Re: [PATCH v2 2/4] modpost: inform compilers that fatal() never returns

[Masahiro Yamada]

On Tue, Jan 23, 2024 at 8:02 AM Nathan Chancellor <nathan@xxxxxxxxxx> wrote:
>
> On Mon, Jan 22, 2024 at 10:29:32PM +0900, Masahiro Yamada wrote:
> > +CC: clang-built-linux list, Fangrui
> >
> > On Mon, Jan 22, 2024 at 1:04 PM Aiden Leong <aiden.leong@xxxxxxxxx> wrote:
> > >
> > >
> > >  > The function fatal() never returns because modpost_log() calls exit(1)
> > >
> > >  > when LOG_FATAL is passed.
> > >  >
> > >  > Inform compilers of this fact so that unreachable code flow can be
> > >  > identified at compile time.
> > >  >
> > >  > Signed-off-by: Masahiro Yamada <masahiroy@xxxxxxxxxx>
> > >  > Reviewed-by: Nathan Chancellor <nathan@xxxxxxxxxx>
> > >  > ---
> > >  >
> > >  > Changes in v2:
> > >  >   - Use noreturn attribute together with alias
> > >  >
> > >  >  scripts/mod/modpost.c | 3 +++
> > >  >  scripts/mod/modpost.h | 5 ++++-
> > >  >  2 files changed, 7 insertions(+), 1 deletion(-)
> > >  >
> > >  > diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
> > >  > index ca0a90158f85..c13bc9095df3 100644
> > >  > --- a/scripts/mod/modpost.c
> > >  > +++ b/scripts/mod/modpost.c
> > >  > @@ -90,6 +90,9 @@ void modpost_log(enum loglevel loglevel, const char
> > > *fmt, ...)
> > >  >          error_occurred = true;
> > >  >  }
> > >  >
> > >  > +void __attribute__((alias("modpost_log")))
> > >
> > > Hi Masahiro,
> > > I cross-compile kernel on Apple Silicon MacBook Pro
> > > and every thing works well until this patch.
> > >
> > > My build command:
> > > make ARCH=arm CROSS_COMPILE=arm-none-eabi- \
> > > HOSTCFLAGS="-I/opt/homebrew/opt/openssl/include" \
> > > HOSTLDFLAGS="-L/opt/homebrew/opt/openssl/lib"
> > >
> > > Error message:
> > > scripts/mod/modpost.c:93:21: error: aliases are not supported on darwin
> >
> >
> > It is unfortunate.  Indeed, I see this message in:
> >
> > clang/include/clang/Basic/DiagnosticSemaKinds.td
> >
> >
> > Is this limitation due to macOS executable (PEF),
> > or is it Clang-specific?
>
> Based on my admittedly brief research, this seems related to the Mach-O
> format. That message was added by [1] in response to [2] but the message
> mentioned weak aliases being supported. A further clarification was made
> in [3] to state that all aliases are unsupported as a result of some
> internal Apple bug it seems but I do see a couple of bug reports stating
> that may not be true [4][5] (although that does not seem relevant for
> this report).
>
> [1]: https://github.com/llvm/llvm-project/commit/0017c5fa92ad3b10e15fd34f3865e8e5b850a5ed
> [2]: https://llvm.org/bz8720
> [3]: https://github.com/llvm/llvm-project/commit/4e30b96834cea5682a8e9e024dda06319825000a
> [4]: https://github.com/llvm/llvm-project/issues/11488
> [5]: https://github.com/llvm/llvm-project/issues/71001



Thank you.
As far as I understood, macOS seems to support weak aliases.

With [5] fixed, perhaps we could do the following:

-void __attribute__((alias("modpost_log")))
+void __attribute__((weak, alias("modpost_log")))


But, we do not need to wait for it.
We can do similar without aliases at all.

fix submitted:
https://lore.kernel.org/all/20240127132811.726504-1-masahiroy@xxxxxxxxxx/T/#u




-- 
Best Regards
Masahiro Yamada