9p: fix EBADF errors in cached mode

cached operations sometimes need to do invalid operations (e.g. read
on a write only file)
Historic fscache had added a "writeback fid", a special handle opened
RW as root, for this. The conversion to new fscache missed that bit.

This commit reinstates a slightly lesser variant of the original code
that uses the writeback fid for partial pages backfills if the regular
user fid had been open as WRONLY, and thus would lack read permissions.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: eb497943fa21 ("9p: Convert to using the netfs helper lib to do reads and caching")
Cc: [email protected]
Cc: David Howells <[email protected]>
Reported-By: Christian Schoenebeck <[email protected]>
Reviewed-by: Christian Schoenebeck <[email protected]>
Tested-by: Christian Schoenebeck <[email protected]>
Signed-off-by: Dominique Martinet <[email protected]>

0 files changed, 0 insertions, 0 deletions