Re: [PATCH v2] pstore: Don't use semaphores in always-atomic-context code

From: Kees Cook <keescook@chromium.org>
To: Jann Horn <jannh@google.com>, Anton Vorontsov <anton@enomsg.org>,
	Colin Cross <ccross@android.com>, Tony Luck <tony.luck@intel.com>,
	Ard Biesheuvel <ardb@kernel.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	linux-efi@vger.kernel.org, linux-kernel@vger.kernel.org,
	stable@vger.kernel.org
Subject: Re: [PATCH v2] pstore: Don't use semaphores in always-atomic-context code
Date: Tue, 22 Feb 2022 23:49:54 -0800	[thread overview]
Message-ID: <8D85619E-99BD-4DB5-BDDB-A205B057C910@chromium.org> (raw)
In-Reply-To: <20220218181950.1438236-1-jannh@google.com>

On February 18, 2022 10:19:50 AM PST, Jann Horn <jannh@google.com> wrote:
>pstore_dump() is *always* invoked in atomic context (nowadays in an RCU
>read-side critical section, before that under a spinlock).
>It doesn't make sense to try to use semaphores here.

Ah, very nice. Thanks for the analysis!

>[...]
>-static bool pstore_cannot_wait(enum kmsg_dump_reason reason)
>+bool pstore_cannot_block_path(enum kmsg_dump_reason reason)

Why the rename, extern, and EXPORT? This appears to still only have the same single caller?

> [...]
>-			pr_err("dump skipped in %s path: may corrupt error record\n",
>-				in_nmi() ? "NMI" : why);
>-			return;
>-		}
>-		if (down_interruptible(&psinfo->buf_lock)) {
>-			pr_err("could not grab semaphore?!\n");
>+	if (pstore_cannot_block_path(reason)) {
>+		if (!spin_trylock_irqsave(&psinfo->buf_lock, flags)) {
>+			pr_err("dump skipped in %s path because of concurrent dump\n"
>+				       , in_nmi() ? "NMI" : why);

The pr_err had the comma following the format string moved, and the note about corruption removed. Is that no longer accurate?

Otherwise looks good; thank you!

-- 
Kees Cook