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Message-ID: <20240814211623.GL10433@brightrain.aerifal.cx>
Date: Wed, 14 Aug 2024 17:16:23 -0400
From: Rich Felker <dalias@...c.org>
To: Brad House <brad@...d-house.com>
Cc: musl@...ts.openwall.com
Subject: Re: [PATCH 1/1] IN6_IS_ADDR_LOOPBACK() and similar macros
warn on -Wcast-qual
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
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