Re: linux-next: build warnings after merge of the powerpc-objtool tree

[Sathvika Vasireddy]

On 29/11/22 20:58, Christophe Leroy wrote:
> Le 29/11/2022 à 16:13, Sathvika Vasireddy a écrit :
> > Hi all,
> >
> > On 25/11/22 09:00, Stephen Rothwell wrote:
> > > Hi all,
> > >
> > > After merging the powerpc-objtool tree, today's linux-next build (powerpc
> > > pseries_le_defconfig) produced these warnings:
> > >
> > > arch/powerpc/kernel/head_64.o: warning: objtool: end_first_256B():
> > > can't find starting instruction
> > > arch/powerpc/kernel/optprobes_head.o: warning: objtool:
> > > optprobe_template_end(): can't find starting instruction
> > >
> > > I have no idea what started this (they may have been there yesterday).
> > I was able to recreate the above mentioned warnings with
> > pseries_le_defconfig and powernv_defconfig. The regression report also
> > mentions a warning
> > (https://lore.kernel.org/oe-kbuild-all/202211282102.QUr7HHrW-lkp@xxxxxxxxx/) seen with arch/powerpc/kernel/kvm_emul.S assembly file.
> >
> >    [1] arch/powerpc/kernel/optprobes_head.o: warning: objtool:
> > optprobe_template_end(): can't find starting instruction
> >    [2] arch/powerpc/kernel/kvm_emul.o: warning: objtool:
> > kvm_template_end(): can't find starting instruction
> >    [3] arch/powerpc/kernel/head_64.o: warning: objtool: end_first_256B():
> > can't find starting instruction
> >
> > The warnings [1] and [2] go away after adding 'nop' instruction. Below
> > diff fixes it for me:
> You have to add NOPs just because those labels are at the end of the
> files. That's a bit odd.
> I think either we are missing some kind of flagging for the symbols, or
> objtool has a bug. In both cases, I'm not sure adding an artificial
> 'nop' is the solution. At least there should be a big hammer warning
> explaining why.

I don't see these warnings with powerpc/topic/objtool branch. However, 
they are seen with linux-next master branch.
Commit dbcdbdfdf137b49144204571f1a5e5dc01b8aaad objtool: Rework 
instruction -> symbol mapping in linux-next is resulting in objtool 
can't find starting instruction warnings on powerpc.

Reverting this particular hunk (pasted below), resolves it and we don't 
see the problem anymore.

@@ -427,7 +427,10 @@ static int decode_instructions(struct objtool_file 
*file)
                }

                list_for_each_entry(func, &sec->symbol_list, list) {
-                       if (func->type != STT_FUNC || func->alias != func)
+                       if (func->type != STT_NOTYPE && func->type != 
STT_FUNC)
+                               continue;
+
+                       if (func->return_thunk || func->alias != func)
                                continue;

                        if (!find_insn(file, sec, func->offset)) {

Peterz, can we ignore STT_NOTYPE symbols?

> > diff --git a/arch/powerpc/kernel/optprobes_head.S
> > b/arch/powerpc/kernel/optprobes_head.S
> > index cd4e7bc32609..ea4e3bd82f4f 100644
> > --- a/arch/powerpc/kernel/optprobes_head.S
> > +++ b/arch/powerpc/kernel/optprobes_head.S
> > @@ -134,3 +134,4 @@ optprobe_template_ret:
> >
> >           .global optprobe_template_end
> >    optprobe_template_end:
> > +       nop
> >
> > diff --git a/arch/powerpc/kernel/kvm_emul.S
> > b/arch/powerpc/kernel/kvm_emul.S
> > index 7af6f8b50c5d..41fd664e3ba0 100644
> > --- a/arch/powerpc/kernel/kvm_emul.S
> > +++ b/arch/powerpc/kernel/kvm_emul.S
> > @@ -352,3 +352,4 @@ kvm_tmp_end:
> >
> >    .global kvm_template_end
> >    kvm_template_end:
> > +       nop
> >
> > For warning [3], objtool is throwing can't find starting instruction
> > warning because it finds that the symbol (end_first_256B) is zero sized,
> > and such symbols are not added to the rbtree. I tried to fix it by
> > adding a 'nop' instruction (pasted diff below), but that resulted in a
> > kernel build failure.
> What's the failure ?
>
>
> > diff --git a/arch/powerpc/kernel/head_64.S b/arch/powerpc/kernel/head_64.S
> > index 874efd25cc45..d48850fe159f 100644
> > --- a/arch/powerpc/kernel/head_64.S
> > +++ b/arch/powerpc/kernel/head_64.S
> > @@ -192,6 +192,7 @@ __secondary_hold:
> >           EMIT_BUG_ENTRY 0b, __FILE__, __LINE__, 0
> >    #endif
> >    CLOSE_FIXED_SECTION(first_256B)
> > +nop
> >
> >    /*
> >     * On server, we include the exception vectors code here as it
> >
> > diff --git a/arch/powerpc/kernel/exceptions-64s.S
> > b/arch/powerpc/kernel/exceptions-64s.S
> > index 26f8fef53c72..f7517d443e9b 100644
> > --- a/arch/powerpc/kernel/exceptions-64s.S
> > +++ b/arch/powerpc/kernel/exceptions-64s.S
> > @@ -3104,9 +3104,13 @@ __end_interrupts:
> >    DEFINE_FIXED_SYMBOL(__end_interrupts, virt_trampolines)
> >
> >    CLOSE_FIXED_SECTION(real_vectors);
> > +nop
> >    CLOSE_FIXED_SECTION(real_trampolines);
> > +nop
> >    CLOSE_FIXED_SECTION(virt_vectors);
> > +nop
> >    CLOSE_FIXED_SECTION(virt_trampolines);
> > +nop
> What are the NOPs after the CLOSE_FIXED_SECTION() ? You don't explain
> them, and I can't see any related warning in the warnings you show.
After fixing arch/powerpc/kernel/head_64.o: warning: objtool: 
end_first_256B(): can't find starting instruction warning, objtool 
started showing more warnings in the same file.
Below is the list of warnings:
 arch/powerpc/kernel/head_64.o: warning: objtool: end_real_vectors(): 
can't find starting instruction
 arch/powerpc/kernel/head_64.o: warning: objtool: 
end_real_trampolines(): can't find starting instruction
 arch/powerpc/kernel/head_64.o: warning: objtool: end_virt_vectors(): 
can't find starting instruction
 arch/powerpc/kernel/head_64.o: warning: objtool: 
end_virt_trampolines(): can't find starting instruction


Thanks,
Sathvika