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From:   Christophe JAILLET <christophe.jaillet@wanadoo.fr>
To:     Kent Overstreet <kent.overstreet@linux.dev>,
        Brian Foster <bfoster@redhat.com>
Cc:     linux-kernel@vger.kernel.org, kernel-janitors@vger.kernel.org,
        Christophe JAILLET <christophe.jaillet@wanadoo.fr>,
        linux-bcachefs@vger.kernel.org
Subject: [PATCH] bcachefs: Avoid a potential useless over memory allocation in bch2_prt_vprintf()
Date:   Sat, 16 Sep 2023 08:20:24 +0200
Message-Id: <0f40108bed3e084057223bdbe32c4b37f8500ff3.1694845203.git.christophe.jaillet@wanadoo.fr>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

printbuf_remaining() returns the number of characters we can print to the
output buffer - i.e. excluding the terminating null.

vsnprintf() takes the size of the buffer, including the trailing null
space.
It is truncated if the returned value is greater than or equal to the size
of the buffer.

Knowing all that, buffer sizes and overflow checks can be fixed in order
to potentially avoid a useless memory over-allocation.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
---
Un-tested
---
 fs/bcachefs/printbuf.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/fs/bcachefs/printbuf.c b/fs/bcachefs/printbuf.c
index de41f9a14492..77bee9060bfe 100644
--- a/fs/bcachefs/printbuf.c
+++ b/fs/bcachefs/printbuf.c
@@ -54,8 +54,9 @@ void bch2_prt_vprintf(struct printbuf *out, const char *fmt, va_list args)
 		va_list args2;
 
 		va_copy(args2, args);
-		len = vsnprintf(out->buf + out->pos, printbuf_remaining(out), fmt, args2);
-	} while (len + 1 >= printbuf_remaining(out) &&
+		len = vsnprintf(out->buf + out->pos, printbuf_remaining(out) + 1,
+				fmt, args2);
+	} while (len >= printbuf_remaining(out) + 1 &&
 		 !bch2_printbuf_make_room(out, len + 1));
 
 	len = min_t(size_t, len,
@@ -70,9 +71,10 @@ void bch2_prt_printf(struct printbuf *out, const char *fmt, ...)
 
 	do {
 		va_start(args, fmt);
-		len = vsnprintf(out->buf + out->pos, printbuf_remaining(out), fmt, args);
+		len = vsnprintf(out->buf + out->pos, printbuf_remaining(out) + 1,
+				fmt, args);
 		va_end(args);
-	} while (len + 1 >= printbuf_remaining(out) &&
+	} while (len >= printbuf_remaining(out) + 1 &&
 		 !bch2_printbuf_make_room(out, len + 1));
 
 	len = min_t(size_t, len,
-- 
2.34.1