bpf: Avoid get_kernel_nofault() to fetch kprobe entry IP

get_kernel_nofault() (or, rather, underlying copy_from_kernel_nofault())
is not free and it does pop up in performance profiles when
kprobes are heavily utilized with CONFIG_X86_KERNEL_IBT=y config.

Let's avoid using it if we know that fentry_ip - 4 can't cross page
boundary. We do that by masking lowest 12 bits and checking if they are

Another benefit (and actually what caused a closer look at this part of
code) is that now LBR record is (typically) not wasted on
copy_from_kernel_nofault() call and code, which helps tools like
retsnoop that grab LBR records from inside BPF code in kretprobes.

Signed-off-by: Andrii Nakryiko <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Acked-by: Jiri Olsa <[email protected]>
Acked-by: Masami Hiramatsu (Google) <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

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