fstat and similar methods in fhandler_socket_local and fhandler_socket_unix

[Ken Brown]

On 2/20/2021 6:32 PM, Ken Brown via Cygwin-developers wrote:
> fhandler_socket_local::fstat and many similar methods seem to assume that the 
> fhandler is based on a socket *file*.  When this is not the case, they end up 
> using handles inappropriately.  Consider the following test case, for example.
> 
> $ cat socket_stat.c
> #include <time.h>
> #include <sys/socket.h>
> #include <sys/un.h>
> #include <sys/stat.h>
> #include <errno.h>
> #include <stdio.h>
> #include <stdlib.h>
> #include <unistd.h>
> 
> void print_stat (struct stat);
> 
> int
> main ()
> {
>    struct stat st;
>    int fd;
> 
>    fd = socket (AF_UNIX, SOCK_STREAM, 0);
>    if (fd == -1)
>      {
>        perror ("socket");
>        exit (1);
>      }
>    if (fstat (fd, &st) < 0)
>      {
>        perror ("fstat");
>        exit (1);
>      }
>    print_stat (st);
> }
> 
> /* https://man7.org/linux/man-pages/man2/lstat.2.html */
> void
> print_stat (struct stat st)
> {
>    printf("File type:                ");
> 
>    switch (st.st_mode & S_IFMT) {
>    case S_IFBLK:  printf("block device\n");            break;
>    case S_IFCHR:  printf("character device\n");        break;
>    case S_IFDIR:  printf("directory\n");               break;
>    case S_IFIFO:  printf("FIFO/pipe\n");               break;
>    case S_IFLNK:  printf("symlink\n");                 break;
>    case S_IFREG:  printf("regular file\n");            break;
>    case S_IFSOCK: printf("socket\n");                  break;
>    default:       printf("unknown?\n");                break;
>    }
> 
>    printf("I-node number:            %ld\n", (long) st.st_ino);
>    printf("dev:                      %lu\n", st.st_dev);
>    printf("rdev:                     %lu\n", st.st_rdev);
>    printf("Mode:                     %lo (octal)\n",
>           (unsigned long) st.st_mode);
> 
>    printf("Link count:               %ld\n", (long) st.st_nlink);
>    printf("Ownership:                UID=%ld   GID=%ld\n",
>           (long) st.st_uid, (long) st.st_gid);
> 
>    printf("Preferred I/O block size: %ld bytes\n",
>           (long) st.st_blksize);
>    printf("File size:                %lld bytes\n",
>           (long long) st.st_size);
>    printf("Blocks allocated:         %lld\n",
>           (long long) st.st_blocks);
> 
>    printf("Last status change:       %s", ctime(&st.st_ctime));
>    printf("Last file access:         %s", ctime(&st.st_atime));
>    printf("Last file modification:   %s", ctime(&st.st_mtime));
>    printf("\n");
> }
> 
> $ gcc -g -O0 -o socket_stat socket_stat.c
> 
> $ ./socket_stat.exe
> File type:                socket
> I-node number:            4
> dev:                      1966200
> rdev:                     1966200
> Mode:                     140644 (octal)
> Link count:               0
> Ownership:                UID=197609   GID=197121
> Preferred I/O block size: 65536 bytes
> File size:                0 bytes
> Blocks allocated:         0
> Last status change:       Wed Dec 31 19:00:00 1969
> Last file access:         Wed Dec 31 19:00:00 1969
> Last file modification:   Wed Dec 31 19:00:00 1969
> 
> On Linux the output is:
> 
> File type:                socket
> I-node number:            117
> dev:                      0
> rdev:                     0
> Mode:                     140777 (octal)
> Link count:               1
> Ownership:                UID=1000   GID=1000
> Preferred I/O block size: 512 bytes
> File size:                0 bytes
> Blocks allocated:         0
> Last status change:       Sat Feb 20 18:13:50 2021
> Last file access:         Sat Feb 20 18:13:50 2021
> Last file modification:   Sat Feb 20 18:13:50 2021
> 
> I haven't been able to find any documentation about what fstat(2) should do on a 
> socket descriptor, so maybe the difference between Cygwin and Linux is 
> unimportant.  But the code path followed on Cygwin definitely seems wrong. 
> fhandler_socket_local::fstat calls fstat_fs which ends up using the io_handle as 
> though it were a file handle.  It seems to me that fstat_fs should only be 
> called if we have a path_conv handle (which is the case if the fstat call 
> resulted from a stat call) or if the socket descriptor came from open with 
> O_PATH set.
> 
> Similar remarks apply to fstatfvs, fchmod,....  In some of these cases, like 
> fchmod, POSIX explicitly states that the behavior is undefined, so maybe we 
> should just fail when there's no underlying socket file.

Maybe the answer in all these cases is just to revert to the fhandler_socket 
methods when our fhandler_socket_local is not based on a socket file.

Ken