On Fri, Nov 29, 2024 at 12:00:51PM +0000, Daniel P. Berrangé wrote:
> The Linux MADV_DONTDUMP flag was added to Linux kernels > 3.3,
> back in 2012, and the dump-guest-core flag was added to QEMU
> > 1.0 at the same time.
>
> IOW, on Linux we have long been able to assume that QEMU core
> dumps will exclude guest memory, unless the user has overridden
> the host level defaults in the domain XML.
>
> It is desirable to permit QEMU core dumps out of the box to make
> it easier for users to report crashes to their OS vendor without
> having to reconfigure and restart libvirt daemons and their
> running guests.
>
> While there is a risk that an admin may have set 'dump_guest_core'
> to true, while leaving 'max_core' to 0, on balance the benefits
> of easier troubleshooting outweigh the risk of changing the
> defaults to permit core dumps.
>
> Signed-off-by: Daniel P. Berrangé <berrange@xxxxxxxxxx>
Yes yes yes!
Reviewed-by: Richard W.M. Jones <rjones@xxxxxxxxxx>
> ---
> src/qemu/qemu.conf.in | 8 +++++---
> src/qemu/qemu_conf.c | 10 +++++++---
> 2 files changed, 12 insertions(+), 6 deletions(-)
>
> diff --git a/src/qemu/qemu.conf.in b/src/qemu/qemu.conf.in
> index 89c9b6b913..d853136f10 100644
> --- a/src/qemu/qemu.conf.in
> +++ b/src/qemu/qemu.conf.in
> @@ -709,7 +709,9 @@
> # As a special case it can be set to the string "unlimited" to
> # to allow arbitrarily sized core dumps.
> #
> -# By default the core dump size is set to 0 disabling all dumps
> +# By default the core dump size is set to unlimited on
> +# Linux where 'dump_guest_core' defaults to false, and
> +# is set to 0 on other platforms, disabling all dumps
> #
> # Size is a positive integer specifying bytes or the
> # string "unlimited"
> @@ -717,8 +719,8 @@
> #max_core = "unlimited"
>
> # Determine if guest RAM is included in QEMU core dumps. By
> -# default guest RAM will be excluded if a new enough QEMU is
> -# present and host kernel supports it. Setting this to '1' will
> +# default guest RAM will be excluded on Linux platforms,
> +# and included on all other patforms. Setting this to '1' will
> # force guest RAM to always be included in QEMU core dumps.
> #
> # This setting will be ignored if the guest XML has set the
> diff --git a/src/qemu/qemu_conf.c b/src/qemu/qemu_conf.c
> index 7c15c521c7..8b9fe4e381 100644
> --- a/src/qemu/qemu_conf.c
> +++ b/src/qemu/qemu_conf.c
> @@ -291,10 +291,14 @@ virQEMUDriverConfig *virQEMUDriverConfigNew(bool privileged,
> cfg->deprecationBehavior = g_strdup("none");
> cfg->storageUseNbdkit = USE_NBDKIT_DEFAULT;
>
> -#ifndef MADV_DONTDUMP
> +#ifdef MADV_DONTDUMP
> + cfg->maxCore = ULLONG_MAX;
> +#else
> /* QEMU uses Linux extensions to madvise() (MADV_DODUMP/MADV_DONTDUMP) to
> - * include/exclude guest memory from core dump. These might be unavailable
> - * on some systems. Provide sane default. */
> + * include/exclude guest memory from core dump. On non-Linux systems
> + * core dumps will unavoidably include all guest RAM, so toggle the
> + * default to reflect this and warn the admin.
> + */
> VIR_INFO("Host kernel doesn't support MADV_DONTDUMP. Enabling dump_guest_core");
> cfg->dumpGuestCore = true;
> #endif
> --
> 2.46.0
--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
nbdkit - Flexible, fast NBD server with plugins
https://gitlab.com/nbdkit/nbdkit
