Re: [PATCH] fcntl: Add 32bit filesystem mode

On Tue, 31 Mar 2020 at 14:37, Linus Walleij [off-list ref] wrote:

It was brought to my attention that this bug from 2018 was
still unresolved: 32 bit emulators like QEMU were given
64 bit hashes when running 32 bit emulation on 64 bit systems.

This adds a fcntl() operation to set the underlying filesystem
into 32bit mode even if the file hanle was opened using 64bit
mode without the compat syscalls.

Programs that need the 32 bit file system behavior need to
issue a fcntl() system call such as in this example:

  #define F_SET_FILE_32BIT_FS (1024 + 15)

  int main(int argc, char** argv) {
    DIR* dir;
    int err;
    int fd;

    dir = opendir("/boot");
    fd = dirfd(dir);
    err = fcntl(fd, F_SET_FILE_32BIT_FS);
    if (err) {
      printf("fcntl() failed! err=%d\n", err);
      return 1;
    }
    printf("dir=%p\n", dir);
    printf("readdir(dir)=%p\n", readdir(dir));
    printf("errno=%d: %s\n", errno, strerror(errno));
    return 0;
  }

I gave this a try with a modified QEMU, but it doesn't seem
to fix the problem. Here's the relevant chunk of the strace
output from stracing a QEMU that's running a 32-bit guest
binary that issues a getdents64 and fails (it's the 'readdir-bug'
test case from the launchpad bug):

openat(AT_FDCWD, ".", O_RDONLY|O_NONBLOCK|O_CLOEXEC|O_DIRECTORY) = 3
fcntl(3, 0x40f /* F_??? */, 0x3)        = 0
fstat(3, {st_dev=makedev(0, 16), st_ino=4637237, st_mode=S_IFDIR|0755,
st_nlink=12, st_uid=1000, st_gid=1000, st_blksize=8192, st_blocks=8,
st_size=4096, st_atime=1587380917 /*
2020-04-20T11:08:37.756174607+0000 */, st_atime_nsec=756174607,
st_mtime=1587380910 /* 2020-04-20T11:08:30.356230179+0000 */,
st_mtime_nsec=356230179, st_ctime=1587380910 /*
2020-04-20T11:08:30.356230179+0000 */, st_ctime_nsec=356230179}) = 0
fstat(1, {st_dev=makedev(0, 2), st_ino=9017, st_mode=S_IFCHR|0600,
st_nlink=1, st_uid=0, st_gid=0, st_blksize=4096, st_blocks=0,
st_rdev=makedev(5, 1), st_atime=1587381196 /* 2020-04-20T11:13:16+0000
*/, st_atime_nsec=0, st_mtime=1587381196 /* 2020-04-20T11:13:16+0000
*/, st_mtime_nsec=0, st_ctime=1587381042 /*
2020-04-20T11:10:42.484981152+0000 */, st_ctime_nsec=484981152}) = 0
ioctl(1, TCGETS, {c_iflags=0x2502, c_oflags=0x5, c_cflags=0xcbd,
c_lflags=0x8a3b, c_line=0,
c_cc="\x03\x1c\x7f\x15\x04\x00\x01\x00\x11\x13\x1a\x00\x12\x0f\x17\x16\x00\x00\x00"})
= 0
write(1, "dir=0x76128\n", 12)           = 12
getdents64(3, [{d_ino=1, d_off=273341893525646730, d_reclen=24,
d_type=DT_DIR, d_name=".."}, {d_ino=4637239, d_off=849308795555391993,
d_reclen=24, d_type=DT_DIR, d_name="etc"}, {d_ino=4587984,
d_off=1620709961571101518, d_reclen=24, d_type=DT_LNK, d_name="usr"},
{d_ino=4637238, d_off=2787937917159437645, d_reclen=24, d_type=DT_DIR,
d_name="dev"}, {d_ino=4637244, d_off=3015508490233103491, d_reclen=24,
d_type=DT_DIR, d_name="sys"}, {d_ino=4587608,
d_off=3551089360661460833, d_reclen=24, d_type=DT_LNK, d_name="lib"},
{d_ino=4637246, d_off=3857320197951442970, d_reclen=24, d_type=DT_DIR,
d_name="var"}, {d_ino=4637242, d_off=4103122318823701457, d_reclen=24,
d_type=DT_DIR, d_name="proc"}, {d_ino=4587541,
d_off=4252201186220906002, d_reclen=24, d_type=DT_LNK, d_name="bin"},
{d_ino=4637245, d_off=4386533378951587638, d_reclen=24, d_type=DT_DIR,
d_name="tmp"}, {d_ino=4637241, d_off=4883206313583644962, d_reclen=24,
d_type=DT_DIR, d_name="host"}, {d_ino=4637237,
d_off=4941119754928488586, d_reclen=24, d_type=DT_DIR, d_name="."},
{d_ino=4637243, d_off=5301154723342888169, d_reclen=24, d_type=DT_DIR,
d_name="root"}, {d_ino=4587838, d_off=6989908915879243400,
d_reclen=32, d_type=DT_LNK, d_name="lib64"}, {d_ino=4587679,
d_off=7356513223657690979, d_reclen=32, d_type=DT_REG,
d_name="strace.log"}, {d_ino=4587847, d_off=7810090083157553519,
d_reclen=24, d_type=DT_LNK, d_name="sbin"}, {d_ino=4637240,
d_off=8254997891991845677, d_reclen=24, d_type=DT_DIR, d_name="home"},
{d_ino=4637248, d_off=9223372036854775807, d_reclen=24, d_type=DT_DIR,
d_name="virt"}], 32768) = 448
write(1, "readdir(dir)=(nil)\n", 19)    = 19
write(1, "errno=75: Value too large for de"..., 48) = 48
exit_group(0)                           = ?


We open fd 3 to read '.'; we issue the new fcntl, which
succeeds. Then there's some unrelated stuff operating on
stdout. Then we do a getdents64(), but the d_off values
we get back are still 64 bits. The guest binary doesn't
like those, so it fails. My expectation was that we would
get back d_off values here that were in the 32 bit range.

(To be clear, the guest binary here is doing a getdents64(),
which QEMU translates into a host getdents64().)

thanks
-- PMM