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Date:   Tue, 22 Feb 2022 23:49:54 -0800
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: =?US-ASCII?Q?Re=3A_=5BPATCH_v2=5D_pstore=3A_Don=27t_use_sem?= =?US-ASCII?Q?aphores_in_always-atomic-context_code?=
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On February 18, 2022 10:19:50 AM PST, Jann Horn <jannh@google=2Ecom> wrote=
:
>pstore_dump() is *always* invoked in atomic context (nowadays in an RCU
>read-side critical section, before that under a spinlock)=2E
>It doesn't make sense to try to use semaphores here=2E

Ah, very nice=2E Thanks for the analysis!

>[=2E=2E=2E]
>-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 sa=
me single caller?

> [=2E=2E=2E]
>-			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 a=
bout corruption removed=2E Is that no longer accurate?

Otherwise looks good; thank you!

--=20
Kees Cook