linux-kernel - Re: [PATCH v2 bpf-next 6/7] bpf: add support for %pT format specifier for bpf_trace

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Date:   Mon, 18 May 2020 10:10:44 +0100 (BST)
From:   Alan Maguire <alan.maguire@...cle.com>
To:     Yonghong Song <yhs@...com>
cc:     Alan Maguire <alan.maguire@...cle.com>, ast@...nel.org,
        daniel@...earbox.net, bpf@...r.kernel.org, joe@...ches.com,
        linux@...musvillemoes.dk, arnaldo.melo@...il.com, kafai@...com,
        songliubraving@...com, andriin@...com, john.fastabend@...il.com,
        kpsingh@...omium.org, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org
Subject: Re: [PATCH v2 bpf-next 6/7] bpf: add support for %pT format specifier
 for bpf_trace_printk() helper

On Wed, 13 May 2020, Yonghong Song wrote:

> 
> > +				while (isbtffmt(fmt[i]))
> > +					i++;
> 
> The pointer passed to the helper may not be valid pointer. I think you
> need to do a probe_read_kernel() here. Do an atomic memory allocation
> here should be okay as this is mostly for debugging only.
> 

Are there other examples of doing allocations in program execution
context? I'd hate to be the first to introduce one if not. I was hoping
I could get away with some per-CPU scratch space. Most data structures
will fit within a small per-CPU buffer, but if multiple copies
are required, performance isn't the key concern. It will make traversing
the buffer during display a bit more complex but I think avoiding 
allocation might make that complexity worth it. The other thought I had 
was we could carry out an allocation associated with the attach, 
but that's messy as it's possible run-time might determine the type for
display (and thus the amount of the buffer we need to copy safely).

Great news about LLVM support for __builtin_btf_type_id()!

Thanks!

Alan