Re: The trouble with __weak and objtool got worse

[Josh Poimboeuf <jpoimboe@redhat.com>]

On Fri, Apr 15, 2022 at 10:40:22AM -0700, Nick Desaulniers wrote:
> On Fri, Apr 15, 2022 at 8:26 AM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Fri, Apr 15, 2022 at 01:19:04PM +0200, Peter Zijlstra wrote:
> > > Alternatively:
> > >
> > >   https://sourceware.org/pipermail/binutils/2020-December/114671.html
> > >
> > > seems to suggest: -Wa,--generate-unused-section-symbols=yes, ought to
> > > work, except I'm getting:
> > >
> > > $ gcc -O2 -fcf-protection=none -fno-asynchronous-unwind-tables -Wa,--generate-unused-section-symbols=yes -c foo*.c
> > > as: unrecognized option '--generate-unused-section-symbols=yes'
> > > as: unrecognized option '--generate-unused-section-symbols=yes'
> >
> > Reading so hard...
> >
> >  $ gcc -O2 -fcf-protection=none -fno-asynchronous-unwind-tables -Wa,-generate-unused-section-symbols=yes -c foo*.c
> >
> > seems to actually work.
> 
> $ clang hello.c -Wa,-generate-unused-section-symbols=yes
> clang-15: error: unsupported argument
> '-generate-unused-section-symbols=yes' to option '-Wa,'
> 
> :)
> 
> I recall LLVM not emitting STT_SECTION symbols being problematic for
> objtool in the past.
> Old pre-lore archive:
> https://groups.google.com/g/clang-built-linux/c/1C6YoJKBsQQ/m/a8IS1NjGAgAJ
> via https://github.com/ClangBuiltLinux/linux/issues/1209.
> (It doesn't look too bad for me to implement)

Right, and that objtool "fix" -- silently falling back to
non-section-symbol relocations instead of erroring out -- is causing
this issue, when it's done for weak symbols.

If LLVM assembler doesn't support this option then we may have to go
with something like this?  I can't seem to recreate so I'm not able to
test.

We'd also need some objtool checks to make sure the non-section-symbol
fallback isn't being done for a weak symbol.  Or just remove the
fallback altogether and force section symbols whereever they're needed,
similar to the below (untested).


diff --git a/include/linux/static_call_types.h b/include/linux/static_call_types.h
index 5a00b8b2cf9f..77040ce575fa 100644
--- a/include/linux/static_call_types.h
+++ b/include/linux/static_call_types.h
@@ -52,9 +52,14 @@ struct static_call_site {
 #define __STATIC_CALL_ADDRESSABLE(name) \
 	__ADDRESSABLE(STATIC_CALL_KEY(name))
 
+extern unsigned long __addressable_ip;
+
+#define __STATIC_CALL_SITE_ADDRESSABLE() __addressable_ip = _THIS_IP_;
+
 #define __static_call(name)						\
 ({									\
 	__STATIC_CALL_ADDRESSABLE(name);				\
+	__STATIC_CALL_SITE_ADDRESSABLE();				\
 	__raw_static_call(name);					\
 })
 
diff --git a/kernel/static_call_inline.c b/kernel/static_call_inline.c
index dc5665b62814..f6e3e0463efb 100644
--- a/kernel/static_call_inline.c
+++ b/kernel/static_call_inline.c
@@ -17,6 +17,8 @@ extern struct static_call_tramp_key __start_static_call_tramp_key[],
 
 static bool static_call_initialized;
 
+unsigned long __section(".discard.addressable") __addressable_ip;
+
 /* mutex to protect key modules/sites */
 static DEFINE_MUTEX(static_call_mutex);