Re: [PATCH v4] kallsyms: strip LTO suffixes from static functions

["Fāng-ruì Sòng" <maskray@google.com>]

On Wed, Jul 7, 2021 at 11:34 AM Nathan Chancellor <nathan@kernel.org> wrote:
>
> On 7/7/2021 11:18 AM, Nick Desaulniers wrote:
> > Similar to:
> > commit 8b8e6b5d3b01 ("kallsyms: strip ThinLTO hashes from static
> > functions")
> >
> > It's very common for compilers to modify the symbol name for static
> > functions as part of optimizing transformations. That makes hooking
> > static functions (that weren't inlined or DCE'd) with kprobes difficult.
> >
> > LLVM has yet another name mangling scheme used by thin LTO.  Strip off
> > these suffixes so that we can continue to hook such static functions.
> >
> > Reported-by: KE.LI(Lieke) <like1@oppo.com>
> > Suggested-by: Nathan Chancellor <nathan@kernel.org>
> > Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
>
> Code looks fine, small comment about a comment below.
>
> Reviewed-by: Nathan Chancellor <nathan@kernel.org>
>
> > ---
> > Changes v3 -> v4:
> > * Convert this function to use IS_ENABLED rather than provide multiple
> >    definitions based on preprocessor checks.
> > * Add Nathan's suggested-by.
> >
> > Changes v2 -> v3:
> > * Un-nest preprocessor checks, as per Nathan.
> >
> > Changes v1 -> v2:
> > * Both mangling schemes can occur for thinLTO + CFI, this new scheme can
> >    also occur for thinLTO without CFI. Split cleanup_symbol_name() into
> >    two function calls.
> > * Drop KE.LI's tested by tag.
> > * Do not carry Fangrui's Reviewed by tag.
> > * Drop the inline keyword; it is meaningless.
> >
> >   kernel/kallsyms.c | 43 ++++++++++++++++++++++++++++++-------------
> >   1 file changed, 30 insertions(+), 13 deletions(-)
> >
> > diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
> > index 4067564ec59f..a10dab216f4f 100644
> > --- a/kernel/kallsyms.c
> > +++ b/kernel/kallsyms.c
> > @@ -171,26 +171,43 @@ static unsigned long kallsyms_sym_address(int idx)
> >       return kallsyms_relative_base - 1 - kallsyms_offsets[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.
> > - */
> > -static inline bool cleanup_symbol_name(char *s)
> > +static bool cleanup_symbol_name(char *s)
> >   {
> >       char *res;
> >
> > +     /*
> > +      * LLVM appends a suffix for local variables that must be promoted to
>
> This says local variables but the example uses a function? Is that correct?

local functions/variables.

Both functions and variables can have a .llvm.[0-9]+ suffix.


Aside from this, the updated description looks good to me

Reviewed-by: Fangrui Song <maskray@google.com>

> > +      * global scope as part of ThinLTO. foo() becomes
> > +      * foo.llvm.974640843467629774. This can break hooking of static
> > +      * functions with kprobes.
> > +      */
> > +     if (!IS_ENABLED(CONFIG_LTO_CLANG_THIN))
> > +             return false;
> > +
> > +     res = strstr(s, ".llvm.");
> > +     if (res) {
> > +             *res = '\0';
> > +             return true;
> > +     }
> > +
> > +     /*
> > +      * LLVM appends a hash to static function names when ThinLTO and CFI
> > +      * are both enabled, i.e. foo() becomes
> > +      * foo$707af9a22804d33c81801f27dcfe489b. This causes confusion and
> > +      * potentially breaks user space tools, so we strip the suffix from
> > +      * expanded symbol names.
> > +      */
> > +     if (!IS_ENABLED(CONFIG_CFI_CLANG))
> > +             return false;
> > +
> >       res = strrchr(s, '$');
> > -     if (res)
> > +     if (res) {
> >               *res = '\0';
> > +             return true;
> > +     }
> >
> > -     return res != NULL;
> > +     return false;
> >   }
> > -#else
> > -static inline bool cleanup_symbol_name(char *s) { return false; }
> > -#endif
> >
> >   /* Lookup the address for this symbol. Returns 0 if not found. */
> >   unsigned long kallsyms_lookup_name(const char *name)
> >