automation/eclair_analysis: deviate linker symbols for Rule 18.2

MISRA C Rule 18.2 states: "Subtraction between pointers shall
only be applied to pointers that address elements of the same array".

Subtractions between pointer where at least one symbol is a
symbol defined by the linker are safe and thus deviated, because
the compiler cannot exploit the undefined behaviour that would
arise from violating the rules in this case.

To create an ECLAIR configuration that contains the list of
linker-defined symbols, the script "linker-symbols.sh" is used
after a build of xen (without static analysis) is performed.
The generated file "linker_symbols.ecl" is then used as part of the
static analysis configuration.

Additional changes to the ECLAIR integration are:
- perform a build of xen without static analysis during prepare.sh
- run the scripts to generated ECL configuration during the prepare.sh,
  rather than analysis.sh
- export ECLAIR_PROJECT_ROOT earlier, to allow such generation

Additionally, the macro page_to_mfn performs a subtraction that is safe,
so its uses are deviated.

No functional changes.

Signed-off-by: Nicola Vetrini <nicola.vetrini@bugseng.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>

automation/eclair_analysis/ECLAIR/deviations.ecl

@@ -533,6 +533,17 @@ safe."
 # Series 18.
 #
 
+-doc_begin="Subtractions between pointers involving at least one of the linker symbols specified by the regex below
+are guaranteed not to be exploited by a compiler that relies on the absence of
+C99 Undefined Behaviour 45: Pointers that do not point into, or just beyond, the same array object are subtracted (6.5.6)."
+-eval_file=linker_symbols.ecl
+-config=MC3R1.R18.2,reports+={safe, "any_area(stmt(operator(sub)&&child(lhs||rhs, skip(__non_syntactic_paren_stmts, ref(linker_symbols)))))"}
+-doc_end
+
+-doc_begin="The following macro performs a subtraction between pointers to obtain the mfn, but does not lead to undefined behaviour."
+-config=MC3R1.R18.2,reports+={safe, "any_area(any_loc(any_exp(macro(^page_to_mfn$))))"}
+-doc_end
+
 -doc_begin="Flexible array members are deliberately used and XEN developers are aware of the dangers related to them:
 unexpected result when the structure is given as argument to a sizeof() operator and the truncation in assignment between structures."
 -config=MC3R1.R18.7,reports+={deliberate, "any()"}