Re: [RFC PATCH 2/2] bpf: Initialise mod[] in bpf_trace_printk

[James Hogan]

On 8 August 2017 17:48:57 BST, David Miller <davem@xxxxxxxxxxxxx> wrote:
>From: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
>Date: Tue, 08 Aug 2017 10:46:52 +0200
>
>> 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@xxxxxxxxxx>
>>> Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
>>> Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
>>> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
>>> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
>>> Cc: netdev@xxxxxxxxxxxxxxx
>>> ---
>>> 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?
>
>This change is not necessary at all.
>
>An empty initializer must clear the whole object to zero.
>
>"theoretical" fix indeed... :-(

cool, i hadn't realised unmentioned elements in an initialiser are always zeroed, even when non-global/static, so had interpreted the whole array as uninitialised. learn something new every day :-) sorry for the noise.

cheers
James