Re: [PATCH] tools/nolibc: make __nolibc_enosys() a compile time error

[Willy Tarreau <w@1wt.eu>]

Hi Thomas,

On Sat, Apr 04, 2026 at 05:26:05PM +0200, Thomas Weißschuh wrote:
> Functions which are known at compile-time to result in ENOSYS can be
> surprising to the user. For example using old UAPI headers might mean
> that stat() will always fail although the kernel would have the system
> call available at runtime. Nowadays __nolibc_enosys() should never be
> called for normal applications.
> 
> Switch the silent ENOSYS return into a compile-time error, so the user
> is aware about the issue. Prefer the 'error' attribute as it provides
> the best diagnostics. If the users defines NOLIBC_COMPILE_TIME_ENOSYS
> the old, silent fallback is kept.
> 
> Also add a test which validates that the error can be optimized away.
> 
> Reported-by: Willy Tarreau <w@1wt.eu>
> Closes: https://lore.kernel.org/lkml/acizRIq2xrFUNHNS@1wt.eu/
> Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
> ---
> This should probably go into the next cycle.
> ---
>  tools/include/nolibc/sys.h                   | 18 ++++++++++++++++--
>  tools/testing/selftests/nolibc/nolibc-test.c | 18 ++++++++++++++++++
>  2 files changed, 34 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/include/nolibc/sys.h b/tools/include/nolibc/sys.h
> index 6335fd51f07f..fd7a2ee780e8 100644
> --- a/tools/include/nolibc/sys.h
> +++ b/tools/include/nolibc/sys.h
> @@ -45,16 +45,30 @@
>  		: __sysret_arg;                         /* return original value */ \
>  })
>  
> -/* Syscall ENOSYS helper: Avoids unused-parameter warnings and provides a
> - * debugging hook.
> +/* Syscall ENOSYS helper: Avoids unused-parameter warnings, provides compile
> + * time validation and a debugging hook.
>   */
>  
> +#if defined(NOLIBC_COMPILE_TIME_ENOSYS)
>  static __inline__ int __nolibc_enosys(const char *syscall, ...)
>  {
>  	(void)syscall;
>  	return -ENOSYS;
>  }
>  
> +#elif __nolibc_has_attribute(error)
> +__attribute__((error("system call not implemented")))
> +extern int __nolibc_enosys(const char *syscall, ...);

I didn't know it was possible to force the error message like this,
I like it!

> +#else
> +static __inline__ int __nolibc_enosys(const char *syscall, ...)
> +{
> +	extern int __nolibc_enosys_error;
> +	(void)syscall;
> +
> +	return __nolibc_enosys_error;
> +}
> +#endif
>  
>  /* Functions in this file only describe syscalls. They're declared static so
>   * that the compiler usually decides to inline them while still being allowed
> diff --git a/tools/testing/selftests/nolibc/nolibc-test.c b/tools/testing/selftests/nolibc/nolibc-test.c
> index dd10402267ee..fb7eaa26ca93 100644
> --- a/tools/testing/selftests/nolibc/nolibc-test.c
> +++ b/tools/testing/selftests/nolibc/nolibc-test.c
> @@ -1300,6 +1300,23 @@ int test_openat(void)
>  	return 0;
>  }
>  
> +int test_nolibc_enosys(void)
> +{
> +	if (true)
> +		return 0;
> +
> +#if defined(NOLIBC)
> +	/*
> +	 * __nolibc_enosys() will fail the compilation.
> +	 * Make sure it can be optimized away if not actually called.
> +	 */
> +	if (__nolibc_enosys("something") != -ENOSYS)
> +		return 1;
> +#endif
> +
> +	return 0;
> +}
> +
>  int test_namespace(void)
>  {
>  	int original_ns, new_ns, ret;
> @@ -1468,6 +1485,7 @@ int run_syscall(int min, int max)
>  		CASE_TEST(munmap_bad);        EXPECT_SYSER(1, munmap(NULL, 0), -1, EINVAL); break;
>  		CASE_TEST(mmap_munmap_good);  EXPECT_SYSZR(1, test_mmap_munmap()); break;
>  		CASE_TEST(nanosleep);         ts.tv_nsec = -1; EXPECT_SYSER(1, nanosleep(&ts, NULL), -1, EINVAL); break;
> +		CASE_TEST(nolibc_enosys);     EXPECT_ZR(is_nolibc, test_nolibc_enosys()); break;
>  		CASE_TEST(open_tty);          EXPECT_SYSNE(1, tmp = open("/dev/null", O_RDONLY), -1); if (tmp != -1) close(tmp); break;
>  		CASE_TEST(open_blah);         EXPECT_SYSER(1, tmp = open("/proc/self/blah", O_RDONLY), -1, ENOENT); if (tmp != -1) close(tmp); break;
>  		CASE_TEST(openat_dir);        EXPECT_SYSZR(1, test_openat()); break;
> 
> ---
> base-commit: 74986b90eaf3f0ec38bd54de4851063616b49486
> change-id: 20260404-nolibc-enosys-d601cc75ab29
> 
> Best regards,
> --  
> Thomas Weißschuh <linux@weissschuh.net>

Acked-by: Willy Tarreau <w@1wt.eu>

Thanks!
Willy