linux-kernel - Re: [PATCH] uprobe: Warn if unable to install breakpoint

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Message-ID: <87r2vr57jq.fsf@concordia.ellerman.id.au>
Date:   Fri, 01 Sep 2017 15:09:29 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
        Ingo Molnar <mingo@...nel.org>
Cc:     Anton Blanchard <anton@...ba.org>, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, Oleg Nesterov <oleg@...hat.com>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Subject: Re: [PATCH] uprobe: Warn if unable to install breakpoint

"Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com> writes:

> When we try to install a uprobe breakpoint in uprobe_mmap(), we ignore
> all errors encountered in the process per this comment at the top of
> the function:
>     /*
>      * Called from mmap_region/vma_adjust with mm->mmap_sem acquired.
>      *
>      * Currently we ignore all errors and always return 0, the callers
>      * can't handle the failure anyway.
>      */
>
> However, this is very confusing for users since no probe hits are
> recorded nor is an error logged in dmesg.
>
> Fix this by logging an error in dmesg so that users can discover that
> there was an issue with the uprobe. To facilitate use of uprobe_warn(),
> we move that function to the top of the file.
>
> With this patch, we see a message similar to this in dmesg:
>     [  201.449213] uprobe: uprobe_t:9740 failed to setup probe at 0x95c (-524)
>
> Reported-by: Anton Blanchard <anton@...ba.org>
> Signed-off-by: Naveen N. Rao <naveen.n.rao@...ux.vnet.ibm.com>
> ---
>  kernel/events/uprobes.c | 21 ++++++++++++++-------
>  1 file changed, 14 insertions(+), 7 deletions(-)
>
> diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> index 0e137f98a50c..587c591a535c 100644
> --- a/kernel/events/uprobes.c
> +++ b/kernel/events/uprobes.c
> @@ -112,6 +112,12 @@ struct xol_area {
>  	unsigned long 			vaddr;		/* Page(s) of instruction slots */
>  };
>  
> +static void uprobe_warn(struct task_struct *t, const char *msg)
> +{
> +	pr_warn("uprobe: %s:%d failed to %s\n",
> +			current->comm, current->pid, msg);

That should probably be ratelimited no?

cheers