Re: [zanussi-trace:ftrace/synth-event-gen-v2 10/12] kernel/trace/trace_kprobe.c:974:17: warning: passing an object that undergoes default argument promotion to 'va_start' has undefined behavior

* Re: [zanussi-trace:ftrace/synth-event-gen-v2 10/12] kernel/trace/trace_kprobe.c:974:17: warning: passing an object that undergoes default argument promotion to 'va_start' has undefined behavior
       [not found] <202001112058.4xALCb72%lkp@intel.com>
@ 2020-01-13 17:20 ` Nick Desaulniers
  2020-01-13 20:11   ` Tom Zanussi
  0 siblings, 1 reply; 3+ messages in thread
From: Nick Desaulniers @ 2020-01-13 17:20 UTC (permalink / raw)
  To: kbuild-all

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Hi Tom,
Below is a report from a 0day bot build w/ Clang, can you please take
a look? (Apologies if this has been previously reported).  In the
past, -Wvarargs warnings are usually related to the last parameter of
a va_arg function undergoing implicit promotion (which is explicitly
UB, IIRC).

On Sat, Jan 11, 2020 at 4:35 AM kbuild test robot <lkp@intel.com> wrote:
>
> CC: kbuild-all(a)lists.01.org
> TO: Tom Zanussi <zanussi@kernel.org>
>
> tree:   https://git.kernel.org/pub/scm/linux/kernel/git/zanussi/linux-trace.git ftrace/synth-event-gen-v2
> head:   91ee64186d5894724e276c4e7fad70446d7a02a7
> commit: 5f052546541d6cc5ad00e28aca6376c221db5c7e [10/12] tracing: Add kprobe event command generation functions
> config: x86_64-defconfig (attached as .config)
> compiler: clang version 10.0.0 (git://gitmirror/llvm_project 016bf03ef6fcd9dce43b0c17971f76323f07a684)
> reproduce:
>         git checkout 5f052546541d6cc5ad00e28aca6376c221db5c7e
>         # save the attached .config to linux build tree
>         make ARCH=x86_64
>
> If you fix the issue, kindly add following tag
> Reported-by: kbuild test robot <lkp@intel.com>
>
> All warnings (new ones prefixed by >>):
>
> >> kernel/trace/trace_kprobe.c:974:17: warning: passing an object that undergoes default argument promotion to 'va_start' has undefined behavior [-Wvarargs]
>            va_start(args, kretprobe);
>                           ^
>    kernel/trace/trace_kprobe.c:947:30: note: parameter of type 'bool' (aka '_Bool') is declared here
>                         const char *loc, bool kretprobe, ...)
>                                               ^
>    1 warning generated.
>
> vim +/va_start +974 kernel/trace/trace_kprobe.c
>
>    925
>    926  /**
>    927   * __gen_kprobe_cmd - Generate a synthetic event command from arg list
>    928   * @cmd: A pointer to the dynevent_cmd struct representing the new event
>    929   * @name: The name of the kprobe event
>    930   * @loc: The location of the kprobe event
>    931   * @kretprobe: Is this a return probe?
>    932   * @args: Variable number of arg (pairs), one pair for each field
>    933   *
>    934   * NOTE: Users normally won't want to call this function directly, but
>    935   * rather use the gen_kprobe_cmd() wrapper, which automatically adds a
>    936   * NULL to the end of the arg list.  If this function is used
>    937   * directly, make suer he last arg in the variable arg list is NULL.
>    938   *
>    939   * Generate a kprobe event command to be executed by
>    940   * create_dynevent().  This function can be used to generate the
>    941   * complete command or only the first part of it; in the latter case,
>    942   * add_probe_fields() can be used to add more fields following this.
>    943   *
>    944   * Return: 0 if successful, error otherwise.
>    945   */
>    946  int __gen_kprobe_cmd(struct dynevent_cmd *cmd, const char *name,
>    947                       const char *loc, bool kretprobe, ...)
>    948  {
>    949          char buf[MAX_EVENT_NAME_LEN];
>    950          struct dynevent_arg arg;
>    951          va_list args;
>    952          int ret;
>    953
>    954          if (cmd->type != DYNEVENT_TYPE_KPROBE)
>    955                  return -EINVAL;
>    956
>    957          if (kretprobe)
>    958                  snprintf(buf, MAX_EVENT_NAME_LEN, "r:%s", name);
>    959          else
>    960                  snprintf(buf, MAX_EVENT_NAME_LEN, "p:%s", name);
>    961
>    962          dynevent_arg_init(&arg, NULL, 0);
>    963          arg.str = buf;
>    964          ret = add_dynevent_arg(cmd, &arg);
>    965          if (ret)
>    966                  return ret;
>    967
>    968          dynevent_arg_init(&arg, NULL, 0);
>    969          arg.str = loc;
>    970          ret = add_dynevent_arg(cmd, &arg);
>    971          if (ret)
>    972                  return ret;
>    973
>  > 974          va_start(args, kretprobe);
>    975          for (;;) {
>    976                  const char *field;
>    977
>    978                  field = va_arg(args, const char *);
>    979                  if (!field)
>    980                          break;
>    981
>    982                  if (++cmd->n_fields > MAX_TRACE_ARGS) {
>    983                          ret = -EINVAL;
>    984                          break;
>    985                  }
>    986
>    987                  dynevent_arg_init(&arg, NULL, 0);
>    988                  arg.str = field;
>    989                  ret = add_dynevent_arg(cmd, &arg);
>    990                  if (ret)
>    991                          break;
>    992          }
>    993          va_end(args);
>    994
>    995          return ret;
>    996  }
>    997  EXPORT_SYMBOL_GPL(__gen_kprobe_cmd);
>    998
>
> ---
> 0-DAY kernel test infrastructure                 Open Source Technology Center
> https://lists.01.org/hyperkitty/list/kbuild-all(a)lists.01.org Intel Corporation

-- 
Thanks,
~Nick Desaulniers

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