Re: [PATCH 2/3] chardev: preserve the logfile fd across cpr-exec #VSTOR-148304

[Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>]

On 10/2/26 7:48 PM, Denis V. Lunev wrote:
> From: Denis V. Lunev <den@openvz.org>
> 
> A chardev with logfile= opens the log in qemu_char_open() on every
> start, including the start of the new QEMU after cpr-exec. The backend
> fds are taken from the cpr fd registry there, the log fd is not, so the
> new process reopens the path itself.
> 
> libvirt normally opens the log through virtlogd and passes a pipe in an
> fdset. The qemu-update command line cannot carry that fdset, so it
> names the log file path. The exec'd QEMU, running as the qemu user,
> then fails to create a log it has no write access to, e.g. a nova
> console.log on node-local storage:
> 
>   qemu-kvm: -chardev pty,id=charserial0,logfile=.../console.log,
>   logappend=on: Could not create '.../console.log': Permission denied
> 
> and the VM is destroyed. Where the open succeeds, the new QEMU writes
> the file directly, behind virtlogd's back.
> 
> Save the log fd in the cpr registry under the chardev label and take it
> from there on cpr-exec incoming, as the backends do. The logfile path
> is not opened at all then. If the source did not save it, run without
> a log rather than reopen the path. A failed chardev-change re-claims
> the log through the base class cpr_claim_fds(), since the replacement's
> log has displaced the entry.
> 
> Signed-off-by: Denis V. Lunev <den@openvz.org>
> ---
>  [...]

LGTM.

Nit: include/chardev/char.h:308 - comment probably needs updating (this
patch or previous one), stating that overrides must call the parent's
hook first.

Reviewed-by: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>