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

[Song Liu]

Hi Steven,

> On Aug 6, 2024, at 11:44 AM, Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> 
> On Fri,  2 Aug 2024 14:08:35 -0700
> Song Liu <song@xxxxxxxxxx> 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@xxxxxxxxxx>
>> ---
>> 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?

We still want to give priority to full match. For example, we have:

[root@~]# grep c_next /proc/kallsyms
ffffffff81419dc0 t c_next.llvm.7567888411731313343
ffffffff81680600 t c_next
ffffffff81854380 t c_next.llvm.14337844803752139461

If the goal is to explicitly trace c_next.llvm.7567888411731313343, the
user can provide the full name. If we always match _without_suffix, all
of the 3 will match to the first one. 

Does this make sense?

Thanks,
Song