Date: Fri, 4 Mar 2016 23:32:31 +0100 From: Jilles Tjoelker <jilles@stack.nl> To: "Pedro F. Giffuni" <pfg@FreeBSD.org> Cc: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r296386 - head/lib/libc/rpc Message-ID: <20160304223231.GA47504@stack.nl> In-Reply-To: <201603041530.u24FUfu5021609@repo.freebsd.org> References: <201603041530.u24FUfu5021609@repo.freebsd.org>
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On Fri, Mar 04, 2016 at 03:30:41PM +0000, Pedro F. Giffuni wrote: > Author: pfg > Date: Fri Mar 4 15:30:41 2016 > New Revision: 296386 > URL: https://svnweb.freebsd.org/changeset/base/296386 > Log: > Work around aliasing issues detected in modern GCC. > Avoid casting gymnastics that lead to pointer aliasing by introducing an > inline function as done in NetBSD (but without #if0'd WIP code). > Obtained from: NetBSD (CVS Rev. 1.24, 1.25) > Modified: > head/lib/libc/rpc/clnt_vc.c > Modified: head/lib/libc/rpc/clnt_vc.c > ============================================================================== > --- head/lib/libc/rpc/clnt_vc.c Fri Mar 4 14:23:34 2016 (r296385) > +++ head/lib/libc/rpc/clnt_vc.c Fri Mar 4 15:30:41 2016 (r296386) > @@ -502,6 +502,20 @@ clnt_vc_abort(CLIENT *cl) > { > } > > +static __inline void > +htonlp(void *dst, const void *src, uint32_t incr) > +{ > + /* We are aligned, so we think */ > + *(uint32_t *)dst = htonl(*(const uint32_t *)src + incr); > +} > + > +static __inline void > +ntohlp(void *dst, const void *src) > +{ > + /* We are aligned, so we think */ > + *(uint32_t *)dst = htonl(*(const uint32_t *)src); > +} > + > static bool_t > clnt_vc_control(CLIENT *cl, u_int request, void *info) > { > @@ -576,14 +590,12 @@ clnt_vc_control(CLIENT *cl, u_int reques > * first element in the call structure > * This will get the xid of the PREVIOUS call > */ > - *(u_int32_t *)info = > - ntohl(*(u_int32_t *)(void *)&ct->ct_u.ct_mcalli); > + ntohlp(info, &ct->ct_u.ct_mcalli); > break; > case CLSET_XID: > /* This will set the xid of the NEXT call */ > - *(u_int32_t *)(void *)&ct->ct_u.ct_mcalli = > - htonl(*((u_int32_t *)info) + 1); > /* increment by 1 as clnt_vc_call() decrements once */ > + htonlp(&ct->ct_u.ct_mcalli, info, 1); > break; > case CLGET_VERS: > /* > @@ -592,15 +604,11 @@ clnt_vc_control(CLIENT *cl, u_int reques > * begining of the RPC header. MUST be changed if the > * call_struct is changed > */ > - *(u_int32_t *)info = > - ntohl(*(u_int32_t *)(void *)(ct->ct_u.ct_mcallc + > - 4 * BYTES_PER_XDR_UNIT)); > + ntohlp(info, ct->ct_u.ct_mcallc + 4 * BYTES_PER_XDR_UNIT); > break; > > case CLSET_VERS: > - *(u_int32_t *)(void *)(ct->ct_u.ct_mcallc + > - 4 * BYTES_PER_XDR_UNIT) = > - htonl(*(u_int32_t *)info); > + htonlp(ct->ct_u.ct_mcallc + 4 * BYTES_PER_XDR_UNIT, info, 0); > break; > > case CLGET_PROG: > @@ -610,15 +618,11 @@ clnt_vc_control(CLIENT *cl, u_int reques > * begining of the RPC header. MUST be changed if the > * call_struct is changed > */ > - *(u_int32_t *)info = > - ntohl(*(u_int32_t *)(void *)(ct->ct_u.ct_mcallc + > - 3 * BYTES_PER_XDR_UNIT)); > + ntohlp(info, ct->ct_u.ct_mcallc + 3 * BYTES_PER_XDR_UNIT); > break; > > case CLSET_PROG: > - *(u_int32_t *)(void *)(ct->ct_u.ct_mcallc + > - 3 * BYTES_PER_XDR_UNIT) = > - htonl(*(u_int32_t *)info); > + htonlp(ct->ct_u.ct_mcallc + 3 * BYTES_PER_XDR_UNIT, info, 0); > break; > > default: This change just deceives GCC's warning logic without changing whether there is an aliasing violation or not (I don't think there is any when clnt_vc.c is compiled separately, since *ct does not have an effective type in that case). To avoid casts, ct_mcalli could be changed to an array (of MCALL_MSG_SIZE / 4 elements) and used. This also avoids depending on the unspecified value of the elements 4 to 23 of ct_mcallc after storing to ct_mcalli. Note that C89 did not permit type punning via a union. NetBSD might care but we do not. C89 compilers are unlikely to be cunning enough for this to be a problem anyway. It may be that this code is too old for this kind of change. -- Jilles Tjoelker
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