musl - Re: [PATCH] add pthread_setname

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Date: Fri, 16 Sep 2016 12:03:16 +0300 (MSK)
From: Alexander Monakov <amonakov@...ras.ru>
To: musl@...ts.openwall.com
Subject: Re: [PATCH] add pthread_setname_np

Hi,

On Thu, 15 Sep 2016, Felix Janda wrote:
> --- /dev/null
> +++ b/src/thread/pthread_setname_np.c
> @@ -0,0 +1,25 @@
> +#include <fcntl.h>
> +#include <string.h>
> +#include <unistd.h>
> +#include <sys/prctl.h>
> +
> +#include "pthread_impl.h"

This should bring in the declaration of pthread_setname_np from pthread.h by
#defin'ing _GNU_SOURCE prior to inclusion.
(today, this rule is not enforced with warnings and thus not always followed in
musl, for example the recent pthread_tryjoin_np patch missed that as well)

> +int pthread_setname_np(pthread_t thread, const char *name)
> +{
> +	int fd, cs, status = 0;
> +	char f[sizeof "/proc/self/task//comm" + 3*sizeof(int)];
> +	size_t len;
> +
> +	if ((len = strnlen(name, 16)) > 15) return ERANGE;
> +
> +	if (thread == pthread_self())

This likely should use the static inline function __pthread_self()?

> +		return prctl(PR_SET_NAME, (unsigned long)name, 0, 0, 0) ? errno : 0;

First, prctl is declared as a variadic function, so zeros should be passed with
the right type (0ul); passing fewer than 5 arguments will cause musl's
implementation of prctl to invoke UB, since it always retrieves 4 variadic arguments.

Second, while I don't have a strong opinion on whether musl wants to use prctl
here (my previous comment in this thread was based on the wrong idea that prctl
would be sufficient in all cases - sorry about that), I think some tuning would
be nice in case the decision is to use prctl. So, either

  return -__syscall(SYS_prctl, PR_SET_NAME, name);

(not sure if Rich would like it), or

  if (thread == pthread_self()) {
    status = prctl(...);
  } else {
    ...
    fd = status = open(f, O_WRONLY);
    if (fd >= 0) {
      status = write(fd, name, len);
      close(fd);
    }
    pthread_setcancelstate(cs, 0);
  }
  return status ? errno : 0;

> +	snprintf(f, sizeof f, "/proc/self/task/%d/comm", thread->tid);
> +	pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, &cs);
> +	if ((fd = open(f, O_WRONLY)) < 0 || write(fd, name, len) < 0) status = errno;
> +	if (fd >= 0) close(fd);
> +	pthread_setcancelstate(cs, 0);
> +	return status;
> +}

Thanks.
Alexander

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