Re: [musl] [PATCH 1/1] IN6_IS_ADDR_LOOPBACK() and similar macros warn on -Wcast-qual

[enh <enh@google.com>]

not quite the question you asked, but the new implementation is what
bionic has shipped since 2016, and had the historical castful
implementation before that. (citation needed?
https://android-review.googlesource.com/c/platform/bionic/+/250098)

On Wed, Aug 14, 2024 at 5:16 PM Rich Felker <dalias@libc.org> wrote:
>
> On Fri, Aug 02, 2024 at 08:02:16PM -0400, Brad House wrote:
> > On 8/2/24 7:38 PM, Rich Felker wrote:
> >
> > >On Fri, Aug 02, 2024 at 05:27:26PM -0400, Brad House wrote:
> > >>I'm the maintainer of c-ares (https://c-ares.org) and have been
> > >>scanning the CI build logs for various systems to catch warnings,
> > >>and on Alpine Linux (which obviously uses musl c) we get these
> > >>warnings, specifically when using clang (but not oddly not on gcc):
> > >>
> > >>/__w/c-ares/c-ares/src/lib/ares__sortaddrinfo.c:93:9: warning: cast
> > >>from 'const struct in6_addr *' to 'unsigned char *' drops const
> > >>qualifier [-Wcast-qual]
> > >>    93 |     if (IN6_IS_ADDR_MULTICAST(&addr6->sin6_addr)) {
> > >>       |         ^
> > >>/usr/include/netinet/in.h:120:48: note: expanded from macro
> > >>'IN6_IS_ADDR_MULTICAST'
> > >>   120 | #define IN6_IS_ADDR_MULTICAST(a) (((uint8_t *) (a))[0] == 0xff)
> > >>       |                                                ^
> > >>...       ^
> > >>
> > >>Full build output: https://github.com/c-ares/c-ares/actions/runs/10219723015/job/28278549865
> > >>
> > >>I've attached a patch that will silence this warning by always
> > >>casting to the comparison to const, but otherwise not impact the
> > >>behavior.
> > >>
> > >>-Brad
> > >>diff --git a/include/netinet/in.h b/include/netinet/in.h
> > >>index fb628b61..f04657f3 100644
> > >>--- a/include/netinet/in.h
> > >>+++ b/include/netinet/in.h
> > >>@@ -108,46 +108,63 @@ uint16_t ntohs(uint16_t);
> > >>  #define IPPROTO_MAX      263
> > >>...
> > >>  #define IN6_IS_ADDR_LOOPBACK(a) \
> > >>-        (((uint32_t *) (a))[0] == 0 && ((uint32_t *) (a))[1] == 0 && \
> > >>-         ((uint32_t *) (a))[2] == 0 && \
> > >>-         ((uint8_t *) (a))[12] == 0 && ((uint8_t *) (a))[13] == 0 && \
> > >>-         ((uint8_t *) (a))[14] == 0 && ((uint8_t *) (a))[15] == 1 )
> > >>+        (((const uint32_t *) (a))[0] == 0 && \
> > >>+         ((const uint32_t *) (a))[1] == 0 && \
> > >>+         ((const uint32_t *) (a))[2] == 0 && \
> > >>+         ((const uint8_t *) (a))[12] == 0 && \
> > >>+         ((const uint8_t *) (a))[13] == 0 && \
> > >>+         ((const uint8_t *) (a))[14] == 0 && \
> > >>+         ((const uint8_t *) (a))[15] == 1 )
> > >>...
> > >It looks like there's a lot wrong with these macros. They should not
> > >be doing random pointer casts like they are. Per the standard, they
> > >take an argument of type const struct in6_addr *, so they should
> > >almost surely be operating on that type directly. That would make them
> > >actually type-safe (diagnostic if called with wrong argument type).
> > >
> > >I guess we should look at whether there's any good reason they were
> > >written the way they were..
> > >
> > >Rich
> >
> > Yep, I see what you mean.  There are already accessors for 8, 16,
> > and 32bit into struct in6_addr so its odd not to use those.  I've
> > attached a v2 patch that uses those instead which also cleans up the
> > warnings.
> >
> > -Brad
>
> > diff --git a/include/netinet/in.h b/include/netinet/in.h
> > index fb628b61..c6afeed8 100644
> > --- a/include/netinet/in.h
> > +++ b/include/netinet/in.h
> > @@ -108,51 +108,68 @@ uint16_t ntohs(uint16_t);
> >  #define IPPROTO_MAX      263
> >
> >  #define IN6_IS_ADDR_UNSPECIFIED(a) \
> > -        (((uint32_t *) (a))[0] == 0 && ((uint32_t *) (a))[1] == 0 && \
> > -         ((uint32_t *) (a))[2] == 0 && ((uint32_t *) (a))[3] == 0)
> > +        (((a)->s6_addr32)[0] == 0 && \
> > +         ((a)->s6_addr32)[1] == 0 && \
> > +         ((a)->s6_addr32)[2] == 0 && \
> > +         ((a)->s6_addr32)[3] == 0)
> >
> >  #define IN6_IS_ADDR_LOOPBACK(a) \
> > -        (((uint32_t *) (a))[0] == 0 && ((uint32_t *) (a))[1] == 0 && \
> > -         ((uint32_t *) (a))[2] == 0 && \
> > -         ((uint8_t *) (a))[12] == 0 && ((uint8_t *) (a))[13] == 0 && \
> > -         ((uint8_t *) (a))[14] == 0 && ((uint8_t *) (a))[15] == 1 )
> > +        (((a)->s6_addr32)[0] == 0 && \
> > +         ((a)->s6_addr32)[1] == 0 && \
> > +         ((a)->s6_addr32)[2] == 0 && \
> > +         ((a)->s6_addr)[12] == 0 && \
> > +         ((a)->s6_addr)[13] == 0 && \
> > +         ((a)->s6_addr)[14] == 0 && \
> > +         ((a)->s6_addr)[15] == 1 )
> >
> > -#define IN6_IS_ADDR_MULTICAST(a) (((uint8_t *) (a))[0] == 0xff)
> > +#define IN6_IS_ADDR_MULTICAST(a) (((a)->s6_addr)[0] == 0xff)
> >
> >  #define IN6_IS_ADDR_LINKLOCAL(a) \
> > -        ((((uint8_t *) (a))[0]) == 0xfe && (((uint8_t *) (a))[1] & 0xc0) == 0x80)
> > +        ((((a)->s6_addr)[0]) == 0xfe && \
> > +         (((a)->s6_addr)[1] & 0xc0) == 0x80)
> >
> >  #define IN6_IS_ADDR_SITELOCAL(a) \
> > -        ((((uint8_t *) (a))[0]) == 0xfe && (((uint8_t *) (a))[1] & 0xc0) == 0xc0)
> > +        ((((a)->s6_addr)[0]) == 0xfe && \
> > +         (((a)->s6_addr)[1] & 0xc0) == 0xc0)
> >
> >  #define IN6_IS_ADDR_V4MAPPED(a) \
> > -        (((uint32_t *) (a))[0] == 0 && ((uint32_t *) (a))[1] == 0 && \
> > -         ((uint8_t *) (a))[8] == 0 && ((uint8_t *) (a))[9] == 0 && \
> > -         ((uint8_t *) (a))[10] == 0xff && ((uint8_t *) (a))[11] == 0xff)
> > +        (((a)->s6_addr32)[0] == 0 && \
> > +         ((a)->s6_addr32)[1] == 0 && \
> > +         ((a)->s6_addr)[8] == 0 && \
> > +         ((a)->s6_addr)[9] == 0 && \
> > +         ((a)->s6_addr)[10] == 0xff && \
> > +         ((a)->s6_addr)[11] == 0xff)
> >
> >  #define IN6_IS_ADDR_V4COMPAT(a) \
> > -        (((uint32_t *) (a))[0] == 0 && ((uint32_t *) (a))[1] == 0 && \
> > -         ((uint32_t *) (a))[2] == 0 && ((uint8_t *) (a))[15] > 1)
> > +        (((a)->s6_addr32)[0] == 0 && \
> > +         ((a)->s6_addr32)[1] == 0 && \
> > +         ((a)->s6_addr32)[2] == 0 && \
> > +         ((a)->s6_addr)[15] > 1)
> >
> >  #define IN6_IS_ADDR_MC_NODELOCAL(a) \
> > -        (IN6_IS_ADDR_MULTICAST(a) && ((((uint8_t *) (a))[1] & 0xf) == 0x1))
> > +        (IN6_IS_ADDR_MULTICAST(a) && \
> > +         ((((a)->s6_addr)[1] & 0xf) == 0x1))
> >
> >  #define IN6_IS_ADDR_MC_LINKLOCAL(a) \
> > -        (IN6_IS_ADDR_MULTICAST(a) && ((((uint8_t *) (a))[1] & 0xf) == 0x2))
> > +        (IN6_IS_ADDR_MULTICAST(a) && \
> > +         ((((a)->s6_addr)[1] & 0xf) == 0x2))
> >
> >  #define IN6_IS_ADDR_MC_SITELOCAL(a) \
> > -        (IN6_IS_ADDR_MULTICAST(a) && ((((uint8_t *) (a))[1] & 0xf) == 0x5))
> > +        (IN6_IS_ADDR_MULTICAST(a) && \
> > +         ((((a)->s6_addr)[1] & 0xf) == 0x5))
> >
> >  #define IN6_IS_ADDR_MC_ORGLOCAL(a) \
> > -        (IN6_IS_ADDR_MULTICAST(a) && ((((uint8_t *) (a))[1] & 0xf) == 0x8))
> > +        (IN6_IS_ADDR_MULTICAST(a) && \
> > +         ((((a)->s6_addr)[1] & 0xf) == 0x8))
> >
> >  #define IN6_IS_ADDR_MC_GLOBAL(a) \
> > -        (IN6_IS_ADDR_MULTICAST(a) && ((((uint8_t *) (a))[1] & 0xf) == 0xe))
> > +        (IN6_IS_ADDR_MULTICAST(a) && \
> > +         ((((a)->s6_addr)[1] & 0xf) == 0xe))
> >
> >  #define __ARE_4_EQUAL(a,b) \
> >       (!( (0[a]-0[b]) | (1[a]-1[b]) | (2[a]-2[b]) | (3[a]-3[b]) ))
> >  #define IN6_ARE_ADDR_EQUAL(a,b) \
> > -     __ARE_4_EQUAL((const uint32_t *)(a), (const uint32_t *)(b))
> > +     __ARE_4_EQUAL((a)->s6_addr32, (b)->s6_addr32)
> >
> >  #define      IN_CLASSA(a)            ((((in_addr_t)(a)) & 0x80000000) == 0)
> >  #define      IN_CLASSA_NET           0xff000000
>
> I think this looks fine. Anyone willing to point a second set of eyes
> at it (or maybe write a test to check whether codegen is same before
> and after) and make sure before I merge it?
>
> Rich