Re: Endless loop in udp with MSG_SPLICE_READ - Re: [syzbot] [fs?] INFO: task hung in pipe_release (4)

[David Howells <dhowells@redhat.com>]

Hi Willem,

Here's a reduced testcase.  I doesn't require anything special; the key is
that the amount of data placed in the packet by the send() - it's related to
the MTU size.  It needs to stuff in sufficient data to go over the
fragmentation limit (I think).

In this case, my interface's MTU is 8192.  send() is sticking in 8161 bytes of
data and then the output from the aforeposted debugging patch is:

	==>splice_to_socket() 6630
	udp_sendmsg(8,8)
	__ip_append_data(copy=-1,len=8, mtu=8192 skblen=8189 maxfl=8188)
	pagedlen 9 = 9 - 0
	copy -1 = 9 - 0 - 1 - 9
	length 8 -= -1 + 0
	__ip_append_data(copy=8172,len=9, mtu=8192 skblen=20 maxfl=8188)
	copy=8172 len=9
	skb_splice_from_iter(8,9)
	__ip_append_data(copy=8164,len=1, mtu=8192 skblen=28 maxfl=8188)
	copy=8164 len=1
	skb_splice_from_iter(0,1)
	__ip_append_data(copy=8164,len=1, mtu=8192 skblen=28 maxfl=8188)
	copy=8164 len=1
	skb_splice_from_iter(0,1)
	__ip_append_data(copy=8164,len=1, mtu=8192 skblen=28 maxfl=8188)
	copy=8164 len=1
	skb_splice_from_iter(0,1)
	__ip_append_data(copy=8164,len=1, mtu=8192 skblen=28 maxfl=8188)
	copy=8164 len=1
	skb_splice_from_iter(0,1)
	copy=8164 len=1
	skb_splice_from_iter(0,1)

It looks like send() pushes 1 byte over the fragmentation limit, then the
splice sees -1 crop up, the length to be copied is increased by 1, but
insufficient data is available and we go into an endless loop.

---
#define _GNU_SOURCE
#include <arpa/inet.h>
#include <fcntl.h>
#include <netinet/in.h>
#include <stdarg.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/socket.h>
#include <sys/mman.h>
#include <sys/uio.h>

#define OSERROR(R, S) do { if ((long)(R) == -1L) { perror((S)); exit(1); } } while(0)

int main()
{
	struct sockaddr_storage ss;
	struct sockaddr_in sin;
	void *buffer;
	unsigned int tmp;
	int pfd[2], sfd;
	int res;

	OSERROR(pipe(pfd), "pipe");

	sfd = socket(AF_INET, SOCK_DGRAM, 0);
	OSERROR(sfd, "socket/2");

	memset(&sin, 0, sizeof(sin));
	sin.sin_family = AF_INET;
	sin.sin_port = htons(0);
	sin.sin_addr.s_addr = htonl(0xc0a80601);
#warning you might want to set the address here - this is 192.168.6.1
	OSERROR(connect(sfd, (struct sockaddr *)&sin, sizeof(sin)), "connect");

	buffer = mmap(NULL, 1024*1024, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANON, -1, 0);
	OSERROR(buffer, "mmap");

	OSERROR(send(sfd, buffer, 8161, MSG_CONFIRM|MSG_MORE), "send");
#warning you need to adjust the length on the above line to match your MTU

	OSERROR(write(pfd[1], buffer, 8), "write");

	OSERROR(splice(pfd[0], 0, sfd, 0, 0x4ffe0ul, 0), "splice");
	return 0;
}