On Sun, 7 Apr 2024 11:59:04 +0800
Zheng Yejian <zhengyeji...@huawei.com> wrote:
> There is once warn in __arm_kprobe_ftrace() on:
>
> ret = ftrace_set_filter_ip(ops, (unsigned long)p->addr, 0, 0);
> if (WARN_ONCE(..., "Failed to arm kprobe-ftrace at %pS (error %d)\n", ...)
> return ret;
>
> This warning is due to 'p->addr' is not a valid ftrace_location and
> that invalid 'p->addr' was bypassed in check_kprobe_address_safe():
Ah, this is a guard warning to detect changes on ftrace_set_filter() and/or
preparation steps. (A kind of debug message.)
The ftrace address check is done by check_ftrace_location() at the beginning of
check_kprobe_address_safe(). At that point, ftrace_location(addr) == addr should
return true if the module is loaded. But there is a small window that we check
the ftrace_location() and get the module refcount, even if the "addr" was ftrace
location, the module is unloaded and failed to get the module refcount and fail
to register the kprobe.
> check_kprobe_address_safe() {
> ...
> // 1. p->addr is in some module, this check passed
> is_module_text_address((unsigned long) p->addr)
> ...
> // 2. If the moudle is removed, the *probed_mod is NULL, but then
> // the return value would still be 0 !!!
> *probed_mod = __module_text_address((unsigned long) p->addr);
> ...
> }
>
> So adjust the module text check to fix it.
This seems just optimizing code (skip the 2nd module search), right?
Also some comments needs to be updated.
>
> Signed-off-by: Zheng Yejian <zhengyeji...@huawei.com>
> ---
> kernel/kprobes.c | 13 +++++++++----
> 1 file changed, 9 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> index 9d9095e81792..e0612cc3e2a3 100644
> --- a/kernel/kprobes.c
> +++ b/kernel/kprobes.c
> @@ -1567,10 +1567,16 @@ static int check_kprobe_address_safe(struct kprobe *p,
> jump_label_lock();
> preempt_disable();
>
/* Ensure the address is in a text area, and find a module if exists. */
> + *probed_mod = NULL;
> + if (!core_kernel_text((unsigned long) p->addr)) {
> + *probed_mod = __module_text_address((unsigned long) p->addr);
> + if (!(*probed_mod)) {
> + ret = -EINVAL;
> + goto out;
> + }
nit: this should get the module refcount soon after getting probed_mod.
(I think this should be an atomic operation, but we don't have such interface.)
> + }
> /* Ensure it is not in reserved area nor out of text */
/* Ensure it is not in reserved area. */
> - if (!(core_kernel_text((unsigned long) p->addr) ||
> - is_module_text_address((unsigned long) p->addr)) ||
Note that this part is doing same thing. If the probe address is !kernel_text
and !module_text, it will be rejected.
> - in_gate_area_no_mm((unsigned long) p->addr) ||
> + if (in_gate_area_no_mm((unsigned long) p->addr) ||
> within_kprobe_blacklist((unsigned long) p->addr) ||
> jump_label_text_reserved(p->addr, p->addr) ||
> static_call_text_reserved(p->addr, p->addr) ||
> @@ -1581,7 +1587,6 @@ static int check_kprobe_address_safe(struct kprobe *p,
> }
>
> /* Check if 'p' is probing a module. */
/* Get module refcount and reject __init functions for loaded modules.
*/
> - *probed_mod = __module_text_address((unsigned long) p->addr);
> if (*probed_mod) {
> /*
> * We must hold a refcount of the probed module while updating
> --
> 2.25.1
>
Thank you,
--
Masami Hiramatsu (Google) <mhira...@kernel.org>
