Re: [PATCH 01/17] Fix "x86/alternatives: Lockdep-enforce text_mutex in text_poke*()"

On Wed, Jan 16, 2019 at 04:32:43PM -0800, Rick Edgecombe wrote:

From: Nadav Amit <redacted>

text_mutex is currently expected to be held before text_poke() is
called, but we kgdb does not take the mutex, 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.

I guess that variable name is "kgdb_do_roundup" ?

This patch creates two wrapper functions, text_poke() and

Avoid having "This patch" or "This commit" in the commit message. It is
tautologically useless.

Also, do

$ git grep 'This patch' Documentation/process

for more details.

text_poke_kgdb() which do or do not run the lockdep assertion
respectively.

While we are at it, change the return code of text_poke() to something
meaningful. One day, callers might actually respect it and the existing
BUG_ON() when patching fails could be removed. For kgdb, the return
value can actually be used.

Cc: Andy Lutomirski <luto@kernel.org>
Cc: Kees Cook <redacted>
Cc: Dave Hansen <redacted>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Fixes: 9222f606506c ("x86/alternatives: Lockdep-enforce text_mutex in text_poke*()")
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Acked-by: Jiri Kosina <redacted>
Signed-off-by: Nadav Amit <redacted>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
---
 arch/x86/include/asm/text-patching.h |  1 +
 arch/x86/kernel/alternative.c        | 52 ++++++++++++++++++++--------
 arch/x86/kernel/kgdb.c               | 11 +++---
 3 files changed, 45 insertions(+), 19 deletions(-)

...

+/**
+ * text_poke_kgdb - Update instructions on a live kernel by kgdb
+ * @addr: address to modify
+ * @opcode: source of the copy
+ * @len: length to copy
+ *
+ * Only atomic text poke/set should be allowed when not doing early patching.
+ * It means the size must be writable atomically and the address must be aligned
+ * in a way that permits an atomic write. It also makes sure we fit on a single
+ * page.
+ *
+ * Context: should only be used by kgdb, which ensures no other core is running,
+ *	    despite the fact it does not hold the text_mutex.
+ */
+void *text_poke_kgdb(void *addr, const void *opcode, size_t len)

text_poke_unlocked() I guess. I don't think kgdb is that special that it
needs its own function flavor.

-- 
Regards/Gruss,
    Boris.

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