[patch V2a 2/6] kbuild: Disable CC_HAS_ASM_GOTO_OUTPUT on clang < version 17

[Geert Uytterhoeven]

Hi Thomas,

On Wed, 17 Sept 2025 at 07:51, Thomas Gleixner <tglx at linutronix.de> wrote:
> clang < 17 fails to use scope local labels with CONFIG_CC_HAS_ASM_GOTO_OUTPUT=y:
>
>      {
>         __label__ local_lbl;
>         ...
>         unsafe_get_user(uval, uaddr, local_lbl);
>         ...
>         return 0;
>         local_lbl:
>                 return -EFAULT;
>      }
>
> when two such scopes exist in the same function:
>
>   error: cannot jump from this asm goto statement to one of its possible targets
>
> There are other failure scenarios. Shuffling code around slightly makes it
> worse and fail even with one instance.
>
> That issue prevents using local labels for a cleanup based user access
> mechanism.
>
> After failed attempts to provide a simple enough test case for the 'depends
> on' test in Kconfig, the initial cure was to mark ASM goto broken on clang
> versions < 17 to get this road block out of the way.
>
> But Nathan pointed out that this is a known clang issue and indeed affects
> clang < version 17 in combination with cleanup(). It's not even required to
> use local labels for that.
>
> The clang issue tracker has a small enough test case, which can be used as
> a test in the 'depends on' section of CC_HAS_ASM_GOTO_OUTPUT:
>
> void bar(void **);
> void* baz();
>
> int  foo (void) {
>     {
>             asm goto("jmp %l0"::::l0);
>             return 0;
> l0:
>             return 1;
>     }
>     void *x __attribute__((cleanup(bar))) = baz();
>     {
>             asm goto("jmp %l0"::::l1);
>             return 42;
> l1:
>             return 0xff;
>     }
> }
>
> Add another dependency to config CC_HAS_ASM_GOTO_OUTPUT for it and use the
> clang issue tracker test case for detection by condensing it to obfuscated
> C-code contest format. This reliably catches the problem on clang < 17 and
> did not show any issues on the non known to be broken GCC versions.
>
> That test might be sufficient to catch all issues and therefore could
> replace the existing test, but keeping that around does no harm either.
>
> Thanks to Nathan for pointing to the relevant clang issue!
>
> Suggested-by: Nathan Chancellor <nathan at kernel.org>
> Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
> Cc: Nathan Chancellor <nathan at kernel.org>
> Link: https://github.com/ClangBuiltLinux/linux/issues/1886
> Link: https://github.com/llvm/llvm-project/commit/f023f5cdb2e6c19026f04a15b5a935c041835d14

Thanks for your patch, which is now commit e2ffa15b9baa447e ("kbuild:
Disable CC_HAS_ASM_GOTO_OUTPUT on clang < 17") in v6.17.

> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -99,7 +99,10 @@ config GCC_ASM_GOTO_OUTPUT_BROKEN
>  config CC_HAS_ASM_GOTO_OUTPUT
>         def_bool y
>         depends on !GCC_ASM_GOTO_OUTPUT_BROKEN
> +       # Find basic issues
>         depends on $(success,echo 'int foo(int x) { asm goto ("": "=r"(x) ::: bar); return x; bar: return 0; }' | $(CC) -x c - -c -o /dev/null)
> +       # Detect buggy clang, fixed in clang-17
> +       depends on $(success,echo 'void b(void **);void* c();int f(void){{asm goto("jmp %l0"::::l0);return 0;l0:return 1;}void *x __attribute__((cleanup(b))) = c();{asm goto("jmp %l0"::::l1);return 2;l1:return 1;}}' | $(CC) -x c - -c -o /dev/null)

This is supposed to affect only clang builds, right?  I am using
gcc version 13.3.0 (Ubuntu 13.3.0-6ubuntu2~24.04) to build for
arm32/arm64/riscv, and thus have:

    CONFIG_CC_IS_GCC=y

Still, this commit causes

    CONFIG_CC_HAS_ASM_GOTO_OUTPUT=y
    CONFIG_CC_HAS_ASM_GOTO_TIED_OUTPUT=y

to disappear from my configs? Is that expected?

Thanks!

Gr{oetje,eeting}s,

                        Geert


--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds