[tor-bugs] #15817 [Tor]: Allow clang runtime sanitizers to be used on tor unit tests

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#15817: Allow clang runtime sanitizers to be used on tor unit tests
------------------------------+------------------------------------
 Reporter:  teor              |          Owner:
     Type:  defect            |         Status:  new
 Priority:  minor             |      Milestone:  Tor: 0.2.7.x-final
Component:  Tor               |        Version:  Tor: 0.2.7
 Keywords:  unit-tests clang  |  Actual Points:
Parent ID:                    |         Points:
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 The clang runtime address sanitizer causes the memwipe and backtrace tests
 to fail, because it catches the undefined behavior that is invoked as an
 unavoidable part of these tests.

 This issue can be resolved by:
 1. blacklisting the involved functions so the sanitizer doesn't check
 them, by adding to the command line `-fsanitize-
 blacklist=sanitize_blacklist.txt` containing:
 {{{
 # test-memwipe.c checks if a freed buffer was properly wiped
 fun:vmemeq

 # test_bt_cl.c stores to a NULL pointer to trigger a crash
 fun:crash

 # we need to exempt the entire file, otherwise address sanitizer munges
 # the expected output
 src:test_bt_cl.c
 }}}
 2. allowing the backtrace handler to catch SIGSEGV (rather than address
 sanitizer), by setting the environmental variable
 `ASAN_OPTIONS=allow_user_segv_handler=1`

 The attached file is a sanitizer blacklist file which documents this
 resolution, and implements the blacklisting (but not the environmental
 variable).

 Nick, can we put this in contrib?
 I think that's the best way to deal with the special case of "clang users
 running tests under AddressSanitizer"

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/15817>
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