[PATCH bpf-next] bpf: avoid get_kernel_nofault() to fetch kprobe entry IP

[Andrii Nakryiko <andrii@kernel.org>]

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
>= 4, in which case we can do direct memory read.

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.

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
---
 kernel/trace/bpf_trace.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index 0a5c4efc73c3..f81adabda38c 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -1053,9 +1053,15 @@ static unsigned long get_entry_ip(unsigned long fentry_ip)
 {
 	u32 instr;
 
-	/* Being extra safe in here in case entry ip is on the page-edge. */
-	if (get_kernel_nofault(instr, (u32 *) fentry_ip - 1))
-		return fentry_ip;
+	/* We want to be extra safe in case entry ip is on the page edge,
+	 * but otherwise we need to avoid get_kernel_nofault()'s overhead.
+	 */
+	if ((fentry_ip & ~PAGE_MASK) < ENDBR_INSN_SIZE) {
+		if (get_kernel_nofault(instr, (u32 *)(fentry_ip - ENDBR_INSN_SIZE)))
+			return fentry_ip;
+	} else {
+		instr = *(u32 *)(fentry_ip - ENDBR_INSN_SIZE);
+	}
 	if (is_endbr(instr))
 		fentry_ip -= ENDBR_INSN_SIZE;
 	return fentry_ip;
-- 
2.43.0