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nfsd4_encode_readv() uses xdr->buf->page_len as a starting point for
the nfsd_iter_read() sink buffer -- page_len is going to be offset
by the parts of the COMPOUND that have already been encoded into
xdr->buf->pages.
However, that value must be captured /before/
xdr_reserve_space_vec() advances page_len by the expected size of
the read payload. Otherwise, the whole front part of the first
page of the payload in the reply will be uninitialized.
Mantas hit this because sec=krb5i forces RQ_SPLICE_OK off, which
invokes the readv part of the nfsd4_encode_read() path. Also,
older Linux NFS clients appear to send shorter READ requests
for files smaller than a page, whereas newer clients just send
page-sized requests and let the server send as many bytes as
are in the file.
Reported-by: Mantas Mikulėnas <grawity@gmail.com>
Closes: https://lore.kernel.org/linux-nfs/f1d0b234-e650-0f6e-0f5d-126b3d51d1eb@gmail.com/
Fixes: 703d752155 ("NFSD: Hoist rq_vec preparation into nfsd_read() [step two]")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.