From 5b3ddb17baec13b4386620b533527d0f53ddeddf Mon Sep 17 00:00:00 2001
From: Jason Wessel <jason.wessel@windriver.com>
Date: Tue, 23 Jun 2020 14:36:54 -0700
Subject: [PATCH] fs/fat/fat.c: Do not perform zero block reads if there are no
 blocks left

While using u-boot with qemu's virtio driver I stumbled across a
problem reading files less than sector size.  On the real hardware the
block reader seems ok with reading zero blocks, and while we could fix
the virtio host side of qemu to deal with a zero block read instead of
crashing, the u-boot fat driver should not be doing zero block reads
in the first place.  If you ask hardware to read zero blocks you are
just going to get zero data.  There may also be other hardware that
responds similarly to the virtio interface so this is worth fixing.

Without the patch I get the following and have to restart qemu because
it dies.
---------------------------------
=> fatls virtio 0:1
       30   cmdline.txt
=> fatload virtio 0:1 ${loadaddr} cmdline.txt
qemu-system-aarch64: virtio: zero sized buffers are not allowed
---------------------------------

With the patch I get the expected results.
---------------------------------
=> fatls virtio 0:1
       30   cmdline.txt
=> fatload virtio 0:1 ${loadaddr} cmdline.txt
30 bytes read in 11 ms (2 KiB/s)
=> md.b ${loadaddr} 0x1E
40080000: 64 77 63 5f 6f 74 67 2e 6c 70 6d 5f 65 6e 61 62    dwc_otg.lpm_enab
40080010: 6c 65 3d 30 20 72 6f 6f 74 77 61 69 74 0a          le=0 rootwait.

---------------------------------

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
---
 fs/fat/fat.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/fs/fat/fat.c b/fs/fat/fat.c
index 9578b74bae..28aa5aaa9f 100644
--- a/fs/fat/fat.c
+++ b/fs/fat/fat.c
@@ -278,7 +278,10 @@ get_cluster(fsdata *mydata, __u32 clustnum, __u8 *buffer, unsigned long size)
 		}
 	} else {
 		idx = size / mydata->sect_size;
-		ret = disk_read(startsect, idx, buffer);
+		if (idx == 0)
+			ret = 0;
+		else
+			ret = disk_read(startsect, idx, buffer);
 		if (ret != idx) {
 			debug("Error reading data (got %d)\n", ret);
 			return -1;
-- 
2.39.5