[PATCH] kselftest: Mark functions that unconditionally call exit() as __noreturn

[Nathan Chancellor <nathan@kernel.org>]

After commit 6d029c25b71f ("selftests/timers/posix_timers: Reimplement
check_timer_distribution()"), clang warns:

  tools/testing/selftests/timers/../kselftest.h:398:6: warning: variable 'major' is used uninitialized whenever '||' condition is true [-Wsometimes-uninitialized]
    398 |         if (uname(&info) || sscanf(info.release, "%u.%u.", &major, &minor) != 2)
        |             ^~~~~~~~~~~~
  tools/testing/selftests/timers/../kselftest.h:401:9: note: uninitialized use occurs here
    401 |         return major > min_major || (major == min_major && minor >= min_minor);
        |                ^~~~~
  tools/testing/selftests/timers/../kselftest.h:398:6: note: remove the '||' if its condition is always false
    398 |         if (uname(&info) || sscanf(info.release, "%u.%u.", &major, &minor) != 2)
        |             ^~~~~~~~~~~~~~~
  tools/testing/selftests/timers/../kselftest.h:395:20: note: initialize the variable 'major' to silence this warning
    395 |         unsigned int major, minor;
        |                           ^
        |                            = 0

This is a false positive because if uname() fails, ksft_exit_fail_msg()
will be called, which unconditionally calls exit(), a noreturn function.
However, clang does not know that ksft_exit_fail_msg() will call exit()
at the point in the pipeline that the warning is emitted because
inlining has not occurred, so it assumes control flow will resume
normally after ksft_exit_fail_msg() is called.

Make it clear to clang that all of the functions that call exit()
unconditionally in kselftest.h are noreturn transitively by marking them
explicitly with '__attribute__((__noreturn__))', which clears up the
warning above and any future warnings that may appear for the same
reason.

Fixes: 6d029c25b71f ("selftests/timers/posix_timers: Reimplement check_timer_distribution()")
Reported-by: John Stultz <jstultz@google.com>
Closes: https://lore.kernel.org/all/20240410232637.4135564-2-jstultz@google.com/
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
---
I have based this change on timers/urgent, as the commit that introduces
this particular warning is there and it is marked for stable, even
though this appears to be a generic kselftest issue. I think it makes
the most sense for this change to go via timers/urgent with Shuah's ack.
While __noreturn with a return type other than 'void' does not make much
sense semantically, there are many places that these functions are used
as the return value for other functions such as main(), so I did not
change the return type of these functions from 'int' to 'void' to
minimize the necessary changes for a backport (it is an existing issue
anyways).

I see there is another instance of this problem that will need to be
addressed in -next, introduced by commit f07041728422 ("selftests: add
ksft_exit_fail_perror()").
---
 tools/testing/selftests/kselftest.h | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/tools/testing/selftests/kselftest.h b/tools/testing/selftests/kselftest.h
index 973b18e156b2..0591974b57e0 100644
--- a/tools/testing/selftests/kselftest.h
+++ b/tools/testing/selftests/kselftest.h
@@ -80,6 +80,9 @@
 #define KSFT_XPASS 3
 #define KSFT_SKIP  4
 
+#ifndef __noreturn
+#define __noreturn       __attribute__((__noreturn__))
+#endif
 #define __printf(a, b)   __attribute__((format(printf, a, b)))
 
 /* counters */
@@ -300,13 +303,13 @@ void ksft_test_result_code(int exit_code, const char *test_name,
 	va_end(args);
 }
 
-static inline int ksft_exit_pass(void)
+static inline __noreturn int ksft_exit_pass(void)
 {
 	ksft_print_cnts();
 	exit(KSFT_PASS);
 }
 
-static inline int ksft_exit_fail(void)
+static inline __noreturn int ksft_exit_fail(void)
 {
 	ksft_print_cnts();
 	exit(KSFT_FAIL);
@@ -333,7 +336,7 @@ static inline int ksft_exit_fail(void)
 		  ksft_cnt.ksft_xfail +	\
 		  ksft_cnt.ksft_xskip)
 
-static inline __printf(1, 2) int ksft_exit_fail_msg(const char *msg, ...)
+static inline __noreturn __printf(1, 2) int ksft_exit_fail_msg(const char *msg, ...)
 {
 	int saved_errno = errno;
 	va_list args;
@@ -348,19 +351,19 @@ static inline __printf(1, 2) int ksft_exit_fail_msg(const char *msg, ...)
 	exit(KSFT_FAIL);
 }
 
-static inline int ksft_exit_xfail(void)
+static inline __noreturn int ksft_exit_xfail(void)
 {
 	ksft_print_cnts();
 	exit(KSFT_XFAIL);
 }
 
-static inline int ksft_exit_xpass(void)
+static inline __noreturn int ksft_exit_xpass(void)
 {
 	ksft_print_cnts();
 	exit(KSFT_XPASS);
 }
 
-static inline __printf(1, 2) int ksft_exit_skip(const char *msg, ...)
+static inline __noreturn __printf(1, 2) int ksft_exit_skip(const char *msg, ...)
 {
 	int saved_errno = errno;
 	va_list args;

---
base-commit: 076361362122a6d8a4c45f172ced5576b2d4a50d
change-id: 20240411-mark-kselftest-exit-funcs-noreturn-17d8ff729a7a

Best regards,
-- 
Nathan Chancellor <nathan@kernel.org>