x/net: incorrect parsing of rt_msghdr errno on Darwin

[spikecurtis]

Go version

x/net v0.31.0

Output of go env in your module/workspace:

GO111MODULE=''
GOARCH='arm64'
GOBIN=''
GOCACHE='/Users/spike/Library/Caches/go-build'
GOENV='/Users/spike/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='arm64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMODCACHE='/Users/spike/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='darwin'
GOPATH='/Users/spike/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/opt/homebrew/Cellar/[email protected]/1.21.8/libexec'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/opt/homebrew/Cellar/[email protected]/1.21.8/libexec/pkg/tool/darwin_arm64'
GOVCS=''
GOVERSION='go1.21.8'
GCCGO='gccgo'
AR='ar'
CC='cc'
CXX='c++'
CGO_ENABLED='1'
GOMOD='/dev/null'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/53/zffdtv3x7lg_pyhrk85p7_nw0000gn/T/go-build2233440794=/tmp/go-build -gno-record-gcc-switches -fno-common'

What did you do?

I read the source and spotted an error.

What did you see happen?

I believe the parsing of route messages in https://go.googlesource.com/net/+/refs/tags/v0.31.0/route/route_classic.go is incorrect for Darwin (presumably other BSD variants). In particular it parses the errno from 28:32 in the header.

However, the header (https://github.com/apple-oss-distributions/xnu/blob/33de042d024d46de5ff4e89f2471de6608e37fa4/bsd/net/route.h#L158) looks like:

struct rt_msghdr {
	u_short rtm_msglen;     /* to skip over non-understood messages */
	u_char  rtm_version;    /* future binary compatibility */
	u_char  rtm_type;       /* message type */
	u_short rtm_index;      /* index for associated ifp */
	int     rtm_flags;      /* flags, incl. kern & message, e.g. DONE */
	int     rtm_addrs;      /* bitmask identifying sockaddrs in msg */
	pid_t   rtm_pid;        /* identify sender */
	int     rtm_seq;        /* for sender to identify action */
	int     rtm_errno;      /* why failed */
	int     rtm_use;        /* from rtentry */
	u_int32_t rtm_inits;    /* which metrics we are initializing */
	struct rt_metrics rtm_rmx; /* metrics themselves */
};

errno is immediately after the rtm_seq, which are sliced as 20:24, meaning it should be 24:28.

What did you expect to see?

errno should be parsed from bytes 24:28 on Darwin

[gabyhelp]

Related Issues

• x/net/route: ParseRIB fails with errMessageTooShort on an AF_ROUTE message from Darwin #44740 (closed)
• x/net/route: ParseRIB panics on message from Darwin #70528 (closed)

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[GoVeronicaGo]

cc: @neild @ianlancetaylor