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

> + * 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)
>

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