On an ext4 partition, Running

  echo -a /mnt/disk0.0/file test

will crash barebox because fsdrv->truncate in __write is NULL.

Don't let it come to this and verify earlier that the FS implements
->write. While at it, explicitly compare with O_RDONLY (== 0) for
clarity.

Fixes: b3fbfad7aeaf ("fs: dentry cache implementation")
Signed-off-by: Ahmad Fatoum <[email protected]>
---
 fs/fs.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/fs/fs.c b/fs/fs.c
index a023324913c4..881dc2fca0ce 100644
--- a/fs/fs.c
+++ b/fs/fs.c
@@ -321,13 +321,13 @@ static ssize_t __write(FILE *f, const void *buf, size_t 
count)
        struct fs_driver_d *fsdrv;
        int ret;
 
-       if (!(f->flags & O_ACCMODE)) {
+       fsdrv = f->fsdev->driver;
+
+       if ((f->flags & O_ACCMODE) == O_RDONLY || !fsdrv->write) {
                ret = -EBADF;
                goto out;
        }
 
-       fsdrv = f->fsdev->driver;
-
        if (fsdrv != ramfs_driver)
                assert_command_context();
 
-- 
2.29.2


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