x/sys/unix: add Send on Linux

[dandare100]

go version go1.16.5 linux/amd64

GOARCH="amd64"
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"

Hello

https://man7.org/linux/man-pages/man2/sendto.2.html refers.

I cannot find a syscall func in the sys/unix package for

ssize_t send(int sockfd, const void *buf, size_t len, int flags);

The good news is that there is one for

ssize_t sendto(int sockfd, const void *buf, size_t len, int flags, const struct sockaddr *dest_addr, socklen_t addrlen);

It looks like the sendto syscall in linux allows the sockaddr to be NULL and its accompanying length to be 0 for connections in a connected state. (because the destination is known)

The above documentation mentions that the following call
send(sockfd, buf, len, flags);
is equivalent to
sendto(sockfd, buf, len, flags, NULL, 0);

The current implementation of
func Sendto(fd int, p []byte, flags int, to Sockaddr) (err error)
in
syscall_unix.go
has code that relies on "to Sockaddr" not being nil and a "nil pointer dereference" occurs if a nil is passed.
There could be cases where "to Sockaddr" can be nil to achieve a "send"

Would it be worthwhile modifying this function to allow for a nil "to" (leaving the current non nil behaviour intact) and letting the call reach the kernel (like the equivalent example above) ?

I have done a quick test and it works as predicted.

[thanm]

per owners: @ianlancetaylor @bradfitz @tklauser

[ianlancetaylor]

Note that this issue is about the x/sys/unix package.

I don't see a problem with adding Send to x/sys/unix. I don't see a reason to add special treatment for SendTo.

[thanm]

Note that this issue is about the x/sys/unix package.

Right, sorry for the confusion. I'll edit my comment.

[dandare100]

I understand and agree. Send will work perfectly and no need to change SendTo

[dandare100]

I just want to check in with some observations and request some advice before submitting to gerrit please :

It seems like "send" is not included because the implementation of makeZSysnumFile() in mkall.go only parses asm/unistd.h for linux.

This does not have "send", it only has

#define __NR_sendto 44  
#define __NR_sendmsg 46  
#define __NR_sendmmsg 307  

which seems strange(the fact that it has "sendto" and others that are already covered by this go package)

The only place I can find a reference to "send(2)" is in linux/net.h, which has

#define SYS_SEND	9		/* sys_send(2)			*/
#define SYS_SENDTO	11		/* sys_sendto(2)		*/
#define SYS_SENDMSG	16		/* sys_sendmsg(2)		*/
#define SYS_SENDMMSG	20		/* sys_sendmmsg(2)		*/

So in summary what I am asking is :

asm/unistd.h does not contain the send syscall I wish to add but it seems net.h contains all of the send calls.

What is the recommended way forward ?

[ianlancetaylor]

Write a Send function that calls the sendto system call. I assume that that is what the C library does.

[dandare100]

You are correct, thank you.

SYSCALL_DEFINE4(send, int, fd, void __user *, buff, size_t, len,
		unsigned int, flags)
{
	return __sys_sendto(fd, buff, len, flags, NULL, 0);
}

[gopherbot]

Change https://golang.org/cl/336569 mentions this issue: unix: add Send on Linux