/etc.

    Tested-by: Pratyush Anand <panand@redhat.com>
    Signed-off-by: Oleg Nesterov <oleg@redhat.com>
    Acked-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
    Acked-by: Anton Arapov <arapov@gmail.com>
    Cc: Andy Lutomirski <luto@amacapital.net>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Link: http://lkml.kernel.org/r/20150721134021.GA4773@redhat.com
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    ---
    kernel/events/uprobes.c | 29 ++++++++++++++++++-----------
    1 file changed, 18 insertions(+), 11 deletions(-)

    diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
    index c5f316e..93d939c8 100644
    --- a/kernel/events/uprobes.c
    +++ b/kernel/events/uprobes.c
    @@ -1774,6 +1774,7 @@ static void handle_trampoline(struct pt_regs *regs)
    {
    struct uprobe_task *utask;
    struct return_instance *ri, *next;
    + bool valid;

    utask = current->utask;
    if (!utask)
    @@ -1783,18 +1784,24 @@ static void handle_trampoline(struct pt_regs *regs)
    if (!ri)
    goto sigill;

    - next = find_next_ret_chain(ri);
    - /*
    - * TODO: we should throw out return_instance's invalidated by
    - * longjmp(), currently we assume that the probed function always
    - * returns.
    - */
    - instruction_pointer_set(regs, ri->orig_ret_vaddr);
    do {
    - handle_uretprobe_chain(ri, regs);
    - ri = free_ret_instance(ri);
    - utask->depth--;
    - } while (ri != next);
    + /*
    + * We should throw out the frames invalidated by longjmp().
    + * If this chain is valid, then the next one should be alive
    + * or NULL; the latter case means that nobody but ri->func
    + * could hit this trampoline on return. TODO: sigaltstack().
    + */
    + next = find_next_ret_chain(ri);
    + valid = !next || arch_uretprobe_is_alive(next, regs);
    +
    + instruction_pointer_set(regs, ri->orig_ret_vaddr);
    + do {
    + if (valid)
    + handle_uretprobe_chain(ri, regs);
    + ri = free_ret_instance(ri);
    + utask->depth--;
    + } while (ri != next);
    + } while (!valid);

    utask->return_instances = ri;
    return;

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