From 7ea701ffc7cf306a903f2966519b3f5aedfb77ea Mon Sep 17 00:00:00 2001
From: Eric Biggers <ebiggers@google.com>
Date: Tue, 11 Jun 2024 11:29:28 -0700
Subject: xfs_io: fix mread with length 1 mod page size

Fix a weird bug in mread where if you passed it a length that was 1
modulo the page size, for example

        xfs_io -r file -c "mmap -r 0 8192" -c "mread -v 0 4097"

... it never reset its pointer into the buffer into which it copies the
data from the memory map.  This caused an out-of-bounds write, which
depending on the length passed could be very large and reliably
segfault.  Also nothing was printed, despite the use of -v option.

(I don't know if this case gets reached by any existing xfstest, but
presumably not.  I noticed it while working on a patch to an xfstest.)

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
---
 io/mmap.c | 20 ++++++--------------
 1 file changed, 6 insertions(+), 14 deletions(-)

diff --git a/io/mmap.c b/io/mmap.c
index 425957d..a5f30e0 100644
--- a/io/mmap.c
+++ b/io/mmap.c
@@ -472,34 +472,26 @@ mread_f(
 		dumplen = pagesize;
 
 	if (rflag) {
-		for (tmp = length - 1, c = 0; tmp >= 0; tmp--, c = 1) {
-			*bp = *(((char *)mapping->addr) + dumpoffset + tmp);
-			cnt++;
-			if (c && cnt == dumplen) {
+		for (tmp = length - 1; tmp >= 0; tmp--) {
+			bp[cnt++] = ((char *)mapping->addr)[dumpoffset + tmp];
+			if (cnt == dumplen) {
 				if (dump) {
 					dump_buffer(printoffset, dumplen);
 					printoffset += dumplen;
 				}
-				bp = (char *)io_buffer;
 				dumplen = pagesize;
 				cnt = 0;
-			} else {
-				bp++;
 			}
 		}
 	} else {
-		for (tmp = 0, c = 0; tmp < length; tmp++, c = 1) {
-			*bp = *(((char *)mapping->addr) + dumpoffset + tmp);
-			cnt++;
-			if (c && cnt == dumplen) {
+		for (tmp = 0; tmp < length; tmp++) {
+			bp[cnt++] = ((char *)mapping->addr)[dumpoffset + tmp];
+			if (cnt == dumplen) {
 				if (dump)
 					dump_buffer(printoffset + tmp -
 						(dumplen - 1), dumplen);
-				bp = (char *)io_buffer;
 				dumplen = pagesize;
 				cnt = 0;
-			} else {
-				bp++;
 			}
 		}
 	}
-- 
2.43.0