[v1] perf record: Fix event fd races

  The write call may set errno which is problematic if occurring in a
function also setting errno. Save and restore errno around the write
call.

done_fd may be used after close, clear it as part of the close and
check its validity in the signal handler.

Suggested-by: Greg Thelen <gthelen@google.com>
Signed-off-by: Ian Rogers <irogers@google.com>
---
 tools/perf/builtin-record.c | 41 ++++++++++++++++++++++---------------
 1 file changed, 25 insertions(+), 16 deletions(-)
  

Hi Ian,

On Sun, Oct 23, 2022 at 06:10:24PM -0700, Ian Rogers wrote:
> The write call may set errno which is problematic if occurring in a
> function also setting errno. Save and restore errno around the write
> call.
> 
> done_fd may be used after close, clear it as part of the close and
> check its validity in the signal handler.
> 
> Suggested-by: Greg Thelen <gthelen@google.com>
> Signed-off-by: Ian Rogers <irogers@google.com>
> ---
>  tools/perf/builtin-record.c | 41 ++++++++++++++++++++++---------------
>  1 file changed, 25 insertions(+), 16 deletions(-)
> 
> diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
> index 52d254b1530c..e128b855ddde 100644
> --- a/tools/perf/builtin-record.c
> +++ b/tools/perf/builtin-record.c
> @@ -649,7 +649,7 @@ static int record__pushfn(struct mmap *map, void *to, void *bf, size_t size)
>  static volatile int signr = -1;
>  static volatile int child_finished;
>  #ifdef HAVE_EVENTFD_SUPPORT
> -static int done_fd = -1;
> +static volatile int done_fd = -1;

Here is a bit suspecious for adding volatile qualifier.  See the
document: process/volatile-considered-harmful.rst.

I know the document is mainly for kernel programming, but seems to me
it's also valid for C programming in userspace.

I not sure what's the purpose for adding volatile for done_fd, if we
really have concern for reading any stale value for done_fd, should we
use WRITE_ONCE/READ_ONCE?

The rest changes look good to me.

Thanks,
Leo

>  #endif
>  
>  static void sig_handler(int sig)
> @@ -661,19 +661,24 @@ static void sig_handler(int sig)
>  
>  	done = 1;
>  #ifdef HAVE_EVENTFD_SUPPORT
> -{
> -	u64 tmp = 1;
> -	/*
> -	 * It is possible for this signal handler to run after done is checked
> -	 * in the main loop, but before the perf counter fds are polled. If this
> -	 * happens, the poll() will continue to wait even though done is set,
> -	 * and will only break out if either another signal is received, or the
> -	 * counters are ready for read. To ensure the poll() doesn't sleep when
> -	 * done is set, use an eventfd (done_fd) to wake up the poll().
> -	 */
> -	if (write(done_fd, &tmp, sizeof(tmp)) < 0)
> -		pr_err("failed to signal wakeup fd, error: %m\n");
> -}
> +	if (done_fd >= 0) {
> +		u64 tmp = 1;
> +		int orig_errno = errno;
> +
> +		/*
> +		 * It is possible for this signal handler to run after done is
> +		 * checked in the main loop, but before the perf counter fds are
> +		 * polled. If this happens, the poll() will continue to wait
> +		 * even though done is set, and will only break out if either
> +		 * another signal is received, or the counters are ready for
> +		 * read. To ensure the poll() doesn't sleep when done is set,
> +		 * use an eventfd (done_fd) to wake up the poll().
> +		 */
> +		if (write(done_fd, &tmp, sizeof(tmp)) < 0)
> +			pr_err("failed to signal wakeup fd, error: %m\n");
> +
> +		errno = orig_errno;
> +	}
>  #endif // HAVE_EVENTFD_SUPPORT
>  }
>  
> @@ -2834,8 +2839,12 @@ static int __cmd_record(struct record *rec, int argc, const char **argv)
>  
>  out_delete_session:
>  #ifdef HAVE_EVENTFD_SUPPORT
> -	if (done_fd >= 0)
> -		close(done_fd);
> +	if (done_fd >= 0) {
> +		fd = done_fd;
> +		done_fd = -1;
> +
> +		close(fd);
> +	}
>  #endif
>  	zstd_fini(&session->zstd_data);
>  	perf_session__delete(session);
> -- 
> 2.38.0.135.g90850a2211-goog
>
  

On Sun, Oct 23, 2022 at 7:56 PM Leo Yan <leo.yan@linaro.org> wrote:
>
> Hi Ian,
>
> On Sun, Oct 23, 2022 at 06:10:24PM -0700, Ian Rogers wrote:
> > The write call may set errno which is problematic if occurring in a
> > function also setting errno. Save and restore errno around the write
> > call.
> >
> > done_fd may be used after close, clear it as part of the close and
> > check its validity in the signal handler.
> >
> > Suggested-by: Greg Thelen <gthelen@google.com>
> > Signed-off-by: Ian Rogers <irogers@google.com>
> > ---
> >  tools/perf/builtin-record.c | 41 ++++++++++++++++++++++---------------
> >  1 file changed, 25 insertions(+), 16 deletions(-)
> >
> > diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
> > index 52d254b1530c..e128b855ddde 100644
> > --- a/tools/perf/builtin-record.c
> > +++ b/tools/perf/builtin-record.c
> > @@ -649,7 +649,7 @@ static int record__pushfn(struct mmap *map, void *to, void *bf, size_t size)
> >  static volatile int signr = -1;
> >  static volatile int child_finished;
> >  #ifdef HAVE_EVENTFD_SUPPORT
> > -static int done_fd = -1;
> > +static volatile int done_fd = -1;
>
> Here is a bit suspecious for adding volatile qualifier.  See the
> document: process/volatile-considered-harmful.rst.
>
> I know the document is mainly for kernel programming, but seems to me
> it's also valid for C programming in userspace.
>
> I not sure what's the purpose for adding volatile for done_fd, if we
> really have concern for reading any stale value for done_fd, should we
> use WRITE_ONCE/READ_ONCE?

We could just switch to C11 and stdatomic. The volatile is consistent
with the code above and more consistent with the expectation of
writing to a variable that is read in a signal handler.

Thanks,
Ian

> The rest changes look good to me.
>
> Thanks,
> Leo
>
> >  #endif
> >
> >  static void sig_handler(int sig)
> > @@ -661,19 +661,24 @@ static void sig_handler(int sig)
> >
> >       done = 1;
> >  #ifdef HAVE_EVENTFD_SUPPORT
> > -{
> > -     u64 tmp = 1;
> > -     /*
> > -      * It is possible for this signal handler to run after done is checked
> > -      * in the main loop, but before the perf counter fds are polled. If this
> > -      * happens, the poll() will continue to wait even though done is set,
> > -      * and will only break out if either another signal is received, or the
> > -      * counters are ready for read. To ensure the poll() doesn't sleep when
> > -      * done is set, use an eventfd (done_fd) to wake up the poll().
> > -      */
> > -     if (write(done_fd, &tmp, sizeof(tmp)) < 0)
> > -             pr_err("failed to signal wakeup fd, error: %m\n");
> > -}
> > +     if (done_fd >= 0) {
> > +             u64 tmp = 1;
> > +             int orig_errno = errno;
> > +
> > +             /*
> > +              * It is possible for this signal handler to run after done is
> > +              * checked in the main loop, but before the perf counter fds are
> > +              * polled. If this happens, the poll() will continue to wait
> > +              * even though done is set, and will only break out if either
> > +              * another signal is received, or the counters are ready for
> > +              * read. To ensure the poll() doesn't sleep when done is set,
> > +              * use an eventfd (done_fd) to wake up the poll().
> > +              */
> > +             if (write(done_fd, &tmp, sizeof(tmp)) < 0)
> > +                     pr_err("failed to signal wakeup fd, error: %m\n");
> > +
> > +             errno = orig_errno;
> > +     }
> >  #endif // HAVE_EVENTFD_SUPPORT
> >  }
> >
> > @@ -2834,8 +2839,12 @@ static int __cmd_record(struct record *rec, int argc, const char **argv)
> >
> >  out_delete_session:
> >  #ifdef HAVE_EVENTFD_SUPPORT
> > -     if (done_fd >= 0)
> > -             close(done_fd);
> > +     if (done_fd >= 0) {
> > +             fd = done_fd;
> > +             done_fd = -1;
> > +
> > +             close(fd);
> > +     }
> >  #endif
> >       zstd_fini(&session->zstd_data);
> >       perf_session__delete(session);
> > --
> > 2.38.0.135.g90850a2211-goog
> >
  

On Sun, Oct 23, 2022 at 10:33:30PM -0700, Ian Rogers wrote:

[...]

> > > +static volatile int done_fd = -1;
> >
> > Here is a bit suspecious for adding volatile qualifier.  See the
> > document: process/volatile-considered-harmful.rst.
> >
> > I know the document is mainly for kernel programming, but seems to me
> > it's also valid for C programming in userspace.
> >
> > I not sure what's the purpose for adding volatile for done_fd, if we
> > really have concern for reading any stale value for done_fd, should we
> > use WRITE_ONCE/READ_ONCE?
> 
> We could just switch to C11 and stdatomic. The volatile is consistent
> with the code above and more consistent with the expectation of
> writing to a variable that is read in a signal handler.

Thanks for the info for C11 and stdatomic.h.  The documentation [1] says
the safe way is for accessing shared data in signal handler is:

  static volatile sig_atomic_t done_fd = -1;

It's fine if you want to use another patch to address this issue, this
patch for fixing errno is fine for me:

Reviewed-by: Leo Yan <leo.yan@linaro.org>

[1] https://wiki.sei.cmu.edu/confluence/display/c/SIG31-C.+Do+not+access+shared+objects+in+signal+handlers
  

Em Mon, Oct 24, 2022 at 05:16:56PM +0800, Leo Yan escreveu:
> On Sun, Oct 23, 2022 at 10:33:30PM -0700, Ian Rogers wrote:
> 
> [...]
> 
> > > > +static volatile int done_fd = -1;
> > >
> > > Here is a bit suspecious for adding volatile qualifier.  See the
> > > document: process/volatile-considered-harmful.rst.
> > >
> > > I know the document is mainly for kernel programming, but seems to me
> > > it's also valid for C programming in userspace.
> > >
> > > I not sure what's the purpose for adding volatile for done_fd, if we
> > > really have concern for reading any stale value for done_fd, should we
> > > use WRITE_ONCE/READ_ONCE?
> > 
> > We could just switch to C11 and stdatomic. The volatile is consistent
> > with the code above and more consistent with the expectation of
> > writing to a variable that is read in a signal handler.
> 
> Thanks for the info for C11 and stdatomic.h.  The documentation [1] says
> the safe way is for accessing shared data in signal handler is:
> 
>   static volatile sig_atomic_t done_fd = -1;
> 
> It's fine if you want to use another patch to address this issue, this
> patch for fixing errno is fine for me:
> 
> Reviewed-by: Leo Yan <leo.yan@linaro.org>



Thanks, applied.

- Arnaldo

 
> [1] https://wiki.sei.cmu.edu/confluence/display/c/SIG31-C.+Do+not+access+shared+objects+in+signal+handlers