On 03/04/16 17:32, Jilles Tjoelker wrote:
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).
Yes, I am trying to keep some changes in sync with NetBSD. They have
some #ifdef 0'd code with memsets but the code is already ugly to
worry about code that apparently is not ready yet.
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.
Hmm.. so C89 is probably the reason the linux guys are also doing the
same, or uglier, workarounds. :(.
It may be that this code is too old for this kind of change.
I am just trying to put the forks in somewhat more consistent shape,
and finding surprises in the way. I will probably put the more
controversial changes in phabricator over the weekend.
Pedro.
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