linux-kernel - Re: [for-next][PATCH 04/18] x86/ftrace: Do not jump to direct code in created trampolines

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Date:   Fri, 3 Jul 2020 10:10:00 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [for-next][PATCH 04/18] x86/ftrace: Do not jump to direct code
 in created trampolines

On Thu, Jul 02, 2020 at 05:58:16PM -0400, Steven Rostedt wrote:

> +	/* No need to test direct calls on created trampolines */
> +	if (ops->flags & FTRACE_OPS_FL_SAVE_REGS) {
> +		/* NOP the jnz 1f; but make sure it's a 2 byte jnz */
> +		ip = trampoline + (jmp_offset - start_offset);
> +		if (WARN_ON(*(char *)ip != 0x75))
> +			goto fail;
> +		ret = copy_from_kernel_nofault(ip, ideal_nops[2], 2);

I really don't get this paranoia, what's wrong with memcpy() ?

> +		if (ret < 0)
> +			goto fail;
> +	}

How about something like this?

--- a/arch/x86/kernel/ftrace.c
+++ b/arch/x86/kernel/ftrace.c
@@ -359,17 +359,11 @@ create_trampoline(struct ftrace_ops *ops
 	npages = DIV_ROUND_UP(*tramp_size, PAGE_SIZE);
 
 	/* Copy ftrace_caller onto the trampoline memory */
-	ret = copy_from_kernel_nofault(trampoline, (void *)start_offset, size);
-	if (WARN_ON(ret < 0))
-		goto fail;
-
-	ip = trampoline + size;
+	memcpy(trampoline, (void *)start_offset, size);
 
 	/* The trampoline ends with ret(q) */
-	retq = (unsigned long)ftrace_stub;
-	ret = copy_from_kernel_nofault(ip, (void *)retq, RET_SIZE);
-	if (WARN_ON(ret < 0))
-		goto fail;
+	ip = trampoline + size;
+	memcpy(ip, text_gen_insn(RET_INSN_OPCODE, NULL, NULL), RET_INSN_SIZE);
 
 	/* No need to test direct calls on created trampolines */
 	if (ops->flags & FTRACE_OPS_FL_SAVE_REGS) {
@@ -377,9 +371,7 @@ create_trampoline(struct ftrace_ops *ops
 		ip = trampoline + (jmp_offset - start_offset);
 		if (WARN_ON(*(char *)ip != 0x75))
 			goto fail;
-		ret = copy_from_kernel_nofault(ip, ideal_nops[2], 2);
-		if (ret < 0)
-			goto fail;
+		memcpy(ip, ideal_nops[2], 2);
 	}
 
 	/*