Re: [Qemu-devel] [PATCH RFC] qemu-error: introduce error_report_once

[Eric Blake <eblake@redhat.com>]

On 05/15/2018 04:13 AM, Peter Xu wrote:
> I stole the printk_once() macro.
> 
> I always wanted to be able to print some error directly if there is a
> buffer to dump, however we can't use error_report() really quite often
> when there can be any DDOS attack.  To avoid that, we can introduce a
> print-once function for it.
> 
> CC: Markus Armbruster <armbru@redhat.com>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
> We can for sure introduce similar functions for the rest of the
> error_*() functions, it's just an idea to see whether we'd like it
> in general.
> ---
>   include/qemu/error-report.h | 12 ++++++++++++
>   1 file changed, 12 insertions(+)
> 
> diff --git a/include/qemu/error-report.h b/include/qemu/error-report.h
> index e1c8ae1a52..efebb80e2c 100644
> --- a/include/qemu/error-report.h
> +++ b/include/qemu/error-report.h
> @@ -44,6 +44,18 @@ void error_report(const char *fmt, ...) GCC_FMT_ATTR(1, 2);
>   void warn_report(const char *fmt, ...) GCC_FMT_ATTR(1, 2);
>   void info_report(const char *fmt, ...) GCC_FMT_ATTR(1, 2);
>   
> +#define error_report_once(fmt, ...)             \
> +    ({                                          \
> +        static bool __print_once;               \

Double-underscore names are reserved for the compiler's use, not ours. 
Better would be naming this:

static bool print_once_;

with a trailing underscore, or at most a single leading underscore.

> +        bool __ret_print_once = !__print_once;  \

Same comment for this variable.

> +                                                \
> +        if (!__print_once) {					\
> +            __print_once = true;				\
> +            error_report(fmt, ##__VA_ARGS__);   \
> +        }                                       \
> +        unlikely(__ret_print_once);				\
> +    })
> +
>   const char *error_get_progname(void);
>   extern bool enable_timestamp_msg;
>   
> 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org