linux-kernel - Re: [PATCH -tip v2 2/3] x86/callthunks: Handle %rip-relative relocations in call thunk template

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Message-ID: <CAFULd4YWa+Xfue-JEebR9JgjxKtQy7byr68Q+tTOdR+Jf1DBEg@mail.gmail.com>
Date:   Fri, 1 Dec 2023 08:48:10 +0100
From:   Uros Bizjak <ubizjak@...il.com>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>, llvm@...ts.linux.dev
Subject: Re: [PATCH -tip v2 2/3] x86/callthunks: Handle %rip-relative
 relocations in call thunk template

On Fri, Dec 1, 2023 at 4:55 AM Nathan Chancellor <nathan@...nel.org> wrote:
>
> Hi Uros,
>
> On Sun, Nov 05, 2023 at 10:34:36PM +0100, Uros Bizjak wrote:
> > Contrary to alternatives, relocations are currently not supported in
> > call thunk templates.  Re-use the existing infrastructure from
> > alternative.c to allow %rip-relative relocations when copying call
> > thunk template from its storage location.
> >
> > The patch allows unification of ASM_INCREMENT_CALL_DEPTH, which already
> > uses PER_CPU_VAR macro, with INCREMENT_CALL_DEPTH, used in call thunk
> > template, which is currently limited to use absolute address.
> >
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Cc: Ingo Molnar <mingo@...nel.org>
> > Cc: Borislav Petkov <bp@...en8.de>
> > Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> > Cc: "H. Peter Anvin" <hpa@...or.com>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Signed-off-by: Uros Bizjak <ubizjak@...il.com>
> ...
> > diff --git a/arch/x86/kernel/callthunks.c b/arch/x86/kernel/callthunks.c
> > index e9ad518a5003..ef9c04707b3c 100644
> > --- a/arch/x86/kernel/callthunks.c
> > +++ b/arch/x86/kernel/callthunks.c
> ...
> > @@ -291,20 +298,27 @@ void *callthunks_translate_call_dest(void *dest)
> >  static bool is_callthunk(void *addr)
> >  {
> >       unsigned int tmpl_size = SKL_TMPL_SIZE;
> > -     void *tmpl = skl_call_thunk_template;
> > +     u8 insn_buff[MAX_PATCH_LEN];
> >       unsigned long dest;
> > +     u8 *pad;
> >
> >       dest = roundup((unsigned long)addr, CONFIG_FUNCTION_ALIGNMENT);
> >       if (!thunks_initialized || skip_addr((void *)dest))
> >               return false;
> >
> > -     return !bcmp((void *)(dest - tmpl_size), tmpl, tmpl_size);
> > +     *pad = dest - tmpl_size;
>
> Clang warns (or errors with CONFIG_WERROR=y):

Uh, GCC didn't warn at all (and there is some mixup with types here,
so a thinko slipped through.

The attached patch fixes the oversight. I'll post a formal patch later
today after some more testing.

Thanks,
Uros.

View attachment "p.diff.txt" of type "text/plain" (479 bytes)