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From: Yonghong Song <yhs@fb.com>
To: Alan Maguire <alan.maguire@oracle.com>
Cc: <ast@kernel.org>, <daniel@iogearbox.net>, <bpf@vger.kernel.org>,
	<joe@perches.com>, <linux@rasmusvillemoes.dk>,
	<arnaldo.melo@gmail.com>, <kafai@fb.com>, <songliubraving@fb.com>,
	<andriin@fb.com>, <john.fastabend@gmail.com>,
	<kpsingh@chromium.org>, <linux-kernel@vger.kernel.org>,
	<netdev@vger.kernel.org>
Subject: Re: [PATCH v2 bpf-next 6/7] bpf: add support for %pT format specifier for bpf_trace_printk() helper
Date: Mon, 18 May 2020 07:47:56 -0700	[thread overview]
Message-ID: <91960eea-fb43-c26d-f8bb-256d37d5903f@fb.com> (raw)
In-Reply-To: <alpine.LRH.2.21.2005181000520.893@localhost>



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
> 

  reply	other threads:[~2020-05-18 14:48 UTC|newest]

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-12  5:56 [PATCH v2 bpf-next 0/7] bpf, printk: add BTF-based type printing Alan Maguire
2020-05-12  5:56 ` [PATCH v2 bpf-next 1/7] bpf: provide function to get vmlinux BTF information Alan Maguire
2020-05-12  5:56 ` [PATCH v2 bpf-next 2/7] bpf: move to generic BTF show support, apply it to seq files/strings Alan Maguire
2020-05-13 23:04   ` Yonghong Song
2020-05-18  9:46     ` Alan Maguire
2020-05-19  6:21       ` Yonghong Song
2020-05-12  5:56 ` [PATCH v2 bpf-next 3/7] checkpatch: add new BTF pointer format specifier Alan Maguire
2020-05-12  5:56 ` [PATCH v2 bpf-next 4/7] printk: add type-printing %pT format specifier which uses BTF Alan Maguire
2020-05-13 23:05   ` Joe Perches
2020-05-13 23:07     ` Alexei Starovoitov
2020-05-13 23:22       ` Joe Perches
2020-05-14 23:43         ` Joe Perches
2020-05-15  0:09           ` Alexei Starovoitov
2020-05-15  0:21             ` Joe Perches
2020-05-14  0:45   ` Yonghong Song
2020-05-14 22:37     ` Alan Maguire
2020-05-15  0:39       ` Yonghong Song
2020-05-12  5:56 ` [PATCH v2 bpf-next 5/7] printk: extend test_printf to test %pT BTF-based format specifier Alan Maguire
2020-05-12  5:56 ` [PATCH v2 bpf-next 6/7] bpf: add support for %pT format specifier for bpf_trace_printk() helper Alan Maguire
2020-05-14  0:53   ` Yonghong Song
2020-05-18  9:10     ` Alan Maguire
2020-05-18 14:47       ` Yonghong Song [this message]
2020-05-12  5:56 ` [PATCH v2 bpf-next 7/7] bpf: add tests for %pT format specifier Alan Maguire
2020-05-15  0:21   ` Andrii Nakryiko
2020-05-13 22:24 ` [PATCH v2 bpf-next 0/7] bpf, printk: add BTF-based type printing Alexei Starovoitov
2020-05-13 22:48   ` Joe Perches
2020-05-13 22:50     ` Alexei Starovoitov
2020-05-13 23:23       ` Joe Perches
2020-05-14 17:46   ` Alan Maguire
2020-05-15 18:59   ` Yonghong Song

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