linux-kernel - Re: [PATCH v2 3/5] objtool: Add support for annotated jump tables

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Message-ID: <20241011155634.se7xifgne6z5mqmx@treble>
Date: Fri, 11 Oct 2024 08:56:34 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Ard Biesheuvel <ardb@...nel.org>
Cc: Ard Biesheuvel <ardb+git@...gle.com>, linux-kernel@...r.kernel.org,
	llvm@...ts.linux.dev, keescook@...omium.org,
	linux-hardening@...r.kernel.org, nathan@...nel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Jan Beulich <jbeulich@...e.com>,
	"Jose E. Marchesi" <jemarch@....org>, Kees Cook <kees@...nel.org>
Subject: Re: [PATCH v2 3/5] objtool: Add support for annotated jump tables

On Fri, Oct 11, 2024 at 08:29:48AM +0200, Ard Biesheuvel wrote:
> On Thu, 10 Oct 2024 at 22:15, Josh Poimboeuf <jpoimboe@...nel.org> wrote:
> >
> > On Thu, Oct 10, 2024 at 02:28:05PM +0200, Ard Biesheuvel wrote:
> > > +++ b/tools/objtool/arch/x86/special.c
> > > @@ -115,30 +115,51 @@ struct reloc *arch_find_switch_table(struct objtool_file *file,
> > >       struct reloc  *text_reloc, *rodata_reloc;
> > >       struct section *table_sec;
> > >       unsigned long table_offset;
> > > +     struct symbol *sym;
> > >
> > >       /* look for a relocation which references .rodata */
> > >       text_reloc = find_reloc_by_dest_range(file->elf, insn->sec,
> > >                                             insn->offset, insn->len);
> >
> > Hm, we can probably put insn_reloc() in check.h and use that here to
> > take advantage of its caching for the no_reloc case.
> >
> 
> insn_reloc() filters out R_*_NONE relocations, for the reasons pointed
> out in the commit log.

True, unless this becomes the fallback code for the generic "no
annotations" case as I suggested elsewhere.

Regardless we should be sure to write and read insn->no_reloc where we
can to minimize duplicate reloc lookups.

-- 
Josh