[PATCH 1/6] x86/alternatives: clarify text_mutex use in text_poke

From: Nadav Amit
Date: Thu Aug 30 2018 - 13:34:11 EST

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@xxxxxxxxxx>
Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>
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)