[RFC 0/3] Enable GCM support

[RFC 0/3] Enable GCM support

[Will Morrison]

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This set of patches is the work we've done so far on adding integrity
protection. We've been using a modified version of the test suite,
available at
https://code.launchpad.net/~zmanji/ecryptfs/1270455/+merge/202197 to
test GCM using the existing tests. This is done by passing -o
"ecryptfs_cipher_mode=gcm" to the test script along with other flags.

When GCM mode is used, most of the test suite passes, but several tests
involving truncating a file to a larger size fail. We believe this to be
a result of calling read_mapping_page at mmap.c:49, which sometimes
calls ecryptfs_readpage with a fresh page. This new page does not have a
valid auth tag, so E_BADMSG is returned. We think this was fine before,
as whatever was in the page would get overwritten and ignored,
but now we're doing integrity checks, so we run into issues.

We'd like to know if there is an alternative to read_mapping_page that
preserves the necessary semantics, or if there is some field in the page
that gets passed to ecryptfs_readpage that would allow us to distinguish
between pages we have never written to before and pages that were just
fetched from the cache or disk.

-Will


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