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

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Date:   Mon, 14 Aug 2017 12:44:35 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Daniel Borkmann' <daniel@...earbox.net>,
        James Hogan <james.hogan@...tec.com>,
        David Miller <davem@...emloft.net>
CC:     "ast@...nel.org" <ast@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: [RFC PATCH 2/2] bpf: Initialise mod[] in bpf_trace_printk

From: Daniel Borkmann
> Sent: 11 August 2017 17:47
> On 08/09/2017 10:34 PM, Daniel Borkmann wrote:
> > On 08/09/2017 09:39 AM, James Hogan wrote:
> > [...]
> >> time (but please consider looking at the other patch which is certainly
> >> a more real issue).
> >
> > Sorry for the delay, started looking into that and whether we
> > have some other options, I'll get back to you on this.
> 
> Could we solve this more generically (as in: originally intended) in
> the sense that we don't need to trust the gcc va_list handling; I feel
> this is relying on an implementation detail? Perhaps something like
> below poc patch?

That patch still has 'cond ? arg : cond1 ? (long)arg : (u32)arg' so
probably has the same warning as the original version.

The va_list handling is defined by the relevant ABI, not gcc.

It is ok on x86-64 because all 32bit values are extended to 64bits
before being passed as arguments (either in registers, or on the stack).
Nothing in the C language requires that, so other 64bit architectures
could pass 32bit values in 4 bytes of stack.

	David