Re: [PATCH v2 3/3] tracing/kprobes: Use APIs that matches symbols without .XXX suffix

[Steven Rostedt <rostedt@goodmis.org>]

On Fri,  2 Aug 2024 14:08:35 -0700
Song Liu <song@kernel.org> wrote:

> Use the new kallsyms APIs that matches symbols name with .XXX
> suffix. This allows userspace tools to get kprobes on the expected
> function name, while the actual symbol has a .llvm.<hash> suffix.
> 
> This only effects kernel compile with CONFIG_LTO_CLANG.
> 
> Signed-off-by: Song Liu <song@kernel.org>
> ---
>  kernel/kprobes.c            |  6 +++++-
>  kernel/trace/trace_kprobe.c | 11 ++++++++++-
>  2 files changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> index e85de37d9e1e..99102283b076 100644
> --- a/kernel/kprobes.c
> +++ b/kernel/kprobes.c
> @@ -70,7 +70,11 @@ static DEFINE_PER_CPU(struct kprobe *, kprobe_instance);
>  kprobe_opcode_t * __weak kprobe_lookup_name(const char *name,
>  					unsigned int __unused)
>  {
> -	return ((kprobe_opcode_t *)(kallsyms_lookup_name(name)));
> +	unsigned long addr = kallsyms_lookup_name(name);
> +
> +	if (IS_ENABLED(CONFIG_LTO_CLANG) && !addr)
> +		addr = kallsyms_lookup_name_without_suffix(name);
> +	return ((kprobe_opcode_t *)(addr));
>  }
>  
>  /*
> diff --git a/kernel/trace/trace_kprobe.c b/kernel/trace/trace_kprobe.c
> index 61a6da808203..d2ad0c561c83 100644
> --- a/kernel/trace/trace_kprobe.c
> +++ b/kernel/trace/trace_kprobe.c
> @@ -203,6 +203,10 @@ unsigned long trace_kprobe_address(struct trace_kprobe *tk)
>  	if (tk->symbol) {
>  		addr = (unsigned long)
>  			kallsyms_lookup_name(trace_kprobe_symbol(tk));
> +
> +		if (IS_ENABLED(CONFIG_LTO_CLANG) && !addr)
> +			addr = kallsyms_lookup_name_without_suffix(trace_kprobe_symbol(tk));
> +

So you do the lookup twice if this is enabled?

Why not just use "kallsyms_lookup_name_without_suffix()" the entire time,
and it should work just the same as "kallsyms_lookup_name()" if it's not
needed?

>  		if (addr)
>  			addr += tk->rp.kp.offset;
>  	} else {
> @@ -766,8 +770,13 @@ static unsigned int number_of_same_symbols(const char *mod, const char *func_nam
>  {
>  	struct sym_count_ctx ctx = { .count = 0, .name = func_name };
>  
> -	if (!mod)
> +	if (!mod) {
>  		kallsyms_on_each_match_symbol(count_symbols, func_name, &ctx.count);
> +		if (IS_ENABLED(CONFIG_LTO_CLANG) && !ctx.count) {
> +			kallsyms_on_each_match_symbol_without_suffix(
> +				count_symbols, func_name, &ctx.count);

Same here.

-- Steve

> +		}
> +	}
>  
>  	module_kallsyms_on_each_symbol(mod, count_mod_symbols, &ctx);
>