Re: [PATCH v2] kallsyms: strip LTO suffixes from static functions

[Nathan Chancellor]

On 6/28/2021 12:05 PM, 'Nick Desaulniers' via Clang Built Linux 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@xxxxxxxx>
> Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> ---
> 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 | 33 +++++++++++++++++++++++++++++----
>   1 file changed, 29 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
> index 4067564ec59f..fbce4a1ec700 100644
> --- a/kernel/kallsyms.c
> +++ b/kernel/kallsyms.c
> @@ -171,14 +171,30 @@ 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)
> +#ifdef CONFIG_LTO_CLANG_THIN
> +/*
> + * LLVM appends a suffix for local variables that must be promoted to global
> + * scope as part of thin LTO. foo() becomes foo.llvm.974640843467629774. This
> + * can break hooking of static functions with kprobes.
> + */
> +static bool cleanup_symbol_name_thinlto(char *s)
> +{
> +	char *res;
> +
> +	res = strstr(s, ".llvm.");
> +	if (res)
> +		*res = '\0';
> +
> +	return res != NULL;
> +}
> +#ifdef CONFIG_CFI_CLANG
>   /*
>    * 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_thinlto_cfi(char *s)
>   {
>   	char *res;
>   
> @@ -189,8 +205,17 @@ static inline bool cleanup_symbol_name(char *s)
>   	return res != NULL;
>   }
>   #else
> -static inline bool cleanup_symbol_name(char *s) { return false; }
> -#endif
> +static bool cleanup_symbol_name_thinlto_cfi(char *s) { return false; }
> +#endif /* CONFIG_CFI_CLANG */
> +#else
> +static bool cleanup_symbol_name_thinlto(char *s) { return false; }
> +#endif /* CONFIG_LTO_CLANG_THIN */
> +
> +static bool cleanup_symbol_name(char *s)
> +{
> +	return cleanup_symbol_name_thinlto(s) &&
> +		cleanup_symbol_name_thinlto_cfi(s);

Won't this be a build error when CONFIG_LTO_CLANG_THIN=n and 
CONFIG_CFI_CLANG=n because cleanup_symbol_name_thinlto_cfi() will not be 
defined? Should the cleanup_symbol_name_thinlto_cfi() stub be in the 
last else block?

Cheers,
Nathan

> +}
>   
>   /* Lookup the address for this symbol. Returns 0 if not found. */
>   unsigned long kallsyms_lookup_name(const char *name)