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SubjectRe: [PATCH v3] kallsyms: strip LTO suffixes from static functions
On Mon, Jun 28, 2021 at 2:20 PM Nathan Chancellor <nathan@kernel.org> wrote:
>
> On Mon, Jun 28, 2021 at 01:31:06PM -0700, 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>
> > Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> > ---
> > 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 | 32 +++++++++++++++++++++++++++++---
> > 1 file changed, 29 insertions(+), 3 deletions(-)
> >
> > diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
> > index 4067564ec59f..143c69e7e75d 100644
> > --- a/kernel/kallsyms.c
> > +++ b/kernel/kallsyms.c
> > @@ -171,6 +171,26 @@ static unsigned long kallsyms_sym_address(int idx)
> > return kallsyms_relative_base - 1 - kallsyms_offsets[idx];
> > }
> >
> > +#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;
> > +}
> > +#else
> > +static bool cleanup_symbol_name_thinlto(char *s) { return false; }
> > +#endif /* CONFIG_LTO_CLANG_THIN */
> > +
> > #if defined(CONFIG_CFI_CLANG) && defined(CONFIG_LTO_CLANG_THIN)
> > /*
> > * LLVM appends a hash to static function names when ThinLTO and CFI are
> > @@ -178,7 +198,7 @@ static unsigned long kallsyms_sym_address(int idx)
> > * 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 +209,14 @@ 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 && CONFIG_LTO_CLANG_THIN */
> > +
> > +static bool cleanup_symbol_name(char *s)
> > +{
> > + return cleanup_symbol_name_thinlto(s) &&
> > + cleanup_symbol_name_thinlto_cfi(s);
> > +}
> >
> > /* Lookup the address for this symbol. Returns 0 if not found. */
> > unsigned long kallsyms_lookup_name(const char *name)
> > --
> > 2.32.0.93.g670b81a890-goog
> >
>
> Is there any reason that we cannot eliminate the stubs and combine the
> functions, or am I missing something? Completely untested diff.
>
> diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
> index c851ca0ed357..014b59ad68a3 100644
> --- a/kernel/kallsyms.c
> +++ b/kernel/kallsyms.c
> @@ -161,26 +161,36 @@ 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 inline bool cleanup_symbol_name_thinlto(char *s)

Drop inline while touching this line.

> {
> char *res;
>
> - res = strrchr(s, '$');
> + if (!IS_ENABLED(CONFIG_LTO_CLANG_THIN))
> + return false;
> +
> + /*
> + * LLVM appends a suffix for local variables that must be promoted to global
> + * scope as part of ThinLTO. foo() becomes foo.llvm.974640843467629774. This
> + * can break hooking of static functions with kprobes.
> + */
> + res = strstr(s, ".llvm.");
> if (res)
> *res = '\0';

Sure, this is nicer though within the true block we should `return
true;` early rather than additionally check the $ mangling, I suppose.

>
> + /*
> + * LLVM appends a hash to static function names when ThinLTO and CFI are
> + * both enabled, i.e. foo() becomes foo$707af9a22804d33c81801f27dcfe489b.
> + * This causes confusion and potentially breaks user space tools, so we
> + * strip the suffix from expanded symbol names.
> + */
> + if (IS_ENABLED(CONFIG_CFI_CLANG)) {
> + res = strrchr(s, '$');
> + if (res)
> + *res = '\0';
> + }
> +
> return res != NULL;
> }
> -#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)
> @@ -195,7 +205,7 @@ unsigned long kallsyms_lookup_name(const char *name)
> if (strcmp(namebuf, name) == 0)
> return kallsyms_sym_address(i);
>
> - if (cleanup_symbol_name(namebuf) && strcmp(namebuf, name) == 0)
> + if (cleanup_symbol_name_thinlto(namebuf) && strcmp(namebuf, name) == 0)
> return kallsyms_sym_address(i);
> }
> return module_kallsyms_lookup_name(name);



--
Thanks,
~Nick Desaulniers

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