[PATCH v2] bcachefs: Avoid a potential useless over memory allocation in bch2_prt_[v]printf()

[Christophe JAILLET]

2 issues related to incorrect available space in the output buffer
computation may lead to some extra memory allocation.


First: vsnprintf() takes the size of the buffer, *including* the space for
the trailing null.

But printbuf_remaining() returns the number of characters we can print
to the output buffer, *excluding* the terminating null.

So, use printbuf_remaining_size(), which includes the space for the
terminating null.


Second: if the return value of vsnprintf() is greater than or equal to the
passed size, the resulting string is truncated.
So, in order to see if some extra space is needed, the check needs to be
changed.

Signed-off-by: Christophe JAILLET <christophe.jaillet@xxxxxxxxxx>
---
Un-tested

v2: - Use printbuf_remaining_size() instead of hand-writing it.  [Brian Foster]
    - Reword the commit log, hoping it is clearer.
    - Synch with -next-20240215

v1: https://lore.kernel.org/all/0f40108bed3e084057223bdbe32c4b37f8500ff3.1694845203.git.christophe.jaillet@xxxxxxxxxx/
---
 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 b27d22925929..8cee9c2f88c4 100644
--- a/fs/bcachefs/printbuf.c
+++ b/fs/bcachefs/printbuf.c
@@ -55,9 +55,10 @@ 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);
+		len = vsnprintf(out->buf + out->pos, printbuf_remaining_size(out),
+				fmt, args2);
 		va_end(args2);
-	} while (len + 1 >= printbuf_remaining(out) &&
+	} while (len >= printbuf_remaining_size(out) &&
 		 !bch2_printbuf_make_room(out, len + 1));
 
 	len = min_t(size_t, len,
@@ -72,9 +73,10 @@ 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_size(out),
+				fmt, args);
 		va_end(args);
-	} while (len + 1 >= printbuf_remaining(out) &&
+	} while (len >= printbuf_remaining_size(out) &&
 		 !bch2_printbuf_make_room(out, len + 1));
 
 	len = min_t(size_t, len,
-- 
2.43.2