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    SubjectRe: PATCH] debug: add notifier chain debugging
    On Mon, 25 Aug 2008 22:08:00 -0700
    "Tony Luck" <tony.luck@intel.com> wrote:

    > >> This breaks on ia64 (and pa-risc I think) where function pointers
    > >> don't point directly at the code, they point to a {code,data}
    > >> structure which is itself located in data space, not text space.
    > >
    > > is there a way to go to the actual address? I'm sure this is a bit
    > > more common.... (like kallsyms!)
    >
    > See dereference_function_descriptor() in lib/vsprintf.c (where it will
    > be clear that my memory was wrong and that PPC64 is the other
    > architecture that needs this).
    >

    The patch below fixes this;
    Ingo, please replace the patch with this one.


    From: Arjan van de Ven <arjan@linux.intel.com>
    Subject: [PATCH] debug: add notifier chain debugging

    during some development we suspected a case where we left something
    in a notifier chain that was from a module that was unloaded already...
    and that sort of thing is rather hard to track down.

    This patch adds a very simple sanity check (which isn't all that
    expensive) to make sure the notifier we're about to call is
    actually from either the kernel itself of from a still-loaded
    module, avoiding a hard-to-chase-down crash.

    Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
    Acked-by: Tony Luck <tony.luck@intel.com>
    ---
    include/linux/kernel.h | 3 +++
    kernel/extable.c | 16 ++++++++++++++++
    kernel/notifier.c | 6 ++++++
    lib/vsprintf.c | 2 +-
    4 files changed, 26 insertions(+), 1 deletions(-)

    diff --git a/include/linux/kernel.h b/include/linux/kernel.h
    index 2651f80..4e1366b 100644
    --- a/include/linux/kernel.h
    +++ b/include/linux/kernel.h
    @@ -187,6 +187,9 @@ extern unsigned long long memparse(char *ptr, char **retptr);
    extern int core_kernel_text(unsigned long addr);
    extern int __kernel_text_address(unsigned long addr);
    extern int kernel_text_address(unsigned long addr);
    +extern int func_ptr_is_kernel_text(void *ptr);
    +extern void *dereference_function_descriptor(void *ptr);
    +
    struct pid;
    extern struct pid *session_of_pgrp(struct pid *pgrp);

    diff --git a/kernel/extable.c b/kernel/extable.c
    index a26cb2e..adf0cc9 100644
    --- a/kernel/extable.c
    +++ b/kernel/extable.c
    @@ -66,3 +66,19 @@ int kernel_text_address(unsigned long addr)
    return 1;
    return module_text_address(addr) != NULL;
    }
    +
    +/*
    + * On some architectures (PPC64, IA64) function pointers
    + * are actually only tokens to some data that then holds the
    + * real function address. As a result, to find if a function
    + * pointer is part of the kernel text, we need to do some
    + * special dereferencing first.
    + */
    +int func_ptr_is_kernel_text(void *ptr)
    +{
    + unsigned long addr;
    + addr = (unsigned long) dereference_function_descriptor(ptr);
    + if (core_kernel_text(addr))
    + return 1;
    + return module_text_address(addr) != NULL;
    +}
    diff --git a/kernel/notifier.c b/kernel/notifier.c
    index 823be11..522277c 100644
    --- a/kernel/notifier.c
    +++ b/kernel/notifier.c
    @@ -82,6 +82,12 @@ static int __kprobes notifier_call_chain(struct notifier_block **nl,

    while (nb && nr_to_call) {
    next_nb = rcu_dereference(nb->next);
    + if (!func_ptr_is_kernel_text(nb->notifier_call)) {
    + WARN(1, "Invalid notifier called!");
    + nb = next_nb;
    + continue;
    + }
    +
    ret = nb->notifier_call(nb, val, v);

    if (nr_calls)
    diff --git a/lib/vsprintf.c b/lib/vsprintf.c
    index d8d1d11..f5e5ffb 100644
    --- a/lib/vsprintf.c
    +++ b/lib/vsprintf.c
    @@ -513,7 +513,7 @@ static char *string(char *buf, char *end, char *s, int field_width, int precisio
    return buf;
    }

    -static inline void *dereference_function_descriptor(void *ptr)
    +void *dereference_function_descriptor(void *ptr)
    {
    #if defined(CONFIG_IA64) || defined(CONFIG_PPC64)
    void *p;
    --
    1.5.5.1


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