netdev - Re: [RFC PATCH 2/2] bpf: Initialise mod[] in bpf_trace

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Date:   Tue, 08 Aug 2017 10:46:52 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     James Hogan <james.hogan@...tec.com>,
        Alexei Starovoitov <ast@...nel.org>
CC:     linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>, netdev@...r.kernel.org
Subject: Re: [RFC PATCH 2/2] bpf: Initialise mod[] in bpf_trace_printk

On 08/08/2017 12:25 AM, James Hogan wrote:
> In bpf_trace_printk(), the elements in mod[] are left uninitialised, but
> they are then incremented to track the width of the formats. Zero
> initialise the array just in case the memory contains non-zero values on
> entry.
>
> Fixes: 9c959c863f82 ("tracing: Allow BPF programs to call bpf_trace_printk()")
> Signed-off-by: James Hogan <james.hogan@...tec.com>
> Cc: Alexei Starovoitov <ast@...nel.org>
> Cc: Daniel Borkmann <daniel@...earbox.net>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: netdev@...r.kernel.org
> ---
> When I checked (on MIPS32), the elements tended to have the value zero
> anyway (does BPF zero the stack or something clever?), so this is a
> purely theoretical fix.
> ---
>   kernel/trace/bpf_trace.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index 32dcbe1b48f2..86a52857d941 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -129,7 +129,7 @@ BPF_CALL_5(bpf_trace_printk, char *, fmt, u32, fmt_size, u64, arg1,
>   	   u64, arg2, u64, arg3)
>   {
>   	bool str_seen = false;
> -	int mod[3] = {};
> +	int mod[3] = { 0, 0, 0 };

I'm probably missing something, but is the behavior of gcc wrt
above initializers different on mips (it zeroes just fine on x86
at least)? If yes, we'd probably need a cocci script to also check
rest of the kernel given this is used in a number of places. Hm,
could you elaborate?

>   	int fmt_cnt = 0;
>   	u64 unsafe_addr;
>   	char buf[64];
>