linux-kernel - [PATCH 1/6] x86/alternatives: clarify text_mutex use in text

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Date:   Thu, 30 Aug 2018 10:32:13 -0700
From:   Nadav Amit <namit@...are.com>
To:     Thomas Gleixner <tglx@...utronix.de>
CC:     <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...hat.com>,
        <x86@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        <linux-arch@...r.kernel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Nadav Amit <namit@...are.com>,
        Andy Lutomirski <luto@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Dave Hansen <dave.hansen@...el.com>
Subject: [PATCH 1/6] x86/alternatives: clarify text_mutex use in text_poke

text_mutex is expected to be held before text_poke() is called, but we
cannot add a lockdep assertion since kgdb does not take it, and instead
*supposedly* ensures the lock is not taken and will not be acquired by
any other core while text_poke() is running.

The reason for the "supposedly" comment is that it is not entirely clear
that this would be the case if gdb_do_roundup is zero.

Add a comment to clarify this behavior.

Cc: Andy Lutomirski <luto@...nel.org>
Cc: Masami Hiramatsu <mhiramat@...nel.org>
Cc: Kees Cook <keescook@...omium.org>
Cc: Dave Hansen <dave.hansen@...el.com>
Suggested-by: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Nadav Amit <namit@...are.com>
---
 arch/x86/kernel/alternative.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 014f214da581..d0c0d8b724e1 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -685,7 +685,10 @@ void *__init_or_module text_poke_early(void *addr, const void *opcode,
  * in a way that permits an atomic write. It also makes sure we fit on a single
  * page.
  *
- * Note: Must be called under text_mutex.
+ * Context: Must be called under text_mutex. kgdb is an exception: it does not
+ *	    hold the mutex, as it *supposedly* ensures that no other core is
+ *	    holding the mutex and ensures that none of them will acquire the
+ *	    mutex while the code runs.
  */
 void *text_poke(void *addr, const void *opcode, size_t len)
 {
-- 
2.17.1