Re: [PATCH 3/3] selftests/nolibc: test the FD_* macros and select() on a high fd

["Thomas Weißschuh" <linux@weissschuh.net>]

On 2026-09-23 20:52:22-0500, Danish Khateeb wrote:
> The only select() test that passes an fd_set uses fd 1, so nothing
> noticed that the FD_* macros used the wrong bits for fds 31-63 of each
> word on 64-bit architectures.
> 
> Add a test that FD_SET() and FD_CLR() change exactly one fd, for every
> fd below FD_SETSIZE, and one that select() reports a readable pipe
> duplicated to fd 40, which also checks that the macros agree with the
> kernel on the layout of the set.
> 
> Assisted-by: LLM
> Signed-off-by: Danish Khateeb <danishkhateeb03@gmail.com>
> ---
>  tools/testing/selftests/nolibc/nolibc-test.c | 54 ++++++++++++++++++++
>  1 file changed, 54 insertions(+)
> 
> diff --git a/tools/testing/selftests/nolibc/nolibc-test.c b/tools/testing/selftests/nolibc/nolibc-test.c
> index f62183a46205..d9a8bfd4b21b 100644
> --- a/tools/testing/selftests/nolibc/nolibc-test.c
> +++ b/tools/testing/selftests/nolibc/nolibc-test.c
> @@ -1369,6 +1369,58 @@ int test_pipe(void)
>  	return !!memcmp(buf, msg, len);
>  }
>  
> +int test_fd_set(void)
> +{
> +	fd_set fds;
> +	int fd, i;
> +
> +	for (fd = 0; fd < FD_SETSIZE; fd++) {
> +		/* FD_SET() must set this fd and no other */
> +		FD_ZERO(&fds);
> +		FD_SET(fd, &fds);
> +		for (i = 0; i < FD_SETSIZE; i++)
> +			if (!!FD_ISSET(i, &fds) != (i == fd))
> +				return 1;
> +
> +		/* FD_CLR() must clear this fd and no other */
> +		for (i = 0; i < FD_SETSIZE; i++)
> +			FD_SET(i, &fds);
> +		FD_CLR(fd, &fds);
> +		for (i = 0; i < FD_SETSIZE; i++)
> +			if (!!FD_ISSET(i, &fds) != (i != fd))
> +				return 1;
> +	}
> +
> +	return 0;
> +}
> +
> +int test_select_high_fd(void)
> +{
> +	struct timeval tv = { 0 };
> +	int pipefd[2], fd, ret = 1;
> +	fd_set fds;
> +
> +	if (pipe(pipefd) == -1)
> +		return 1;
> +
> +	/* fd 40 is in the upper half of a 64-bit fd_set word */
> +	fd = dup2(pipefd[0], 40);
> +	if (fd == -1)
> +		goto out;
> +
> +	write(pipefd[1], "x", 1);
> +	FD_ZERO(&fds);
> +	FD_SET(fd, &fds);
> +	if (select(fd + 1, &fds, NULL, NULL, &tv) == 1 && FD_ISSET(fd, &fds))
> +		ret = 0;
> +
> +	close(fd);
> +out:
> +	close(pipefd[0]);
> +	close(pipefd[1]);
> +	return ret;

I am not conviced this testcase provides much value.
If we really want to keep it, it should be easier to dup2(STDOUT_FILENO)
and select() for it to be writable instead of doing the dance with the
pipe.

> +}
> +
>  int test_rlimit(void)
>  {
>  	struct rlimit rlim = {
> @@ -1711,6 +1763,7 @@ int run_syscall(int min, int max)
>  		CASE_TEST(execve_root);       EXPECT_SYSER(1, execve("/", (char*[]){ [0] = (char []){"/"}, [1] = NULL }, NULL), -1, EACCES); break;
>  		CASE_TEST(fchdir_stdin);      EXPECT_SYSER(1, fchdir(STDIN_FILENO), -1, ENOTDIR); break;
>  		CASE_TEST(fchdir_badfd);      EXPECT_SYSER(1, fchdir(-1), -1, EBADF); break;
> +		CASE_TEST(fd_set);            EXPECT_ZR(1, test_fd_set()); break;

This is not really a syscall test, could you move it to the stdlib
tests? Also the test functions should have the same order as the
testcases which are calling them.

>  		CASE_TEST(fdopendir_notdir);  EXPECT_SYSER(1, (uintptr_t)fdopendir(STDIN_FILENO), (uintptr_t)NULL, ENOTDIR); break;
>  		CASE_TEST(file_stream);       EXPECT_SYSZR(1, test_file_stream()); break;
>  		CASE_TEST(file_stream_wsr);   EXPECT_SYSZR(1, test_file_stream_wsr()); break;
> @@ -1753,6 +1806,7 @@ int run_syscall(int min, int max)
>  		CASE_TEST(select_null);       EXPECT_SYSZR(1, ({ struct timeval tv = { 0 }; select(0, NULL, NULL, NULL, &tv); })); break;
>  		CASE_TEST(select_stdout);     EXPECT_SYSNE(1, ({ fd_set fds; FD_ZERO(&fds); FD_SET(1, &fds); select(2, NULL, &fds, NULL, NULL); }), -1); break;
>  		CASE_TEST(select_fault);      EXPECT_SYSER(1, select(1, (void *)1, NULL, NULL, 0), -1, EFAULT); break;
> +		CASE_TEST(select_high_fd);    EXPECT_SYSZR(1, test_select_high_fd()); break;
>  		CASE_TEST(sendfile);          EXPECT_SYSZR(1, test_sendfile()); break;
>  		CASE_TEST(stat_blah);         EXPECT_SYSER(1, stat("/proc/self/blah", &stat_buf), -1, ENOENT); break;
>  		CASE_TEST(stat_fault);        EXPECT_SYSER(1, stat(NULL, &stat_buf), -1, EFAULT); break;
> -- 
> 2.55.0
>