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verifiers: Add the documentation 

Signed-off-by: Vladimir Serbinenko <phcoder@gmail.com>
Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>
Reviewed-by: Ross Philipson <ross.philipson@oracle.com>
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Vladimir Serbinenko 2017-05-09 16:39:38 +02:00 committed by Daniel Kiper
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@ -84,6 +84,7 @@ This edition documents version @value{VERSION}.
* Video Subsystem::
* PFF2 Font File Format::
* Graphical Menu Software Design::
* Verifiers framework::
* Copying This Manual:: Copying This Manual
* Index::
@end menu
@ -1949,6 +1950,63 @@ the graphics mode that was in use before @code{grub_video_setup()} was called
might fix some of the problems.
@node Verifiers framework
@chapter Verifiers framework
To register your own verifier call @samp{grub_verifier_register} with a structure
pointing to your functions.
The interface is inspired by the hash interface with @samp{init}/@samp{write}/@samp{fini}.
There are essentially 2 ways of using it, hashing and whole-file verification.
With the hashing approach:
During @samp{init} you decide whether you want to check the given file and init context.
In @samp{write} you update your hashing state.
In @samp{fini} you check that the hash matches the expected value/passes some check/...
With whole-file verification:
During @samp{init} you decide whether you want to check the given file and init context.
In @samp{write} you verify the file and return an error if it fails.
You don't have @samp{fini}.
Additional @samp{verify_string} receives various strings like kernel parameters
to verify. Returning no error means successful verification and an error stops
the current action.
Detailed description of the API:
Every time a file is opened your @samp{init} function is called with file descriptor
and file type. Your function can have the following outcomes:
@itemize
@item returning no error and setting @samp{*flags} to @samp{GRUB_VERIFY_FLAGS_DEFER_AUTH}.
In this case verification is deferred to other active verifiers. Verification
fails if nobody cares or selected verifier fails.
@item returning no error and setting @samp{*flags} to @samp{GRUB_VERIFY_FLAGS_SKIP_VERIFICATION}.
In this case your verifier will not be called anymore and it is assumed to have
skipped verification.
@item returning no error and not setting @samp{*flags} to @samp{GRUB_VERIFY_FLAGS_SKIP_VERIFICATION}
In this case verification is done as described in the following section.
@item returning an error. Then opening of the file will fail due to failed verification.
@end itemize
In the third case your @samp{write} will be called with chunks of the file. If
you need the whole file in a single chunk then during @samp{init} set the bit
@samp{GRUB_VERIFY_FLAGS_SINGLE_CHUNK} in @samp{*flags}. During @samp{init} you
may set @samp{*context} if you need additional context. At every iteration you
may return an error and the file will be considered as having failed the
verification. If you return no error then verification continues.
Optionally at the end of the file @samp{fini}, if it exists, is called with just
the context. If you return no error during any of @samp{init}, @samp{write} and
@samp{fini} then the file is considered as having succeded verification.
@node Copying This Manual
@appendix Copying This Manual