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

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Message-ID: <91960eea-fb43-c26d-f8bb-256d37d5903f@fb.com>
Date:   Mon, 18 May 2020 07:47:56 -0700
From:   Yonghong Song <yhs@...com>
To:     Alan Maguire <alan.maguire@...cle.com>
CC:     <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 5/18/20 2:10 AM, Alan Maguire wrote:
> 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).

percpu buffer definitely better. In fact, I am using percpu buffer
in bpf_seq_printf() helper. Yes, you will need to handling contention
though. I guess we can do the same thing here, return -EBUSY so bpf
program can react properly (retry, or just print error, etc.)
if there is a contention.

> 
> Great news about LLVM support for __builtin_btf_type_id()!

Thanks. Hopefully this will make implementation easier.

> 
> Thanks!
> 
> Alan
>