I'm seeing the following warning compiling with Clang:
kernel/time/posix-cpu-timers.c:1397:29: warning: shifting a negative
signed value is undefined
[-Wshift-negative-value]
return posix_cpu_clock_get(THREAD_CLOCK, tp);
^~~~~~~~~~~~
kernel/time/posix-cpu-timers.c:1367:22: note: expanded from macro 'THREAD_CLOCK'
#define THREAD_CLOCK MAKE_THREAD_CPUCLOCK(0, CPUCLOCK_SCHED)
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
./include/linux/posix-timers.h:48:2: note: expanded from macro
'MAKE_THREAD_CPUCLOCK'
MAKE_PROCESS_CPUCLOCK((tid), (clock) | CPUCLOCK_PERTHREAD_MASK)
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
./include/linux/posix-timers.h:46:23: note: expanded from macro
'MAKE_PROCESS_CPUCLOCK'
((~(clockid_t) (pid) << 3) | (clockid_t) (clock))
~~~~~~~~~~~~~~~~~~ ^
If I understand C's operator precedence rules
(http://en.cppreference.com/w/c/language/operator_precedence)
correctly, then I suspect the problem is in the sub-expression:
(~(clockid_t) (pid) << 3)
where pid (an argument to the macro) is first cast to a clockid_t (aka
[signed] int), then negated, then shifted by 3 (oops, undefined
behavior).
Should the result after negation be cast to an unsigned int, or should
the left shift happen before negation?
CPUCLOCK_PID and CLOCKID_TO_FD seem to shift then negate, but
FD_TO_CLOCKID seems to have the same issue as MAKE_PROCESS_CPUCLOCK.
Changing the sub-expression to:
(~(clockid_t) ((pid) << 3))
changes what it evaluates to. Changing it to:
(~(unsigned) (pid) << 3)
or
((unsigned) ~(clockid_t) (pid) << 3)
or
(((unsigned) ~(clockid_t) (pid)) << 3) /* ugly */
does not.
I'm happy to send a patch with your suggestion.
On Sat, 23 Dec 2017, Nick Desaulniers wrote:
> I'm seeing the following warning compiling with Clang:
>
> kernel/time/posix-cpu-timers.c:1397:29: warning: shifting a negative
> signed value is undefined
> [-Wshift-negative-value]
> return posix_cpu_clock_get(THREAD_CLOCK, tp);
> ^~~~~~~~~~~~
> kernel/time/posix-cpu-timers.c:1367:22: note: expanded from macro 'THREAD_CLOCK'
> #define THREAD_CLOCK MAKE_THREAD_CPUCLOCK(0, CPUCLOCK_SCHED)
> ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> ./include/linux/posix-timers.h:48:2: note: expanded from macro
> 'MAKE_THREAD_CPUCLOCK'
> MAKE_PROCESS_CPUCLOCK((tid), (clock) | CPUCLOCK_PERTHREAD_MASK)
> ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> ./include/linux/posix-timers.h:46:23: note: expanded from macro
> 'MAKE_PROCESS_CPUCLOCK'
> ((~(clockid_t) (pid) << 3) | (clockid_t) (clock))
> ~~~~~~~~~~~~~~~~~~ ^
>
> If I understand C's operator precedence rules
> (http://en.cppreference.com/w/c/language/operator_precedence)
> correctly, then I suspect the problem is in the sub-expression:
>
> (~(clockid_t) (pid) << 3)
>
> where pid (an argument to the macro) is first cast to a clockid_t (aka
> [signed] int), then negated, then shifted by 3 (oops, undefined
> behavior).
>
> Should the result after negation be cast to an unsigned int, or should
> the left shift happen before negation?
>
> CPUCLOCK_PID and CLOCKID_TO_FD seem to shift then negate, but
> FD_TO_CLOCKID seems to have the same issue as MAKE_PROCESS_CPUCLOCK.
>
> Changing the sub-expression to:
>
> (~(clockid_t) ((pid) << 3))
>
> changes what it evaluates to. Changing it to:
>
> (~(unsigned) (pid) << 3)
>
> or
>
> ((unsigned) ~(clockid_t) (pid) << 3)
>
> or
>
> (((unsigned) ~(clockid_t) (pid)) << 3) /* ugly */
All of these are butt ugly. And the same problem exists for FD_TO_CLOCKID.
My preference would be to replace all these crappy macros with simple
inline functions.
Thanks,
tglx
Shifting a negative signed number is undefined behavior. Looking at the
macros MAKE_PROCESS_CPUCLOCK and FD_TO_CLOCKID, it seems that the
subexpression:
(~(clockid_t) (pid) << 3)
where clockid_t resolves to a signed int, which once negated, is
undefined behavior to shift the value of if the results thus far are
negative.
It was further suggested to make these macros into inline functions.
Suggested-by: Thomas Gleixner <[email protected]>
Signed-off-by: Nick Desaulniers <[email protected]>
---
include/linux/posix-timers.h | 25 +++++++++++++++++++------
kernel/time/posix-clock.c | 2 +-
kernel/time/posix-cpu-timers.c | 4 ++--
tools/testing/selftests/ptp/testptp.c | 4 +---
4 files changed, 23 insertions(+), 12 deletions(-)
diff --git a/include/linux/posix-timers.h b/include/linux/posix-timers.h
index 672c4f3..b3d87c0 100644
--- a/include/linux/posix-timers.h
+++ b/include/linux/posix-timers.h
@@ -42,13 +42,26 @@ struct cpu_timer_list {
#define CLOCKFD CPUCLOCK_MAX
#define CLOCKFD_MASK (CPUCLOCK_PERTHREAD_MASK|CPUCLOCK_CLOCK_MASK)
-#define MAKE_PROCESS_CPUCLOCK(pid, clock) \
- ((~(clockid_t) (pid) << 3) | (clockid_t) (clock))
-#define MAKE_THREAD_CPUCLOCK(tid, clock) \
- MAKE_PROCESS_CPUCLOCK((tid), (clock) | CPUCLOCK_PERTHREAD_MASK)
+static inline clockid_t make_process_cpuclock(const unsigned int pid,
+ const clockid_t clock)
+{
+ return ((~pid) << 3) | clock;
+}
+static inline clockid_t make_thread_cpuclock(const unsigned int tid,
+ const clockid_t clock)
+{
+ return make_process_cpuclock(tid, clock | CPUCLOCK_PERTHREAD_MASK);
+}
-#define FD_TO_CLOCKID(fd) ((~(clockid_t) (fd) << 3) | CLOCKFD)
-#define CLOCKID_TO_FD(clk) ((unsigned int) ~((clk) >> 3))
+inline int fd_to_clockid(const int fd)
+{
+ return (int) make_process_cpuclock((unsigned int) fd, CLOCKFD);
+}
+
+inline int clockid_to_fd(const clockid_t clk)
+{
+ return ~(clk >> 3);
+}
#define REQUEUE_PENDING 1
diff --git a/kernel/time/posix-clock.c b/kernel/time/posix-clock.c
index 17cdc55..cc91d90 100644
--- a/kernel/time/posix-clock.c
+++ b/kernel/time/posix-clock.c
@@ -216,7 +216,7 @@ struct posix_clock_desc {
static int get_clock_desc(const clockid_t id, struct posix_clock_desc *cd)
{
- struct file *fp = fget(CLOCKID_TO_FD(id));
+ struct file *fp = fget(clockid_to_fd(id));
int err = -EINVAL;
if (!fp)
diff --git a/kernel/time/posix-cpu-timers.c b/kernel/time/posix-cpu-timers.c
index 1f27887a..cef79ca 100644
--- a/kernel/time/posix-cpu-timers.c
+++ b/kernel/time/posix-cpu-timers.c
@@ -1363,8 +1363,8 @@ static long posix_cpu_nsleep_restart(struct restart_block *restart_block)
return do_cpu_nanosleep(which_clock, TIMER_ABSTIME, &t);
}
-#define PROCESS_CLOCK MAKE_PROCESS_CPUCLOCK(0, CPUCLOCK_SCHED)
-#define THREAD_CLOCK MAKE_THREAD_CPUCLOCK(0, CPUCLOCK_SCHED)
+#define PROCESS_CLOCK make_process_cpuclock(0, CPUCLOCK_SCHED)
+#define THREAD_CLOCK make_thread_cpuclock(0, CPUCLOCK_SCHED)
static int process_cpu_clock_getres(const clockid_t which_clock,
struct timespec64 *tp)
diff --git a/tools/testing/selftests/ptp/testptp.c b/tools/testing/selftests/ptp/testptp.c
index 5d2eae1..a5d8f0a 100644
--- a/tools/testing/selftests/ptp/testptp.c
+++ b/tools/testing/selftests/ptp/testptp.c
@@ -60,9 +60,7 @@ static int clock_adjtime(clockid_t id, struct timex *tx)
static clockid_t get_clockid(int fd)
{
#define CLOCKFD 3
-#define FD_TO_CLOCKID(fd) ((~(clockid_t) (fd) << 3) | CLOCKFD)
-
- return FD_TO_CLOCKID(fd);
+ return (((unsigned int) ~fd) << 3) | CLOCKFD;
}
static void handle_alarm(int s)
--
2.7.4
sorry, fd_to_clockid() should probably return a clockid_t, let me fix that.
Shifting a negative signed number is undefined behavior. Looking at the
macros MAKE_PROCESS_CPUCLOCK and FD_TO_CLOCKID, it seems that the
subexpression:
(~(clockid_t) (pid) << 3)
where clockid_t resolves to a signed int, which once negated, is
undefined behavior to shift the value of if the results thus far are
negative.
It was further suggested to make these macros into inline functions.
Suggested-by: Thomas Gleixner <[email protected]>
Signed-off-by: Nick Desaulniers <[email protected]>
---
Changes in v2:
* change return type of fd_to_clockid() from int to clockid_t
* mark fd_to_clockid() and clockid_to_fd() as static inline, not just
inline.
include/linux/posix-timers.h | 25 +++++++++++++++++++------
kernel/time/posix-clock.c | 2 +-
kernel/time/posix-cpu-timers.c | 4 ++--
tools/testing/selftests/ptp/testptp.c | 4 +---
4 files changed, 23 insertions(+), 12 deletions(-)
diff --git a/include/linux/posix-timers.h b/include/linux/posix-timers.h
index 672c4f3..c85704f 100644
--- a/include/linux/posix-timers.h
+++ b/include/linux/posix-timers.h
@@ -42,13 +42,26 @@ struct cpu_timer_list {
#define CLOCKFD CPUCLOCK_MAX
#define CLOCKFD_MASK (CPUCLOCK_PERTHREAD_MASK|CPUCLOCK_CLOCK_MASK)
-#define MAKE_PROCESS_CPUCLOCK(pid, clock) \
- ((~(clockid_t) (pid) << 3) | (clockid_t) (clock))
-#define MAKE_THREAD_CPUCLOCK(tid, clock) \
- MAKE_PROCESS_CPUCLOCK((tid), (clock) | CPUCLOCK_PERTHREAD_MASK)
+static inline clockid_t make_process_cpuclock(const unsigned int pid,
+ const clockid_t clock)
+{
+ return ((~pid) << 3) | clock;
+}
+static inline clockid_t make_thread_cpuclock(const unsigned int tid,
+ const clockid_t clock)
+{
+ return make_process_cpuclock(tid, clock | CPUCLOCK_PERTHREAD_MASK);
+}
-#define FD_TO_CLOCKID(fd) ((~(clockid_t) (fd) << 3) | CLOCKFD)
-#define CLOCKID_TO_FD(clk) ((unsigned int) ~((clk) >> 3))
+static inline clockid_t fd_to_clockid(const int fd)
+{
+ return make_process_cpuclock((unsigned int) fd, CLOCKFD);
+}
+
+static inline int clockid_to_fd(const clockid_t clk)
+{
+ return ~(clk >> 3);
+}
#define REQUEUE_PENDING 1
diff --git a/kernel/time/posix-clock.c b/kernel/time/posix-clock.c
index 17cdc55..cc91d90 100644
--- a/kernel/time/posix-clock.c
+++ b/kernel/time/posix-clock.c
@@ -216,7 +216,7 @@ struct posix_clock_desc {
static int get_clock_desc(const clockid_t id, struct posix_clock_desc *cd)
{
- struct file *fp = fget(CLOCKID_TO_FD(id));
+ struct file *fp = fget(clockid_to_fd(id));
int err = -EINVAL;
if (!fp)
diff --git a/kernel/time/posix-cpu-timers.c b/kernel/time/posix-cpu-timers.c
index 1f27887a..cef79ca 100644
--- a/kernel/time/posix-cpu-timers.c
+++ b/kernel/time/posix-cpu-timers.c
@@ -1363,8 +1363,8 @@ static long posix_cpu_nsleep_restart(struct restart_block *restart_block)
return do_cpu_nanosleep(which_clock, TIMER_ABSTIME, &t);
}
-#define PROCESS_CLOCK MAKE_PROCESS_CPUCLOCK(0, CPUCLOCK_SCHED)
-#define THREAD_CLOCK MAKE_THREAD_CPUCLOCK(0, CPUCLOCK_SCHED)
+#define PROCESS_CLOCK make_process_cpuclock(0, CPUCLOCK_SCHED)
+#define THREAD_CLOCK make_thread_cpuclock(0, CPUCLOCK_SCHED)
static int process_cpu_clock_getres(const clockid_t which_clock,
struct timespec64 *tp)
diff --git a/tools/testing/selftests/ptp/testptp.c b/tools/testing/selftests/ptp/testptp.c
index 5d2eae1..a5d8f0a 100644
--- a/tools/testing/selftests/ptp/testptp.c
+++ b/tools/testing/selftests/ptp/testptp.c
@@ -60,9 +60,7 @@ static int clock_adjtime(clockid_t id, struct timex *tx)
static clockid_t get_clockid(int fd)
{
#define CLOCKFD 3
-#define FD_TO_CLOCKID(fd) ((~(clockid_t) (fd) << 3) | CLOCKFD)
-
- return FD_TO_CLOCKID(fd);
+ return (((unsigned int) ~fd) << 3) | CLOCKFD;
}
static void handle_alarm(int s)
--
2.7.4
Commit-ID: 29f1b2b0fecfae69e31833836f1da3136696eee5
Gitweb: https://git.kernel.org/tip/29f1b2b0fecfae69e31833836f1da3136696eee5
Author: Nick Desaulniers <[email protected]>
AuthorDate: Thu, 28 Dec 2017 22:11:36 -0500
Committer: Thomas Gleixner <[email protected]>
CommitDate: Thu, 4 Jan 2018 14:57:10 +0100
posix-timers: Prevent UB from shifting negative signed value
Shifting a negative signed number is undefined behavior. Looking at the
macros MAKE_PROCESS_CPUCLOCK and FD_TO_CLOCKID, it seems that the
subexpression:
(~(clockid_t) (pid) << 3)
where clockid_t resolves to a signed int, which once negated, is
undefined behavior to shift the value of if the results thus far are
negative.
It was further suggested to make these macros into inline functions.
Suggested-by: Thomas Gleixner <[email protected]>
Signed-off-by: Nick Desaulniers <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Cc: Dimitri Sivanich <[email protected]>
Cc: Frederic Weisbecker <[email protected]>
Cc: Al Viro <[email protected]>
Cc: [email protected]
Cc: Shuah Khan <[email protected]>
Cc: Deepa Dinamani <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
---
include/linux/posix-timers.h | 25 +++++++++++++++++++------
kernel/time/posix-clock.c | 2 +-
kernel/time/posix-cpu-timers.c | 4 ++--
tools/testing/selftests/ptp/testptp.c | 4 +---
4 files changed, 23 insertions(+), 12 deletions(-)
diff --git a/include/linux/posix-timers.h b/include/linux/posix-timers.h
index 672c4f3..c85704f 100644
--- a/include/linux/posix-timers.h
+++ b/include/linux/posix-timers.h
@@ -42,13 +42,26 @@ struct cpu_timer_list {
#define CLOCKFD CPUCLOCK_MAX
#define CLOCKFD_MASK (CPUCLOCK_PERTHREAD_MASK|CPUCLOCK_CLOCK_MASK)
-#define MAKE_PROCESS_CPUCLOCK(pid, clock) \
- ((~(clockid_t) (pid) << 3) | (clockid_t) (clock))
-#define MAKE_THREAD_CPUCLOCK(tid, clock) \
- MAKE_PROCESS_CPUCLOCK((tid), (clock) | CPUCLOCK_PERTHREAD_MASK)
+static inline clockid_t make_process_cpuclock(const unsigned int pid,
+ const clockid_t clock)
+{
+ return ((~pid) << 3) | clock;
+}
+static inline clockid_t make_thread_cpuclock(const unsigned int tid,
+ const clockid_t clock)
+{
+ return make_process_cpuclock(tid, clock | CPUCLOCK_PERTHREAD_MASK);
+}
-#define FD_TO_CLOCKID(fd) ((~(clockid_t) (fd) << 3) | CLOCKFD)
-#define CLOCKID_TO_FD(clk) ((unsigned int) ~((clk) >> 3))
+static inline clockid_t fd_to_clockid(const int fd)
+{
+ return make_process_cpuclock((unsigned int) fd, CLOCKFD);
+}
+
+static inline int clockid_to_fd(const clockid_t clk)
+{
+ return ~(clk >> 3);
+}
#define REQUEUE_PENDING 1
diff --git a/kernel/time/posix-clock.c b/kernel/time/posix-clock.c
index 17cdc55..cc91d90 100644
--- a/kernel/time/posix-clock.c
+++ b/kernel/time/posix-clock.c
@@ -216,7 +216,7 @@ struct posix_clock_desc {
static int get_clock_desc(const clockid_t id, struct posix_clock_desc *cd)
{
- struct file *fp = fget(CLOCKID_TO_FD(id));
+ struct file *fp = fget(clockid_to_fd(id));
int err = -EINVAL;
if (!fp)
diff --git a/kernel/time/posix-cpu-timers.c b/kernel/time/posix-cpu-timers.c
index 1f27887a..cef79ca 100644
--- a/kernel/time/posix-cpu-timers.c
+++ b/kernel/time/posix-cpu-timers.c
@@ -1363,8 +1363,8 @@ static long posix_cpu_nsleep_restart(struct restart_block *restart_block)
return do_cpu_nanosleep(which_clock, TIMER_ABSTIME, &t);
}
-#define PROCESS_CLOCK MAKE_PROCESS_CPUCLOCK(0, CPUCLOCK_SCHED)
-#define THREAD_CLOCK MAKE_THREAD_CPUCLOCK(0, CPUCLOCK_SCHED)
+#define PROCESS_CLOCK make_process_cpuclock(0, CPUCLOCK_SCHED)
+#define THREAD_CLOCK make_thread_cpuclock(0, CPUCLOCK_SCHED)
static int process_cpu_clock_getres(const clockid_t which_clock,
struct timespec64 *tp)
diff --git a/tools/testing/selftests/ptp/testptp.c b/tools/testing/selftests/ptp/testptp.c
index 5d2eae1..a5d8f0a 100644
--- a/tools/testing/selftests/ptp/testptp.c
+++ b/tools/testing/selftests/ptp/testptp.c
@@ -60,9 +60,7 @@ static int clock_adjtime(clockid_t id, struct timex *tx)
static clockid_t get_clockid(int fd)
{
#define CLOCKFD 3
-#define FD_TO_CLOCKID(fd) ((~(clockid_t) (fd) << 3) | CLOCKFD)
-
- return FD_TO_CLOCKID(fd);
+ return (((unsigned int) ~fd) << 3) | CLOCKFD;
}
static void handle_alarm(int s)