Re: [PATCH 02/53] vircgroup: introduce virCgroupV2Available

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On 10/02/2018 10:43 AM, Pavel Hrdina wrote:
> We cannot detect only mount points to figure out whether cgroup v2
> is available because systemd uses cgroup v2 for process tracking and
> all controllers are mounted as cgroup v1 controllers.
> 
> To make sure that this is no the situation we need to check
> 'cgroup.controllers' file if it's not empty to make sure that cgroup
> v2 is not mounted only for process tracking.
> 
> Signed-off-by: Pavel Hrdina <phrdina@xxxxxxxxxx>
> ---
>  src/util/vircgroupv2.c | 52 ++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 52 insertions(+)
> 
> diff --git a/src/util/vircgroupv2.c b/src/util/vircgroupv2.c
> index 23bf81dae2..78497bd31d 100644
> --- a/src/util/vircgroupv2.c
> +++ b/src/util/vircgroupv2.c
> @@ -19,6 +19,10 @@
>   */
>  #include <config.h>
>  
> +#ifdef __linux__
> +# include <mntent.h>
> +#endif /* __linux__ */
> +
>  #include "internal.h"
>  
>  #define __VIR_CGROUP_ALLOW_INCLUDE_PRIV_H__
> @@ -28,7 +32,9 @@
>  #include "vircgroup.h"
>  #include "vircgroupbackend.h"
>  #include "vircgroupv2.h"
> +#include "virfile.h"
>  #include "virlog.h"
> +#include "virstring.h"
>  
>  VIR_LOG_INIT("util.cgroup");
>  
> @@ -41,8 +47,54 @@ VIR_ENUM_IMPL(virCgroupV2Controller, VIR_CGROUP_CONTROLLER_LAST,
>  
>  #ifdef __linux__
>  
> +/* We're looking for one 'cgroup2' fs mount which has some
> + * controllers enabled. */
> +static bool
> +virCgroupV2Available(void)
> +{
> +    bool ret = false;
> +    FILE *mounts = NULL;
> +    struct mntent entry;
> +    char buf[CGROUP_MAX_VAL];
> +
> +    if (!(mounts = fopen("/proc/mounts", "r")))
> +        return false;
> +
> +    while (getmntent_r(mounts, &entry, buf, sizeof(buf)) != NULL) {
> +        if (STREQ(entry.mnt_type, "cgroup2")) {
> +            ret = true;
> +            break;
> +        }
> +    }
> +
> +    /* Systemd uses cgroup v2 for process tracking but no controller is
> +     * available. We should consider this configuration as cgroup v2 is
> +     * not available. */
> +    if (ret) {
> +        int rc;
> +        VIR_AUTOFREE(char *) contFile = NULL;
> +        VIR_AUTOFREE(char *) contStr = NULL;
> +
> +        if (virAsprintf(&contFile, "%s/cgroup.controllers", entry.mnt_dir) < 0)
> +            return false;
> +
> +        rc = virFileReadAll(contFile, 1024 * 1024, &contStr);
> +        if (rc < 0)
> +            return false;
> +
> +        if (STREQ(contStr, ""))
> +            return false;
> +    }
> +
> +    VIR_FORCE_FCLOSE(mounts);
> +    return ret;

This is wrong on two levels. Firstly, if any of those conditions under
'if (ret)' fail, then @mounts is leaked. Secondly, @ret is set but then
it's ignored and false is returned regardless.

Michal

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