linux-kernel - Re: [PATCH] bcachefs: Avoid a potential useless over memory allocation in bch2_prt

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Date:   Tue, 19 Sep 2023 20:18:48 +0200
From:   Christophe JAILLET <christophe.jaillet@...adoo.fr>
To:     Brian Foster <bfoster@...hat.com>
Cc:     Kent Overstreet <kent.overstreet@...ux.dev>,
        linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org,
        linux-bcachefs@...r.kernel.org
Subject: Re: [PATCH] bcachefs: Avoid a potential useless over memory
 allocation in bch2_prt_vprintf()

Le 19/09/2023 à 15:17, Brian Foster a écrit :
> On Sat, Sep 16, 2023 at 08:20:24AM +0200, Christophe JAILLET wrote:
>> printbuf_remaining() returns the number of characters we can print to the
>> output buffer - i.e. excluding the terminating null.
>>
>> vsnprintf() takes the size of the buffer, including the trailing null
>> space.
>> It is truncated if the returned value is greater than or equal to the size
>> of the buffer.
>>
>> Knowing all that, buffer sizes and overflow checks can be fixed in order
>> to potentially avoid a useless memory over-allocation.
>>
> 
> For whatever reason I had a hard time parsing this last sentence.  Do
> you mean to say there's an off by one here that leads to an unnecessary
> overallocation?

An off-by-two in fact, IIUC.
But yes, that's my point.

We consider that the string is truncated when it may not be (len+1 vs 
len) and we under-estimate the available space (printbuf_remaining() vs 
printbuf_remaining()+1 or printbuf_remaining_size())

> 
>> Signed-off-by: Christophe JAILLET <christophe.jaillet@...adoo.fr>
>> ---
>> Un-tested
>> ---
>>   fs/bcachefs/printbuf.c | 10 ++++++----
>>   1 file changed, 6 insertions(+), 4 deletions(-)
>>
>> diff --git a/fs/bcachefs/printbuf.c b/fs/bcachefs/printbuf.c
>> index de41f9a14492..77bee9060bfe 100644
>> --- a/fs/bcachefs/printbuf.c
>> +++ b/fs/bcachefs/printbuf.c
>> @@ -54,8 +54,9 @@ void bch2_prt_vprintf(struct printbuf *out, const char *fmt, va_list args)
>>   		va_list args2;
>>   
>>   		va_copy(args2, args);
>> -		len = vsnprintf(out->buf + out->pos, printbuf_remaining(out), fmt, args2);
>> -	} while (len + 1 >= printbuf_remaining(out) &&
>> +		len = vsnprintf(out->buf + out->pos, printbuf_remaining(out) + 1,
>> +				fmt, args2);
>> +	} while (len >= printbuf_remaining(out) + 1 &&
>>   		 !bch2_printbuf_make_room(out, len + 1));
> 
> It's amazing how simple arithmetic can make my eyes cross at times. :) I
> think I follow the fix after reading the commit log a couple times, but
> could we use printbuf_remaining_size() appropriately in these places
> that want to check actual buffer size (i.e. including terminating null)
> instead of doing the manual size fixup?

Sure, it would be much better.
I had not seen this function.

CJ

> 
> Brian
> 
>>   
>>   	len = min_t(size_t, len,
>> @@ -70,9 +71,10 @@ void bch2_prt_printf(struct printbuf *out, const char *fmt, ...)
>>   
>>   	do {
>>   		va_start(args, fmt);
>> -		len = vsnprintf(out->buf + out->pos, printbuf_remaining(out), fmt, args);
>> +		len = vsnprintf(out->buf + out->pos, printbuf_remaining(out) + 1,
>> +				fmt, args);
>>   		va_end(args);
>> -	} while (len + 1 >= printbuf_remaining(out) &&
>> +	} while (len >= printbuf_remaining(out) + 1 &&
>>   		 !bch2_printbuf_make_room(out, len + 1));
>>   
>>   	len = min_t(size_t, len,
>> -- 
>> 2.34.1
>>
> 
>