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 <[email protected]>
Signed-off-by: Ian Rogers <[email protected]>
---
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;
#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
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 <[email protected]>
> Signed-off-by: Ian Rogers <[email protected]>
> ---
> 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 <[email protected]> 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 <[email protected]>
> > Signed-off-by: Ian Rogers <[email protected]>
> > ---
> > 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 <[email protected]>
[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 <[email protected]>
Thanks, applied.
- Arnaldo
> [1] https://wiki.sei.cmu.edu/confluence/display/c/SIG31-C.+Do+not+access+shared+objects+in+signal+handlers
--
- Arnaldo