Re: [PATCH bpf-next v4 05/12] bpf, tracing: load the vmlinux BTF where tracefs and bpffs requests start

With CONFIG_DEBUG_INFO_BTF=m, bpf_get_btf_vmlinux() and
bpf_find_btf_id() do not load the vmlinux BTF: loading waits for user
space, and their callers were not written for that.  Besides the bpf()
system call and /sys/kernel/btf/vmlinux, some tracefs and bpffs requests
need the BTF.  Load it at the start of those, with
bpf_load_btf_vmlinux(), and have the code that only uses the BTF if it
happens to be there peek:

 - Reading a tracepoint's btf_ids file (events/*/btf_ids): built-in
   events use the vmlinux BTF, and module BTF is only registered once
   that is loaded.  event_btf_ids_read() looks the BTF up under
   event_mutex, which the trace notifier of the btf_vmlinux module
   takes, so load it before taking the mutex, on the first read() of
   the file (not again for the one that returns EOF).
 - Probe events with BTF arguments ($argN, argument names, $retval,
[ ... ]

With CONFIG_DEBUG_INFO_BTF=y bpf_load_btf_vmlinux() is
bpf_get_btf_vmlinux() and the BTF is parsed at boot, so nothing changes.

Signed-off-by: Jay Wang <redacted>

Sashiko has reviewed this patch and found no issues. It looks great!

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Sashiko AI review · https://sashiko.dev/#/patchset/20261001225214.12351-1-wanjay@amazon.com?part=5