linux-kernel - Re: [PATCH v2] bcachefs: Avoid a potential useless over memory allocation in bch2_prt

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Date: Mon, 19 Feb 2024 19:10:10 +0100
From: Christophe JAILLET <christophe.jaillet@...adoo.fr>
To: Kent Overstreet <kent.overstreet@...ux.dev>
Cc: Brian Foster <bfoster@...hat.com>, linux-kernel@...r.kernel.org,
 kernel-janitors@...r.kernel.org, linux-bcachefs@...r.kernel.org
Subject: Re: [PATCH v2] bcachefs: Avoid a potential useless over memory
 allocation in bch2_prt_[v]printf()

Le 19/02/2024 à 04:39, Kent Overstreet a écrit :
> On Sun, Feb 18, 2024 at 05:12:28PM +0100, Christophe JAILLET wrote:
>> 2 issues related to incorrect available space in the output buffer
>> computation may lead to some extra memory allocation.
>>
>>
>> First: vsnprintf() takes the size of the buffer, *including* the space for
>> the trailing null.
>>
>> But printbuf_remaining() returns the number of characters we can print
>> to the output buffer, *excluding* the terminating null.
>>
>> So, use printbuf_remaining_size(), which includes the space for the
>> terminating null.
> 
> nope, buggy.
> 
> passing printbuf_remaining_size() to snprintf() is correct, but
> snprintf() returns the number of charecters wrote _excluding_ the null

Hi,

I think that the patch is correct.

snprintf() returns the number of characters wrote _excluding_ the null. 
That is why the test is: len >= size_of_the_buffer   ==>  more space is 
needed.
The case you describe is handled by the == part of >=.

On the contrary, if len is < size_of_the_buffer, then we have at least 1 
place for the terminating NULL. It will then eventually lead to:
   len_of_the_string + space_for_'\0' ==  size_of_the_buffer
So it does not overflow.


Anyway, feel free to ignore the patch completely if it sounds too risky, 
take only half of it (s/printbuf_remaining/printbuf_remaining_size/) if 
you are more confident with it, or the complete patch if the explanation 
above convinced you.

 From my PoV, the 3 options lead to the same run-time output, with more 
or less memory allocated.

Best regards.

CJ


> 
>>
>>
>> Second: if the return value of vsnprintf() is greater than or equal to the
>> passed size, the resulting string is truncated.
>> So, in order to see if some extra space is needed, the check needs to be
>> changed.
>>
>> Signed-off-by: Christophe JAILLET <christophe.jaillet@...adoo.fr>
>> ---
>> Un-tested
>>
>> v2: - Use printbuf_remaining_size() instead of hand-writing it.  [Brian Foster]
>>      - Reword the commit log, hoping it is clearer.
>>      - Synch with -next-20240215
>>
>> v1: https://lore.kernel.org/all/0f40108bed3e084057223bdbe32c4b37f8500ff3.1694845203.git.christophe.jaillet@wanadoo.fr/
>> ---
>>   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 b27d22925929..8cee9c2f88c4 100644
>> --- a/fs/bcachefs/printbuf.c
>> +++ b/fs/bcachefs/printbuf.c
>> @@ -55,9 +55,10 @@ 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);
>> +		len = vsnprintf(out->buf + out->pos, printbuf_remaining_size(out),
>> +				fmt, args2);
>>   		va_end(args2);
>> -	} while (len + 1 >= printbuf_remaining(out) &&
>> +	} while (len >= printbuf_remaining_size(out) &&
>>   		 !bch2_printbuf_make_room(out, len + 1));
>>   
>>   	len = min_t(size_t, len,
>> @@ -72,9 +73,10 @@ 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_size(out),
>> +				fmt, args);
>>   		va_end(args);
>> -	} while (len + 1 >= printbuf_remaining(out) &&
>> +	} while (len >= printbuf_remaining_size(out) &&
>>   		 !bch2_printbuf_make_room(out, len + 1));
>>   
>>   	len = min_t(size_t, len,
>> -- 
>> 2.43.2
>>
> 
>