linux-kernel - Re: [PATCH v2] kallsyms: strip ThinLTO postfix ".cfi

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Date:   Thu, 29 Jul 2021 17:29:04 +0200
From:   Padmanabha Srinivasaiah <treasure4paddy@...il.com>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Jessica Yu <jeyu@...nel.org>, Kees Cook <keescook@...omium.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Miroslav Benes <mbenes@...e.cz>,
        Petr Mladek <pmladek@...e.com>,
        Miguel Ojeda <ojeda@...nel.org>, Joe Perches <joe@...ches.com>,
        Stephen Boyd <swboyd@...omium.org>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH v2] kallsyms: strip ThinLTO postfix ".cfi_jt"

On Wed, Jul 28, 2021 at 01:57:21PM -0700, Sami Tolvanen wrote:
> Hi,
> 
> On Tue, Jul 27, 2021 at 7:07 AM Padmanabha Srinivasaiah
> <treasure4paddy@...il.com> wrote:
> >
> > Clang ThinLTO adds a postfix ".cfi_jt" to a symbols of extern functions.
> 
> These symbols are added with CONFIG_CFI_CLANG no matter which LTO mode
> is selected, so talking about ThinLTO here isn't quite correct.
>
Yes, checked irrespective of the LTO mode choosen ".cfi_jt" postfix
is added with CONFIG_CFI_CLANG flag. Thanks for correcting out,
will make neccessary changes.

> > For example this breaks syscall tracer that doesn't expect such postfix,
> > so strip out the postfix from the output.
> >
> > Signed-off-by: Padmanabha Srinivasaiah <treasure4paddy@...il.com>
> > ---
> > Change in v2:
> >   - Use existing routine in kallsyms to strip postfix ".cfi_jt" from
> >     extern function name.
> >   - Modified the commit message accordingly
> >
> >  kernel/kallsyms.c | 12 ++++++++----
> >  1 file changed, 8 insertions(+), 4 deletions(-)
> >
> > diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
> > index 0ba87982d017..e9148626ae6c 100644
> > --- a/kernel/kallsyms.c
> > +++ b/kernel/kallsyms.c
> > @@ -166,16 +166,20 @@ static unsigned long kallsyms_sym_address(int idx)
> >
> >  #if defined(CONFIG_CFI_CLANG) && defined(CONFIG_LTO_CLANG_THIN)
> >  /*
> > - * LLVM appends a hash to static function names when ThinLTO and CFI are
> > - * both enabled, i.e. foo() becomes foo$707af9a22804d33c81801f27dcfe489b.
> > - * This causes confusion and potentially breaks user space tools, so we
> > - * strip the suffix from expanded symbol names.
> > + * LLVM appends a hash to static function names and just ".cfi_jt" postfix
> > + * for non-static functions when both ThinLTO and CFI are enabled,
> 
> Functions aren't technically speaking renamed to add a .cfi_jt
> postfix. Instead, these are separate symbols that point to the CFI
> jump table. Perhaps the comment should just say that we want to strip
> .cfi_jt from CFI jump table symbols?
> 

Agree, in jest modified existing comment. Will address same.

> > + * i.e. for example foo() becomes foo$707af9a22804d33c81801f27dcfe489b.
> > + * This causes confusion and potentially breaks user space tools and
> > + * built-in components, so we strip the suffix from expanded symbol names.
> >   */
> >  static inline bool cleanup_symbol_name(char *s)
> >  {
> >         char *res;
> >
> >         res = strrchr(s, '$');
> > +       if (!res)
> > +               res = strstr(s, ".cfi_jt");
> > +
> >         if (res)
> >                 *res = '\0';
> 
> This looks otherwise fine to me, but it's going to conflict with
> Nick's earlier patch:
> 
> https://lore.kernel.org/lkml/20210707181814.365496-1-ndesaulniers@google.com/
> 
> Could you please rebase this on top of that, and take into account
> that we should do this when CONFIG_LTO_CLANG is enabled, not only with
> LTO_CLANG_THIN?
>

Thanks Sami for pointing out the link, will rebase and refactor the change.

> Sami