I am seeing a readdir-related problem inside qemu-system-i386, when accessing a
'9p' filesystem which is mapped to a host directory. The readdir call does not return any directory entries, not even '.' or '..'.
When accessing a local filesystem like /tmp, readdir does return the expected entries.

The problem is seen when running Qemu on CentOS 7 with kernel 3.10, and not seen when running the same Qemu version 2.11.0 on Gentoo with kernel 4.14.

The target toolchain used to build the test program running inside Qemu is
consisting of gcc 7.3.0, glibc 2.28, binutils 2.31.1.

Test program takes a directory path as argument and prints out the first three
entries received from readdir. In the bad case, this prints nothing.


#include <dirent.h>
#include <stdio.h>

void readonce(DIR *dirp)
{
    struct dirent *d;
    d = readdir(dirp);
    if (d == NULL) {
        printf("no more entries\n");
        return;
    }

    printf("d_name: %s\n", d->d_name);
}

int main(int argc, char *argv[])
{
    DIR *dirp = opendir(argv[1]);
    for (int i = 0; i < 3 ; i++) {
        readonce(dirp);
    }
}


Output from 'strace -v' on the bad case (accessing the 9p filesystem):

openat(AT_FDCWD, "/mnt/nand-dbase/foo/", O_RDONLY|O_NONBLOCK|O_LARGEFILE|O_DIRECTORY|O_CLOEXEC) = 3
fstat64(3, {st_dev=makedev(0, 19), st_ino=361311, st_mode=S_IFDIR|0755, st_nlink=2, st_uid=10570, st_gid=3610, st_blksize=4096, st_blocks=8, st_size=4096, st_atime=1545319664 /* 2018-12-20T15:27:44.400824994+0000 */, st_atime_nsec=400824994, st_mtime=1545319674 /* 2018-12-20T15:27:54.205814105+0000 */, st_mtime_nsec=205814105, st_ctime=1545319674 /* 2018-12-20T15:27:54.205814105+0000 */, st_ctime_nsec=205814105}) = 0
brk(NULL)                               = 0x9da0000
brk(0x9dc1000)                          = 0x9dc1000
brk(0x9dc2000)                          = 0x9dc2000
getdents64(3, [{d_ino=361275, d_off=1532808351538279948, d_reclen=24, d_type=DT_DIR, d_name=".."}, {d_ino=358605, d_off=8467452789161523084, d_reclen=24, d_type=DT_REG, d_name="bar"}, {d_ino=361311, d_off=9223372036854775807, d_reclen=24, d_type=DT_DIR, d_name="."}], 32768) = 72
fstat64(1, {st_dev=makedev(0, 6), st_ino=4568, st_mode=S_IFCHR|0600, st_nlink=1, st_uid=0, st_gid=0, st_blksize=4096, st_blocks=0, st_rdev=makedev(4, 65), st_atime=1545319680 /* 2018-12-20T15:28:00.769282559+0000 */, st_atime_nsec=769282559, st_mtime=1545319680 /* 2018-12-20T15:28:00.769282559+0000 */, st_mtime_nsec=769282559, st_ctime=1545313074 /* 2018-12-20T13:37:54.834207421+0000 */, st_ctime_nsec=834207421}) = 0
ioctl(1, TCGETS, {c_iflags=0x1500, c_oflags=0x5, c_cflags=0xcbd, c_lflags=0xa3b, c_line=0, c_cc="\x03\x1c\x7f\x15\x04\x00\x01\x00\x11\x13\x1a\x0a\x12\x0f\x17\x16\x00\x00\x00"}) = 0
write(1, "no more entries\n", 16no more entries
)       = 16
getdents64(3, [], 32768)                = 0
write(1, "no more entries\n", 16no more entries
)       = 16
getdents64(3, [], 32768)                = 0
write(1, "no more entries\n", 16no more entries
)       = 16


Output on the same system but a local filesystem (/tmp):

openat(AT_FDCWD, "/tmp/foo", O_RDONLY|O_NONBLOCK|O_LARGEFILE|O_DIRECTORY|O_CLOEXEC) = 3
fstat64(3, {st_dev=makedev(0, 15), st_ino=15109, st_mode=S_IFDIR|0755, st_nlink=2, st_uid=0, st_gid=0, st_blksize=4096, st_blocks=0, st_size=60, st_atime=1545320147 /* 2018-12-20T15:35:47.893535644+0000 */, st_atime_nsec=893535644, st_mtime=1545320150 /* 2018-12-20T15:35:50.597535644+0000 */, st_mtime_nsec=597535644, st_ctime=1545320150 /* 2018-12-20T15:35:50.597535644+0000 */, st_ctime_nsec=597535644}) = 0
brk(NULL)                               = 0x9f11000
brk(0x9f32000)                          = 0x9f32000
brk(0x9f33000)                          = 0x9f33000
getdents64(3, [{d_ino=15109, d_off=1, d_reclen=24, d_type=DT_DIR, d_name="."}, {d_ino=4856, d_off=2, d_reclen=24, d_type=DT_DIR, d_name=".."}, {d_ino=15110, d_off=3, d_reclen=24, d_type=DT_REG, d_name="bar"}], 32768) = 72
fstat64(1, {st_dev=makedev(0, 6), st_ino=4568, st_mode=S_IFCHR|0600, st_nlink=1, st_uid=0, st_gid=0, st_blksize=4096, st_blocks=0, st_rdev=makedev(4, 65), st_atime=1545320152 /* 2018-12-20T15:35:52.769282559+0000 */, st_atime_nsec=769282559, st_mtime=1545320152 /* 2018-12-20T15:35:52.769282559+0000 */, st_mtime_nsec=769282559, st_ctime=1545313074 /* 2018-12-20T13:37:54.834207421+0000 */, st_ctime_nsec=834207421}) = 0
ioctl(1, TCGETS, {c_iflags=0x1500, c_oflags=0x5, c_cflags=0xcbd, c_lflags=0xa3b, c_line=0, c_cc="\x03\x1c\x7f\x15\x04\x00\x01\x00\x11\x13\x1a\x0a\x12\x0f\x17\x16\x00\x00\x00"}) = 0
write(1, "d_name: .\n", 10d_name: .
)             = 10
write(1, "d_name: ..\n", 11d_name: ..
)            = 11
write(1, "d_name: bar\n", 12d_name: bar
)           = 12


The bad case shows very large d_off values in getdents64, and repeated calls to
this function, while in the good case there is just one call with small d_off
values.

Could be related to #23960 and/or #23497.