kern/123095 kern/131602 sendfile

[Kostik Belousov]

On Wed, Jul 07, 2010 at 10:24:41AM -0700, Ming Fu wrote:
> Hi,
> 
> 
> I was trying to use sendfile and hit with problem very similar to the
> 123095 and 131602. 
> It seems that when the file is large enough (in megs), the file can be
> corrupted even if it is open read-only and exist on disk as read-only
> file, though the filesystem is mounted read-write.
> 
> I have a small program to reliably reproduce the problem.
> 
> ---------- corrupt.c -----------------
> 
> #include <sys/types.h>
> #include <sys/socket.h>
> #include <sys/uio.h>
> #include <fcntl.h>
> #include <netinet/in.h>
> #include <sys/select.h>
> #include <sys/stat.h>
> #include <strings.h>
> #include <stdio.h>
> #include <err.h>
> main () {
>         int s, f;
>         struct sockaddr_in addr;
>         int flags;
>         char str[32]="\r\n800\r\n";
>         char *p = str;
>         struct stat sb;
>         int n;
>         fd_set wset;
>         int64_t size;
>         off_t sbytes;
>         off_t sent = 0;
>         int chunk;
> 
>         s = socket(AF_INET, SOCK_STREAM, 0);
>         bzero(&addr, sizeof(addr));
>         addr.sin_family = AF_INET;
>         addr.sin_port = htons(7000);
>         addr.sin_addr.s_addr = inet_addr("10.1.19.16");
> 
>         n = connect(s, (struct sockaddr *)&addr, sizeof (addr));
>         if (n < 0)
>                 warn ("fail to connect");
>         flags = fcntl(s, F_GETFL);
>         flags |= O_NONBLOCK;
>         fcntl(s, F_SETFL);
> 
>         f = open("large", O_RDONLY);
>         if (f<0)
>                 warn("fail to open file");
>         n = fstat(f, &sb);
>         if (n<0)
>                 warn("fstat failed");
> 
>         size = sb.st_size;
>         chunk = 0;
>         while (size > 0) {
>                 FD_ZERO(&wset);
>                 FD_SET(s, &wset);
>                 n = select(f+1, NULL, &wset, NULL, NULL);
>                 if (n < 0)
>                         continue;
>                 if (chunk > 0) {
>                         sbytes = 0;
>                         n = sendfile(f, s, sent, chunk, NULL, &sbytes,
> 0);
>                         if (n < 0)
>                                 continue;
>                         chunk -= sbytes;
>                         size -= sbytes;
>                         sent += sbytes;
>                         continue;
>                 }
>                 if (size > 2048)
>                         chunk = 2048;
>                 else
>                         chunk = size;
>                 n = sprintf(str, "\r\n%x\r\n", 2048);
>                 p = str;
>                 write(s, p, n);
>         }
> }
> 
> ------------- end ---------------------------------------------
> 
> Run nc to receive the sendfile
> $ nc -l 7000
> 
> Copy a large from for sendfile to send
> $ cp /usr/lib/libc_pic.a large
> 
> $ md5 large
> MD5 (large) = 252def82f9d75df11df7123e9fd376f6
> 
> $ cc -o co corrupt.c
> $./co
> $ md5 large 
> MD5 (large) = 81ee84e55f4611434459f637c83b892e
> 
> I run this on 8.0-RELEASE. The same happens on 7.2 and 6.3. The disk are
> SATA ide. I run all these command under unprivileged user account. I
> also run the same program on several different hardware, the result is
> the same. Although the corrupted file is not the same. The corruption
> looks random to me.
> 
> I know a bit of the network side of FreeBSD kernel code, but the I have
> no idea how the filesystem side work. I can dig a bit further if someone
> give me a hint as where to look.

Right, thank you for the easy way to reproduce it. I was able to
trigger this as well. At the
http://people.freebsd.org/~kib/vm/sf_buf_readonly.patch 
is the patch to map some sf buffers
readonly, in particular, for the pages that are mapped by sendfile(2).

Sure enough, it triggered the panic immediately, with backtrace
bcopy
sbappendstream_locked
tcp_do_segment
tcp_input
ip_input
swi_net
(swi because I tested over loopback). To be clear: the backtrace above
points to the code path that causes modifications to the file object
pages inserted (?) into the mbuf that are supposed to be immutable.

Any help from tcp-clueful people is appreciated.

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