[PATCH] pNFS: Fix uninited ptr access in ext_tree_encode_commit

[Sergey Bashirov <sergeybashirov@gmail.com>]

Current implementation of the function assumes that the provided buffer
can always accommodate at least one encoded extent. This patch adds
handling of all theoretically possible values of be_prev, so that
ext_tree_encode_commit makes no assumptions about the provided buffer
size, and static checks pass without warnings.

Fixes: d84c4754f874 ("pNFS: Fix extent encoding in block/scsi layout")
Addresses-Coverity-ID: 1647611 ("Memory - illegal accesses  (UNINIT)")
Signed-off-by: Sergey Bashirov <sergeybashirov@gmail.com>
---
 fs/nfs/blocklayout/extent_tree.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/fs/nfs/blocklayout/extent_tree.c b/fs/nfs/blocklayout/extent_tree.c
index 315949a7e92d..adc1fe190cfc 100644
--- a/fs/nfs/blocklayout/extent_tree.c
+++ b/fs/nfs/blocklayout/extent_tree.c
@@ -585,7 +585,7 @@ static int
 ext_tree_encode_commit(struct pnfs_block_layout *bl, __be32 *p,
 		size_t buffer_size, size_t *count, __u64 *lastbyte)
 {
-	struct pnfs_block_extent *be, *be_prev;
+	struct pnfs_block_extent *be, *be_prev = NULL;
 	int ret = 0;
 
 	spin_lock(&bl->bl_ext_lock);
@@ -611,10 +611,13 @@ ext_tree_encode_commit(struct pnfs_block_layout *bl, __be32 *p,
 	if (!ret) {
 		*lastbyte = (bl->bl_lwb != 0) ? bl->bl_lwb - 1 : U64_MAX;
 		bl->bl_lwb = 0;
-	} else {
+	} else if (be_prev) {
 		*lastbyte = be_prev->be_f_offset + be_prev->be_length;
 		*lastbyte <<= SECTOR_SHIFT;
 		*lastbyte -= 1;
+	} else {
+		/* Buffer too small even for one extent, count is zero */
+		*lastbyte = U64_MAX;
 	}
 	spin_unlock(&bl->bl_ext_lock);
 
-- 
2.43.0