linux-kernel - [PATCH -next v2 12/12] pstore/ram: Do not treat empty buffers as valid

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Message-Id: <20181129230841.29955-13-keescook@chromium.org>
Date:   Thu, 29 Nov 2018 15:08:41 -0800
From:   Kees Cook <keescook@...omium.org>
To:     linux-kernel@...r.kernel.org
Cc:     Kees Cook <keescook@...omium.org>,
        "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        Anton Vorontsov <anton@...msg.org>,
        Colin Cross <ccross@...roid.com>,
        Tony Luck <tony.luck@...el.com>
Subject: [PATCH -next v2 12/12] pstore/ram: Do not treat empty buffers as valid

From: "Joel Fernandes (Google)" <joel@...lfernandes.org>

The ramoops backend currently calls persistent_ram_save_old() even
if a buffer is empty. While this appears to work, it is does not seem
like the right thing to do and could lead to future bugs so lets avoid
that. It also prevents misleading prints in the logs which claim the
buffer is valid.

I got something like:

	found existing buffer, size 0, start 0

When I was expecting:

	no valid data in buffer (sig = ...)

This bails out early (and reports with pr_debug()), since it's an
acceptable state.

Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
Co-developed-by: Kees Cook <keescook@...omium.org>
Signed-off-by: Kees Cook <keescook@...omium.org>
---
 fs/pstore/ram_core.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/fs/pstore/ram_core.c b/fs/pstore/ram_core.c
index e6375439c5ac..c11711c2cc83 100644
--- a/fs/pstore/ram_core.c
+++ b/fs/pstore/ram_core.c
@@ -511,6 +511,11 @@ static int persistent_ram_post_init(struct persistent_ram_zone *prz, u32 sig,
 	sig ^= PERSISTENT_RAM_SIG;
 
 	if (prz->buffer->sig == sig) {
+		if (buffer_size(prz) == 0) {
+			pr_debug("found existing empty buffer\n");
+			return 0;
+		}
+
 		if (buffer_size(prz) > prz->buffer_size ||
 		    buffer_start(prz) > buffer_size(prz)) {
 			pr_info("found existing invalid buffer, size %zu, start %zu\n",
-- 
2.17.1