[libvirt PATCH 12/17] util: Try to get limits from /proc

[Daniel P. Berrangé]

On Fri, Mar 05, 2021 at 08:13:59PM +0100, Andrea Bolognani wrote:
> Calling prlimit() requires elevated privileges, specifically
> CAP_SYS_RESOURCE, and getrlimit() only works for the current
> process which is too limiting for our needs; /proc/$pid/limits,
> on the other hand, can be read by any process, so implement
> parsing that file as a fallback for when prlimit() fails.
> 
> This is useful in containerized environments.
> 
> Signed-off-by: Andrea Bolognani <abologna at redhat.com>
> ---
>  src/util/virprocess.c | 98 +++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 98 insertions(+)
> 
> diff --git a/src/util/virprocess.c b/src/util/virprocess.c
> index abce522bf3..e62ec379a6 100644
> --- a/src/util/virprocess.c
> +++ b/src/util/virprocess.c
> @@ -757,6 +757,95 @@ virProcessSetRLimit(int resource,
>  #endif /* WITH_SETRLIMIT */
>  
>  #if WITH_GETRLIMIT
> +static const char*
> +virProcessLimitResourceToLabel(int resource)
> +{
> +    switch (resource) {
> +# if defined(RLIMIT_MEMLOCK)
> +        case RLIMIT_MEMLOCK:
> +            return "Max locked memory";
> +# endif /* defined(RLIMIT_MEMLOCK) */
> +
> +# if defined(RLIMIT_NPROC)
> +        case RLIMIT_NPROC:
> +            return "Max processes";
> +# endif /* defined(RLIMIT_NPROC) */
> +
> +# if defined(RLIMIT_NOFILE)
> +        case RLIMIT_NOFILE:
> +            return "Max open files";
> +# endif /* defined(RLIMIT_NOFILE) */
> +
> +# if defined(RLIMIT_CORE)
> +        case RLIMIT_CORE:
> +            return "Max core file size";
> +# endif /* defined(RLIMIT_CORE) */
> +
> +        default:
> +            return NULL;
> +    }
> +}
> +
> +static int
> +virProcessGetLimitFromProc(pid_t pid,
> +                           int resource,
> +                           struct rlimit *limit)
> +{
> +    g_autofree char *procfile = NULL;
> +    g_autofree char *buf = NULL;
> +    g_auto(GStrv) lines = NULL;
> +    const char *label;
> +    size_t len;
> +    size_t i;
> +
> +    if (!(label = virProcessLimitResourceToLabel(resource))) {
> +        virReportError(VIR_ERR_INTERNAL_ERROR,
> +                       _("Unknown resource %d requested for process %lld"),
> +                       resource, (long long)pid);
> +        return -1;

Setting errors on -1

> +    }
> +
> +    procfile = g_strdup_printf("/proc/%lld/limits", (long long)pid);
> +
> +    if (!g_file_get_contents(procfile, &buf, &len, NULL))
> +        return -1;

Not setting errors on -1

> +
> +    if (!(lines = g_strsplit(buf, "\n", 0)))
> +        return -1;

....

> +
> +    for (i = 0; lines[i]; i++) {
> +        g_autofree char *softLimit = NULL;
> +        g_autofree char *hardLimit = NULL;
> +        char *line = lines[i];
> +        unsigned long long tmp;
> +
> +        if (!STRPREFIX(line, label))
> +            continue;
> +
> +        line += strlen(label);
> +
> +        if (sscanf(line, "%ms %ms %*s", &softLimit, &hardLimit) < 2)
> +            return -1;
> +
> +        if (STREQ(softLimit, "unlimited")) {
> +            limit->rlim_cur = RLIM_INFINITY;
> +        } else {
> +            if (virStrToLong_ull(softLimit, NULL, 10, &tmp) < 0)
> +                return -1;
> +            limit->rlim_cur = tmp;
> +        }
> +        if (STREQ(hardLimit, "unlimited")) {
> +            limit->rlim_max = RLIM_INFINITY;
> +        } else {
> +            if (virStrToLong_ull(hardLimit, NULL, 10, &tmp) < 0)
> +                return -1;
> +            limit->rlim_max = tmp;
> +        }
> +    }
> +
> +    return 0;
> +}
> +
>  static int
>  virProcessGetLimit(pid_t pid,
>                     int resource,
> @@ -768,6 +857,15 @@ virProcessGetLimit(pid_t pid,
>      if (virProcessPrLimit(pid, resource, NULL, old_limit) == 0)
>          return 0;
>  
> +    /* For whatever reason, using prlimit() on another process - even
> +     * when it's just to obtain the current limit rather than changing
> +     * it - requires CAP_SYS_RESOURCE, which we might not have in a
> +     * containerized environment; on the other hand, no particular
> +     * permission is needed to poke around /proc, so try that if going
> +     * through the syscall didn't work */
> +    if (virProcessGetLimitFromProc(pid, resource, old_limit) == 0)
> +        return 0;

This ought to be conditional for Linux only and error reporting needs
to be made consistent.


> +
>      if (same_process && virProcessGetRLimit(resource, old_limit) == 0)
>          return 0;
>  
> -- 
> 2.26.2
> 

Regards,
Daniel
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