Re: [PATCH v2 2/2] util: Remove possible quadratic behaviour from write_remainder()

[David Gibson <david@gibson.dropbear.id.au>]

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On Wed, Sep 18, 2024 at 08:20:25AM +0200, Markus Armbruster 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 | 20 +++++++++++---------
> >  1 file changed, 11 insertions(+), 9 deletions(-)
> >
> > diff --git a/util.c b/util.c
> > index 7db7c2e7..43e5f5ec 100644
> > --- a/util.c
> > +++ b/util.c
> > @@ -615,28 +615,30 @@ int write_all_buf(int fd, const void *buf, size_t len)
> >   *
> >   * Return: 0 on success, -1 on error (with errno set)
> >   *
> > - * #syscalls write writev
> > + * #syscalls writev
> 
> I'm not familiar with this annotation.  I guess it's system calls used
> in directly this function, not including the ones used in callees.

Basically, yes.  They're just slurped up by a script to generate our
seccomp() filter.

> 
> >   */
> >  int write_remainder(int fd, const struct iovec *iov, size_t iovcnt, size_t skip)
> >  {
> > -	size_t offset, i;
> > +	size_t i = 0, offset;
> >  
> > -	while ((i = iov_skip_bytes(iov, iovcnt, skip, &offset)) < iovcnt) {
> > +	while ((i += iov_skip_bytes(iov + i, iovcnt - i, skip, &offset)) < iovcnt) {
> >  		ssize_t rc;
> >  
> >  		if (offset) {
> > -			rc = write(fd, (char *)iov[i].iov_base + offset,
> > -				   iov[i].iov_len - offset);
> > -		} else {
> > -			rc = writev(fd, &iov[i], iovcnt - i);
> > +			/* Write the remainder of the partially written buffer */
> > +			if (write_all_buf(fd, (char *)iov[i].iov_base + offset,
> > +					  iov[i].iov_len - offset) < 0)
> > +				return -1;
> > +			i++;
> >  		}
> >  
> > +		/* Write as much of the remaining whole buffers as we can */
> > +		rc = writev(fd, &iov[i], iovcnt - i);
> 
> Last argument can be zero now.  Okay.

Yes.

> >  		if (rc < 0)
> >  			return -1;
> >  
> > -		skip += rc;
> > +		skip = rc;
> >  	}
> > -
> >  	return 0;
> >  }
> 
> We could use try to save system calls by only using writev(), with
> a suitably adjusted copy of @iov.  But non-zero @offset should be rare,
> so I guess it's not worth the bother.

Actually, they're not rare - we nearly always pass a non-zero initial
skip value.  But write_remainder() itself is somewhat rare, and making
a copy of the iov is kind of structurally awkward in passt (we don't
allocate).

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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