linux-kernel - Re: [PATCH v3 05/10] objtool: Handle unreachable entry of rodata

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Message-ID: <CAAhV-H5b2=UPzW2W-etkBpCDoRUE+XrTdZNj-L1v2pozHXNmiQ@mail.gmail.com>
Date: Thu, 21 Nov 2024 20:56:55 +0800
From: Huacai Chen <chenhuacai@...nel.org>
To: Jinyang He <hejinyang@...ngson.cn>, wanglei@...ngson.cn
Cc: Tiezhu Yang <yangtiezhu@...ngson.cn>, Josh Poimboeuf <jpoimboe@...nel.org>, 
	Peter Zijlstra <peterz@...radead.org>, loongarch@...ts.linux.dev, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 05/10] objtool: Handle unreachable entry of rodata

Hi, Jinyang,

On Tue, Nov 19, 2024 at 6:47 PM Jinyang He <hejinyang@...ngson.cn> wrote:
>
> On 2024-11-19 14:56, Tiezhu Yang wrote:
>
> > When compling with Clang on LoongArch, there exists unreachable entry
> > of rodata which points to a nop instruction after the function return
> > instruction, this is generated by compiler to fill the non-existent
> > switch case, just skip the entry when parsing the relocation section
> > of rodata.
> >
> > This is preparation for later patch on LoongArch, there is no effect
> > for the other archs with this patch.
> >
> > Signed-off-by: Tiezhu Yang <yangtiezhu@...ngson.cn>
> > ---
> >   tools/objtool/check.c | 8 ++++++++
> >   1 file changed, 8 insertions(+)
> >
> > diff --git a/tools/objtool/check.c b/tools/objtool/check.c
> > index eb4c89501493..17df3738e087 100644
> > --- a/tools/objtool/check.c
> > +++ b/tools/objtool/check.c
> > @@ -2136,6 +2136,14 @@ static int add_jump_table(struct objtool_file *file, struct instruction *insn,
> >               if (!dest_insn)
> >                       break;
> >
> > +             /* Handle the special cases compiled with Clang on LoongArch */
> > +             if (file->elf->ehdr.e_machine == EM_LOONGARCH &&
> > +                 reloc->sym->type == STT_SECTION && dest_insn->type == INSN_NOP &&
> Why it points to NOP, and whether it could be some other instruction
> (e.g. break)?
> It seems fragile. I think it should fix in Clang.

As discussed with Tiezhu and the developer of Clang compiler (Wang
Lei), this is just the implementation of Clang, the non-existent
switch case is filled in the jump table, the table entry points to a
position after the function return instruction, this is not an issue
for Clang.

Currently, objdump the object files of Linux kernel shows that the
position is a nop instruction.

But it is not necessary to check the position is a nop instruction, it
only needs to check whether the position is after the function return
instruction, so "dest_insn->type == INSN_NOP" can be removed. I will
modify it when applying the patch.

Other patches seem to have no fundamental issues.

Huacai

> > +                 (!insn_func(dest_insn) || insn_func(dest_insn)->pfunc != pfunc)) {
> > +                     prev_offset = reloc_offset(reloc);
> > +                     continue;
> > +             }
> > +
> >               /* Make sure the destination is in the same function: */
> >               if (!insn_func(dest_insn) || insn_func(dest_insn)->pfunc != pfunc)
> >                       break;
>