linux-kernel - Re: [PATCH] riscv: Fix early ftrace nop patching

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Date: Thu, 23 May 2024 14:48:55 +0200
From: Björn Töpel <bjorn@...nel.org>
To: Alexandre Ghiti <alexghiti@...osinc.com>, Paul Walmsley
 <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>, Albert Ou
 <aou@...s.berkeley.edu>, Steven Rostedt <rostedt@...dmis.org>, Masami
 Hiramatsu <mhiramat@...nel.org>, Mark Rutland <mark.rutland@....com>,
 Alexandre Ghiti <alexghiti@...osinc.com>, Andrea Parri
 <parri.andrea@...il.com>, Björn Töpel
 <bjorn@...osinc.com>, Conor Dooley
 <conor.dooley@...rochip.com>, linux-riscv@...ts.infradead.org,
 linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH] riscv: Fix early ftrace nop patching

Alexandre Ghiti <alexghiti@...osinc.com> writes:

> Commit c97bf629963e ("riscv: Fix text patching when IPI are used")
> converted ftrace_make_nop() to use patch_insn_write() which does not
> emit any icache flush relying entirely on __ftrace_modify_code() to do
> that.
>
> But we missed that ftrace_make_nop() was called very early directly when
> converting mcount calls into nops (actually on riscv it converts 2B nops
> emitted by the compiler into 4B nops).
>
> This caused crashes on multiple HW as reported by Conor and Björn since
> the booting core could have half-patched instructions in its icache
> which would trigger an illegal instruction trap: fix this by emitting a
> local flush icache when early patching nops.
>
> Fixes: c97bf629963e ("riscv: Fix text patching when IPI are used")
> Signed-off-by: Alexandre Ghiti <alexghiti@...osinc.com>

Nice!

I've manged to reproduce the crash on the VisionFive2 board (however
only triggered when CONFIG_RELOCATABLE=y), and can verify that this fix
solves the issue.

Reviewed-by: Björn Töpel <bjorn@...osinc.com>
Tested-by: Björn Töpel <bjorn@...osinc.com>