Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Mon, 26 Nov 2018 16:43:56 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Josh Poimboeuf <jpoimboe@redhat.com>
Cc:     x86@kernel.org, linux-kernel@vger.kernel.org,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Andy Lutomirski <luto@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Jason Baron <jbaron@akamai.com>, Jiri Kosina <jkosina@suse.cz>,
        David Laight <David.Laight@ACULAB.COM>,
        Borislav Petkov <bp@alien8.de>,
        Julia Cartwright <julia@ni.com>, Jessica Yu <jeyu@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH v2 3/4] x86/static_call: Add out-of-line static call
 implementation
Message-ID: <20181126154356.GP2113@hirez.programming.kicks-ass.net>
References: <cover.1543200841.git.jpoimboe@redhat.com>
 <00b08f2194e80241decbf206624b6580b9b8855b.1543200841.git.jpoimboe@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <00b08f2194e80241decbf206624b6580b9b8855b.1543200841.git.jpoimboe@redhat.com>
User-Agent: Mutt/1.10.1 (2018-07-13)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Mon, Nov 26, 2018 at 07:54:59AM -0600, Josh Poimboeuf wrote:

> +void static_call_bp_handler(void);
> +void *bp_handler_dest;
> +
> +asm(".pushsection .text, \"ax\"						\n"
> +    ".globl static_call_bp_handler					\n"
> +    ".type static_call_bp_handler, @function				\n"
> +    "static_call_bp_handler:						\n"
> +    "ANNOTATE_RETPOLINE_SAFE						\n"
> +    "jmp *bp_handler_dest						\n"
> +    ".popsection							\n");
> +
> +void arch_static_call_transform(void *site, void *tramp, void *func)
> +{
> +	s32 dest_relative;
> +	unsigned long insn;
> +	unsigned char insn_opcode;
> +	unsigned char opcodes[CALL_INSN_SIZE];
> +
> +	insn = (unsigned long)tramp;
> +
> +	mutex_lock(&text_mutex);
> +
> +	insn_opcode = *(unsigned char *)insn;
> +	if (insn_opcode != 0xe8 && insn_opcode != 0xe9) {
> +		WARN_ONCE(1, "unexpected static call insn opcode 0x%x at %pS",
> +			  insn_opcode, (void *)insn);
> +		goto done;
> +	}
> +
> +	dest_relative = (long)(func) - (long)(insn + CALL_INSN_SIZE);
> +
> +	opcodes[0] = insn_opcode;
> +	memcpy(&opcodes[1], &dest_relative, CALL_INSN_SIZE - 1);
> +
> +	/* Set up the variable for the breakpoint handler: */
> +	bp_handler_dest = func;
> +
> +	/* Patch the call site: */
> +	text_poke_bp((void *)insn, opcodes, CALL_INSN_SIZE,
> +		     static_call_bp_handler);

I'm confused by the whole static_call_bp_handler thing; why not jump
straight to @func ?

Also, what guarantees this other thread will have gotten from
static_call_bp_handler and executed the actual indirect JMP instruction
by the time we re-write @bp_handler_dest again?

> +done:
> +	mutex_unlock(&text_mutex);
> +}