Messages in this thread | | | Date | Mon, 8 Jan 2018 16:11:40 +0100 | From | Stefano Brivio <> | Subject | Re: [PATCH v2] net: ipv4: emulate READ_ONCE() on ->hdrincl bit-field in raw_sendmsg() |
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On Mon, 8 Jan 2018 15:54:44 +0100 Nicolai Stange <nstange@suse.de> wrote:
> Commit 8f659a03a0ba ("net: ipv4: fix for a race condition in > raw_sendmsg") fixed the issue of possibly inconsistent ->hdrincl handling > due to concurrent updates by reading this bit-field member into a local > variable and using the thus stabilized value in subsequent tests. > > However, aforementioned commit also adds the (correct) comment that > > /* hdrincl should be READ_ONCE(inet->hdrincl) > * but READ_ONCE() doesn't work with bit fields > */ > > because as it stands, the compiler is free to shortcut or even eliminate > the local variable at its will. > > Note that I have not seen anything like this happening in reality and thus, > the concern is a theoretical one. > > However, in order to be on the safe side, emulate a READ_ONCE() on the > bit-field by doing it on the local 'hdrincl' variable itself: > > int hdrincl = inet->hdrincl; > hdrincl = READ_ONCE(hdrincl); > > This breaks the chain in the sense that the compiler is not allowed > to replace subsequent reads from hdrincl with reloads from inet->hdrincl. > > Fixes: 8f659a03a0ba ("net: ipv4: fix for a race condition in raw_sendmsg") > Signed-off-by: Nicolai Stange <nstange@suse.de>
Reviewed-by: Stefano Brivio <sbrivio@redhat.com>
-- Stefano
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