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SubjectRe: Linux 5.3-rc8
On Tue, Sep 10, 2019 at 12:33:12PM +0100, Linus Torvalds wrote:
> On Tue, Sep 10, 2019 at 5:21 AM Ahmed S. Darwish <darwish.07@gmail.com> wrote:
> >
> > The commit b03755ad6f33 (ext4: make __ext4_get_inode_loc plug), [1]
> > which was merged in v5.3-rc1, *always* leads to a blocked boot on my
> > system due to low entropy.
>
> Exactly what is it that blocks on entropy? Nobody should do that
> during boot, because on some systems entropy is really really low
> (think flash memory with polling IO etc).
>

Ok, I've tracked it down further. It's unfortunately GDM
intentionally blocking on a getrandom(buf, 16, 0).

Booting the system with an straced GDM service
("ExecStart=strace -f /usr/bin/gdm") reveals:

...
[ 3.779375] strace[262]: [pid 323] execve("/usr/lib/gnome-session-binary",
... /* 28 vars */) = 0
...
[ 4.019227] strace[262]: [pid 323] getrandom( <unfinished ...>
[ 79.601433] kernel: random: crng init done
[ 79.601443] kernel: random: 3 urandom warning(s) missed due to ratelimiting
[ 79.601262] strace[262]: [pid 323] <... getrandom resumed>..., 16, 0) = 16
[ 79.601262] strace[262]: [pid 323] getrandom(..., 16, 0) = 16
[ 79.603041] strace[262]: [pid 323] getrandom(..., 16, 0) = 16
[ 79.603041] strace[262]: [pid 323] getrandom(..., 16, 0) = 16
[ 79.603041] strace[262]: [pid 323] getrandom(..., 16, 0) = 16

As can be seen in the timestamps, the GDM boot was only continued
by typing randomly on the keyboard..

> That said, I would have expected that any PC gets plenty of entropy.
> Are you sure it's entropy that is blocking, and not perhaps some odd
> "forgot to unplug" situation?
>

Yes, doing any of below steps makes the problem reliably disappear:

- boot param "random.trust_cpu=on"
- rngd(8) enabled at boot (entropy source: x86 RDRAND + jitter)
- pressing random 3 or 4 keyboard keys while GDM boot is stuck

> > Can this even be considered a user-space breakage? I'm honestly not
> > sure. On my modern RDRAND-capable x86, just running rng-tools rngd(8)
> > early-on fixes the problem. I'm not sure about the status of older
> > CPUs though.
>
> It's definitely breakage, although rather odd. I would have expected
> us to have other sources of entropy than just the disk. Did we stop
> doing low bits of TSC from timer interrupts etc?
>

Exactly.

While gnome-session is obviously at fault here by requiring
*blocking* randomness at the boot path, it's still not requesting
much, just (5 * 16) bytes to be exact.

I guess an x86 laptop should be able to provide that, even without
RDRAND / random.trust_cpu=on (TSC jitter, etc.) ?

thanks,
--darwi

> Ted, either way - ext4 IO patterns or random number entropy - this is
> your code. Comments?
>
> Linus

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