linux-kernel - Re: [PATCH v2 3/6] riscv: ftrace: prepare ftrace for atomic code patching

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Message-ID: <87sev78dnz.fsf@all.your.base.are.belong.to.us>
Date: Wed, 14 Aug 2024 14:57:52 +0200
From: Björn Töpel <bjorn@...nel.org>
To: Andy Chiu <andy.chiu@...ive.com>, Paul Walmsley
 <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>, Albert Ou
 <aou@...s.berkeley.edu>, Alexandre Ghiti <alexghiti@...osinc.com>, Zong Li
 <zong.li@...ive.com>, Steven Rostedt <rostedt@...dmis.org>, Masami
 Hiramatsu <mhiramat@...nel.org>, Mark Rutland <mark.rutland@....com>,
 Nathan Chancellor <nathan@...nel.org>, Nick Desaulniers
 <ndesaulniers@...gle.com>, Bill Wendling <morbo@...gle.com>, Justin Stitt
 <justinstitt@...gle.com>, Puranjay Mohan <puranjay@...nel.org>
Cc: Palmer Dabbelt <palmer@...osinc.com>, linux-riscv@...ts.infradead.org,
 linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
 llvm@...ts.linux.dev, Andy Chiu <andy.chiu@...ive.com>
Subject: Re: [PATCH v2 3/6] riscv: ftrace: prepare ftrace for atomic code
 patching

Björn Töpel <bjorn@...nel.org> writes:

> Andy Chiu <andy.chiu@...ive.com> writes:
>
>> We use an AUIPC+JALR pair to jump into a ftrace trampoline. Since
>> instruction fetch can break down to 4 byte at a time, it is impossible
>> to update two instructions without a race. In order to mitigate it, we
>> initialize the patchable entry to AUIPC + NOP4. Then, the run-time code
>> patching can change NOP4 to JALR to eable/disable ftrcae from a
>                                       enable        ftrace
>
>> function. This limits the reach of each ftrace entry to +-2KB displacing
>> from ftrace_caller.
>>
>> Starting from the trampoline, we add a level of indirection for it to
>> reach ftrace caller target. Now, it loads the target address from a
>> memory location, then perform the jump. This enable the kernel to update
>> the target atomically.
>
> The +-2K limit is for direct calls, right?
>
> ...and this I would say breaks DIRECT_CALLS (which should be implemented
> using call_ops later)?

Thinking a bit more, and re-reading the series.

This series is good work, and it's a big improvement for DYNAMIC_FTRACE,
but

+int ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr)
+{
+	unsigned long distance, orig_addr;
+
+	orig_addr = (unsigned long)&ftrace_caller;
+	distance = addr > orig_addr ? addr - orig_addr : orig_addr - addr;
+	if (distance > JALR_RANGE)
+		return -EINVAL;
+
+	return __ftrace_modify_call(rec->ip, addr, false);
+}
+

breaks WITH_DIRECT_CALLS. The direct trampoline will *never* be within
the JALR_RANGE.

Unless we're happy with a break (I'm not) -- I really think Puranjay's
CALL_OPS patch needs to be baked in in the series!


Björn