Within my UML guest kernel, I'm finding that between 5% and
10% of reads from /dev/hwrng block, seemingly indefinitely.
(I've certainly seem reads block for more than 12 hours.)
All other reads complete in significantly less than 1
second.
I've tried a variety of 2.6.27, .28 and .29 guest UML
kernels. (I've had problems, almost certainly unrelated,
getting 2.6.30 working reliably, so haven't tested this
there.) I have CONFIG_UML_RANDOM=y and /dev/hwrng has the
correct permissions (0644) and device numbers (10 183). I'm
using a 32-bit x86 architecture.
Under strace(1) I see it blocking in a read(2) syscall:
guest:~# strace dd if=/dev/hwrng of=/dev/zero bs=32 count=4
[...]
open("/dev/hwrng", O_RDONLY|O_LARGEFILE) = 3
dup2(3, 0) = 0
close(3) = 0
_llseek(0, 0, [0], SEEK_CUR) = 0
[...]
read(0,
If I recompile the guest kernel with CONFIG_DEBUG_KERNEL=y
and CONFIG_FRAME_POINTER=y and attach gdb to the main
running UML process, I see the following backtrace during a
blocked read:
(gdb) bt
#0 0xb7ff0e9e in read () from /lib/libc.so.6
#1 0x0806a2a1 in os_read_file ()
#2 0x08069684 in rng_dev_read ()
#3 0x080b0c7a in vfs_read ()
#4 0x080b0dac in sys_read ()
#5 0x08061b5f in handle_syscall ()
#6 0x0806e7a8 in userspace ()
#7 0x0805f929 in fork_handler ()
#8 0x00000000 in ?? ()
I've struggled to get gdb to access the function parameters
in the kernel (yes, debugging symbols are there, or so
readelf thinks), but the stack layout in glibc's read
function (i.e. frame #0) is straightforward:
(gdb) x/8x $esp
0xba74e70: 0x0ba74000 0x0806a2a1 0x00000007 0x0ba74ebc
0xba74e80: 0x00000004 0x0ba74ecc 0x08069684 0x00000007
i.e. we've just called read(7, (void*)0xba74ebc, 4). The
host's /proc/$PID/fd/7 confirms that fd 7 is /dev/random.
The code in rng_dev_read starts:
u32 data;
int n, ret = 0, have_data;
while (size) {
n = os_read_file(random_fd, &data, sizeof(data));
... so second argument to read is plausible (it's on the
stack in the right function), and sizeof(data)==4 so the
third argument is correct. In other words, we don't seem to
have passed bogus parameters to the VDS host's read.
When the kernel blocks, the instruction pointer, %eip, is
pointing to the instruction after the
0xb7ff0e9c <read+28>: int $0x80
in glibc's read. I.e. we appear to be blocking in the host
kernel. My obvious first thought is that the host kernel's
entropy is depleted, although it seems hard to believe that
this could explain the guest blocking for 12+ hours.
The host kernel is 2.6.29.1. I haven't yet tried a 2.6.30.5
host kernel, though I plan to. (That said, there seems to
be no relevant changes in drivers/char/random.c between
these two releases.)
The host is running rngd which seems to be effective at
preventing the UML guests from depleting the host kernel's
entropy pool. If I monitor the available host entropy, I
see:
host:~# while true; do tr '\n' ' ' \
< /proc/sys/kernel/random/entropy_avail; sleep 1;
done; echo
3840 3968 3968 3968 3968 3968 3968 2944 3968 3968 3968 3968
3968 3968 3968 3968 3968 2944 3968 3968 3968 3968 3776 3520
3520 3537 3968 3968 3968 3968 3968 3968 3968 2944 3968 3968
3968 3968 3968 3968 3968 3968 3968 2955 3968 3968 3968 3968
3968 3968 3968 3968 3968 3968 3968 3968 3968 3968 3968 3968
[...]
I'm not sure why it should peak at 3968 rather than 4096
(the poolsize) or 3686 (rngd is running with
--fill-watermark=90%), but that doesn't seem cause for
concern. In any case, that seems like plenty of entropy.
I'd be very grateful for any thoughts or suggestions. The
lack of a working /dev/hwrng is preventing me from using
rngd on the UML guests, which in turn is resulting in them
running out of entropy which most usually manifests as ssh
dying or hanging.
Thanks in advance for any advice,
Richard
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