Re: [PATCH 2/3] kallsyms: Add APIs to match symbol without .llmv. suffix.

Song Liu
2024-07-31T01:00:39Z

Hi Masami,

> On Jul 30, 2024, at 6:03 AM, Masami Hiramatsu <mhiramat@kernel.org> wrote:
>
> On Mon, 29 Jul 2024 17:54:32 -0700
> Song Liu <song@kernel.org> wrote:
>
>> With CONFIG_LTO_CLANG=y, the compiler may add suffix to function names
>> to avoid duplication. This causes confusion with users of kallsyms.
>> On one hand, users like livepatch are required to match the symbols
>> exactly. On the other hand, users like kprobe would like to match to
>> original function names.
>>
>> Solve this by splitting kallsyms APIs. Specifically, existing APIs now
>> should match the symbols exactly. Add two APIs that matches the full
>> symbol, or only the part without .llvm.suffix. Specifically, the following
>> two APIs are added:
>>
>> 1. kallsyms_lookup_name_or_prefix()
>> 2. kallsyms_on_each_match_symbol_or_prefix()
>
> Since this API only removes the suffix, "match prefix" is a bit confusing.
> (this sounds like matching "foo" with "foo" and "foo_bar", but in reality,
> it only matches "foo" and "foo.llvm.*")
> What about the name below?
>
> kallsyms_lookup_name_without_suffix()
> kallsyms_on_each_match_symbol_without_suffix()

I am open to name suggestions. I named it as xx or prefix to highlight
that these two APIs will try match full name first, and they only match
the symbol without suffix when there is no full name match.

Maybe we can call them:
- kallsyms_lookup_name_or_without_suffix()
- kallsyms_on_each_match_symbol_or_without_suffix()

Again, I am open to any name selections here.

>
>>
>> These APIs will be used by kprobe.
>
> No other user need this?

AFACIT, kprobe is the only use case here. Sami, please correct
me if I missed any users.

More thoughts on this:

I actually hope we don't need these two new APIs, as they are
confusing. Modern compilers can do many things to the code
(inlining, etc.). So when we are tracing a function, we are not
really tracing "function in the source code". Instead, we are
tracing "function in the binary". If a function is inlined, it
will not show up in the binary. If a function is _partially_
inlined (inlined by some callers, but not by others), it will
show up in the binary, but we won't be tracing it as it appears
in the source code. Therefore, tracing functions by their names
in the source code only works under certain assumptions. And
these assumptions may not hold with modern compilers. Ideally,
I think we cannot promise the user can use name "ping_table" to
trace function "ping_table.llvm.15394922576589127018"

Does this make sense?

Thanks,
Song …