Re: [PATCH v3 2/2] util: Remove possible quadratic behaviour from write_remainder()

[Stefano Brivio <sbrivio@redhat.com>]

On Thu, 19 Sep 2024 12:42:58 +0200
Markus Armbruster <armbru@redhat.com> wrote:

> David Gibson <david@gibson.dropbear.id.au> writes:
> 
> > write_remainder() steps through the buffers in an IO vector writing out
> > everything past a certain byte offset.  However, on each iteration it
> > rescans the buffer from the beginning to find out where we're up to.  With
> > an unfortunate set of write sizes this could lead to quadratic behaviour.
> >
> > In an even less likely set of circumstances (total vector length > maximum
> > size_t) the 'skip' variable could overflow.  This is one factor in a
> > longstanding Coverity error we've seen (although I still can't figure out
> > the remainder of its complaint).
> >
> > Rework write_remainder() to always work out our new position in the vector
> > relative to our old/current position, rather than starting from the
> > beginning each time.  As a bonus this seems to fix the Coverity error.
> >
> > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> > ---
> >  util.c | 27 +++++++++++++++++----------
> >  1 file changed, 17 insertions(+), 10 deletions(-)
> >
> > diff --git a/util.c b/util.c
> > index 7db7c2e7..87309c51 100644
> > --- a/util.c
> > +++ b/util.c
> > @@ -597,10 +597,15 @@ int write_all_buf(int fd, const void *buf, size_t len)
> >  	size_t left = len;
> >  
> >  	while (left) {
> > -		ssize_t rc = write(fd, p, left);
> > +		ssize_t rc;
> > +
> > +		do
> > +			rc = write(fd, p, left);
> > +		while ((rc < 0) && errno == EINTR);
> >  
> >  		if (rc < 0)
> >  			return -1;
> > +
> >  		p += rc;
> >  		left -= rc;
> >  	}  
> 
> Uh, shouldn't this be squashed into PATCH 1?

Oh, oops, in theory yes, but while reviewing this I just had a look at
the result in util.c, and now it's merged. It's not a big issue I'd
say. Thanks for reporting anyway.

-- 
Stefano