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Date: Fri, 20 Sep 2024 01:36:16 +1000
From: David Gibson <david@gibson.dropbear.id.au>
To: Markus Armbruster <armbru@redhat.com>
Subject: Re: [PATCH v3 2/2] util: Remove possible quadratic behaviour from
write_remainder()
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On Thu, Sep 19, 2024 at 12:42:58PM +0200, Markus Armbruster wrote:
> David Gibson <david@gibson.dropbear.id.au> writes:
>=20
> > 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. W=
ith
> > an unfortunate set of write sizes this could lead to quadratic behaviou=
r.
> >
> > In an even less likely set of circumstances (total vector length > maxi=
mum
> > 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 o=
ut
> > the remainder of its complaint).
> >
> > Rework write_remainder() to always work out our new position in the vec=
tor
> > 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 =3D len;
> > =20
> > while (left) {
> > - ssize_t rc =3D write(fd, p, left);
> > + ssize_t rc;
> > +
> > + do
> > + rc =3D write(fd, p, left);
> > + while ((rc < 0) && errno =3D=3D EINTR);
> > =20
> > if (rc < 0)
> > return -1;
> > +
> > p +=3D rc;
> > left -=3D rc;
> > }
>=20
> Uh, shouldn't this be squashed into PATCH 1?
Bother, yes it should. But, it's merged now, so never mind, I guess.
--=20
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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