linux-kernel - confused about kprobes

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Date:   Sun, 6 Aug 2023 13:18:28 +0200
From:   Nam Cao <namcaov@...il.com>
To:     "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
        "David S. Miller" <davem@...emloft.net>,
        Masami Hiramatsu <mhiramat@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: confused about kprobes

Hello,

I am struggling to understand how kprobes works. It would be very nice if someone
can spare the time to explain to me. I'm confused about this function in particular:

/*
 * Return an optimized kprobe whose optimizing code replaces
 * instructions including 'addr' (exclude breakpoint).
 */
static struct kprobe *get_optimized_kprobe(kprobe_opcode_t *addr)
{
	int i;
	struct kprobe *p = NULL;
	struct optimized_kprobe *op;

	/* Don't check i == 0, since that is a breakpoint case. */
	for (i = 1; !p && i < MAX_OPTIMIZED_LENGTH / sizeof(kprobe_opcode_t); i++)
		p = get_kprobe(addr - i);

	if (p && kprobe_optready(p)) {
		op = container_of(p, struct optimized_kprobe, kp);
		if (arch_within_optimized_kprobe(op, addr))
			return p;
	}

	return NULL;
}

The document mentions something about optimizing by replacing trap instructions
with jump instructions, so I am assuming this function is part of that. But I
fail to see what this function is trying to do exactly. The for loop seems to
call get_kprobe at addresses immediately before "addr". But what for? What are
at addresses before "addr"?

Can someone be so kind to give me a line-by-line explanation of this function?

Thanks!

Nam