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

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Message-ID: <csjcyhnh5lhj22emlwdzpr6v4s3mfldsw3um75hsk4ffjsmqmb@otyei3qylhdn>
Date: Mon, 19 Feb 2024 16:36:09 -0500
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: Christophe JAILLET <christophe.jaillet@...adoo.fr>
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()

On Mon, Feb 19, 2024 at 07:10:10PM +0100, Christophe JAILLET wrote:
> 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.

You didn't test it, I did.

In the future, no more patches that you haven't tested - thanks.

> 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
> > > 
> > 
> > 
>