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

[Fangrui Song]

On 2021-06-22, '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.
>
> Full LTO uses a different mangling scheme than thin LTO; full LTO
> imports all code into effectively one big translation unit. It must
> rename static functions to prevent collisions. Strip off these suffixes
> so that we can continue to hook such static functions.

See below. The message needs a change.

I can comment on the LTO side thing, but a maintainer needs to check
about the kernel side logic.

Reviewed-by: Fangrui Song <maskray@xxxxxxxxxx>

> Reported-by: KE.LI(Lieke) <like1@xxxxxxxx>
> Tested-by: KE.LI(Lieke) <like1@xxxxxxxx>
> Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> ---
> kernel/kallsyms.c | 18 ++++++++++++++++++
> 1 file changed, 18 insertions(+)
>
> diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
> index 4067564ec59f..14cf3a6474de 100644
> --- a/kernel/kallsyms.c
> +++ b/kernel/kallsyms.c
> @@ -188,6 +188,24 @@ static inline bool cleanup_symbol_name(char *s)
>
> 	return res != NULL;
> }
> +#elif defined(CONFIG_LTO_CLANG_FULL)
> +/*
> + * LLVM mangles static functions for full LTO so that two static functions with
> + * the same identifier do not collide when all code is combined into one
> + * module. The scheme used converts references to foo into
> + * foo.llvm.974640843467629774, for example. This can break hooking of static
> + * functions with kprobes.
> + */

The comment should say ThinLTO instead.

The .llvm.123 suffix is for global scope promotion for local linkage
symbols. The scheme is ThinLTO specific. This ensures that a local
linkage symbol, when imported into multiple translation units, then
compiled into different object files, during linking, the copies can be
deduplicated. This matters for code size and for correctness when the
function address is taken.

Regular LTO (sometimes called full LTO) uses the regular name.\d+
scheme.

> +static inline bool cleanup_symbol_name(char *s)
> +{
> +	char *res;
> +
> +	res = strstr(s, ".llvm.");
> +	if (res)
> +		*res = '\0';
> +
> +	return res != NULL;
> +}
> #else
> static inline bool cleanup_symbol_name(char *s) { return false; }
> #endif
> --
> 2.32.0.288.g62a8d224e6-goog

I wonder whether it makes sense to strip all `.something` suffixes.
For example, the recent -funique-internal-linkage-name (which can
improve sample profile accuracy) uses the `.__uniq.1234` scheme.

Function specialization/clones can create arbitrary `.123` suffixes.

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