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From:   Rasmus Villemoes <linux@rasmusvillemoes.dk>
To:     Ard Biesheuvel <ardb@kernel.org>
Cc:     Alex Kluver <alex.kluver@hpe.com>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        linux-efi@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: [PATCH] efi: cper: fix scnprintf() use in cper_mem_err_location()
Date:   Wed, 21 Apr 2021 21:31:46 +0200
Message-Id: <20210421193146.1539649-1-linux@rasmusvillemoes.dk>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

The last two if-clauses fail to update n, so whatever they might have
written at &msg[n] would be cut off by the final nul-termination.

That nul-termination is redundant; scnprintf(), just like snprintf(),
guarantees a nul-terminated output buffer, provided the buffer size is
positive.

And there's no need to discount one byte from the initial buffer;
vsnprintf() expects to be given the full buffer size - it's not going
to write the nul-terminator one beyond the given (buffer, size) pair.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
---
 drivers/firmware/efi/cper.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/drivers/firmware/efi/cper.c b/drivers/firmware/efi/cper.c
index e15d484b6a5a..dfa0bd140bef 100644
--- a/drivers/firmware/efi/cper.c
+++ b/drivers/firmware/efi/cper.c
@@ -221,7 +221,7 @@ static int cper_mem_err_location(struct cper_mem_err_compact *mem, char *msg)
 		return 0;
 
 	n = 0;
-	len = CPER_REC_LEN - 1;
+	len = CPER_REC_LEN;
 	if (mem->validation_bits & CPER_MEM_VALID_NODE)
 		n += scnprintf(msg + n, len - n, "node: %d ", mem->node);
 	if (mem->validation_bits & CPER_MEM_VALID_CARD)
@@ -258,13 +258,12 @@ static int cper_mem_err_location(struct cper_mem_err_compact *mem, char *msg)
 		n += scnprintf(msg + n, len - n, "responder_id: 0x%016llx ",
 			       mem->responder_id);
 	if (mem->validation_bits & CPER_MEM_VALID_TARGET_ID)
-		scnprintf(msg + n, len - n, "target_id: 0x%016llx ",
-			  mem->target_id);
+		n += scnprintf(msg + n, len - n, "target_id: 0x%016llx ",
+			       mem->target_id);
 	if (mem->validation_bits & CPER_MEM_VALID_CHIP_ID)
-		scnprintf(msg + n, len - n, "chip_id: %d ",
-			  mem->extended >> CPER_MEM_CHIP_ID_SHIFT);
+		n += scnprintf(msg + n, len - n, "chip_id: %d ",
+			       mem->extended >> CPER_MEM_CHIP_ID_SHIFT);
 
-	msg[n] = '\0';
 	return n;
 }
 
-- 
2.29.2