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Re: [PATCH v3 05/11] esp: introduce esp_fifo_pop_buf() and use it instea

From: Philippe Mathieu-Daudé
Subject: Re: [PATCH v3 05/11] esp: introduce esp_fifo_pop_buf() and use it instead of fifo8_pop_buf()
Date: Thu, 1 Apr 2021 20:05:41 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.0

On 4/1/21 12:51 PM, Mark Cave-Ayland wrote:
> On 01/04/2021 10:34, Philippe Mathieu-Daudé wrote:
>> On 4/1/21 9:49 AM, Mark Cave-Ayland wrote:
>>> The const pointer returned by fifo8_pop_buf() lies directly within
>>> the array used
>>> to model the FIFO. Building with address sanitisers enabled shows
>>> that if the
>> Typo "sanitizers"
> Ha. It's definitely "sanitiser" here in the UK (UK English) as opposed
> to "sanitizer" (US English).

Oh OK, TIL :)

> I don't really mind either way, but I can
> fix this if it needs a v4 following Paolo's comments.

Not needed since it is correct.

>>> caller expects a minimum number of bytes present then if the FIFO is
>>> nearly full,
>>> the caller may unexpectedly access past the end of the array.
>> Why isn't it a problem with the other models? Because the pointed
>> buffer is consumed directly?
> Yes that's correct, which is why Fifo8 currently doesn't support
> wraparound. I haven't analysed how other devices have used it but I
> would imagine there would be an ASan hit if it were being misused this way.
>>> Introduce esp_fifo_pop_buf() which takes a destination buffer and
>>> performs a
>>> memcpy() in it to guarantee that the caller cannot overwrite the FIFO
>>> array and
>>> update all callers to use it. Similarly add underflow protection
>>> similar to
>>> esp_fifo_push() and esp_fifo_pop() so that instead of triggering an
>>> assert()
>>> the operation becomes a no-op.
>> This is OK for your ESP model.
>> Now thinking loudly about the Fifo8 API, shouldn't this be part of it?
>> Something prototype like:
>>    /**
>>     * fifo8_pop_buf:
>>     * @do_copy: If %true, also copy data to @bufptr.
>>     */
>>    size_t fifo8_pop_buf(Fifo8 *fifo,
>>                         void **bufptr,
>>                         size_t buflen,
>>                         bool do_copy);
> That could work, and may even allow support for wraparound in future. I
> suspect things would become clearer after looking at the other Fifo8
> users to see if this is worth an API change/alternative API.

Thanks for the feedback!


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