[sanitizers] Explainer about dyld and weak overrides on Darwin. (NFC)

[rsundahl]

Summary

Explain in the release notes that the Darwin dynamic linker (dyld) requires
that at least one weak symbol be present in any mach-o file that defines an
intended override of a sanitizer dylib weak reference.

rdar://103453678

[rsundahl]

Include a better summary

[yln]

Thanks Roy! I put my suggestions, but none of them are blocking.

llvm/docs/ReleaseNotes.rst

234

Here are my suggestions:

I feel that "weak overrides" is a bit ambiguous (what is weak? the overrider or the overridden?)

• Let's use: "overrides of weak symbols" instead of "weak overrides"
• Use __asan_default_options() which is more commonly used than __asan_on_error()
• Choose one of the 2 options (to avoid confusion)
• Mention from which deployment target / macOS version on this dyld behavior changes

237–239

241–244

[rsundahl]

Reword for consistency.

[rsundahl]

Changed order of examples to meet narrative.

[rsundahl]

I have reworded with your suggestions in mind @yln. Thank you for the input. I didn't add any minimum version information since the guidance is benign going backward and good hygiene moving forward.

[yln]

llvm/docs/ReleaseNotes.rst

238–240

What I meant above:

• Let's be opinionated: instead of listing all the possible options (and providing no guidance which one to pick), we should recommend one.
• Which one is the best? I am making the argument that having a dummy symbol is the preferable option:
  • The symbol name doubles as a short explanation/documentation
  • It's confusing that the "overrider" is weak (it doesn't have to be, we just want to opt-in the executable); my mental model was "strong definitions overwrite weak ones", which is wrong, but I think other people might fall into this trap as well. It confused Vitaly & Dmitry (two experts!) here: D146471
  • What happens if it's defined multiple times? Not important (as long as it compiles); but it would be important for the override (i.e., if they diverge -> trouble)

These 2 points are separate, and I feel more strongly about "recommend single option". Which ones do you agree/disagree with?

243–247

[rsundahl]

Ping - Thank you.

[thetruestblue]

In D146745#4235114, @rsundahl wrote:

Ping - Thank you.

I agree with most of Julian's comments. I think we should pick a "suggested worked around" and use that as an example rather than listing all the options. Those who understand the issue will understand what workarounds need to be added, and those who do not will be provided with a single fix that works, rather than being offered multiple workarounds for something they don't understand.

(When looking at the revision it looks like Julian's edits haven't been addressed cause of the "not done" tag, will you check the boxes of those items that have been completed?)

llvm/docs/ReleaseNotes.rst

243–247

I agree with this edit.

[rsundahl]

Unresolved issues from this differential are resolved in D147526.

llvm/docs/ReleaseNotes.rst

* In the results of commands such as ``expr`` and ``frame var``, type summaries will now
   omit defaulted template parameters. The full template parameter list can still be
   viewed with ``expr --raw-output``/``frame var --raw-output``. (`D141828 <https://reviews.llvm.org/D141828>`_)
 
 * LLDB is now able to show the subtype of signals found in a core file. For example
   memory tagging specific segfaults such as ``SIGSEGV: sync tag check fault``.
 
 Changes to Sanitizers
 ---------------------
+* For Darwin users that override weak symbols, note that the dynamic linker will
+  only consider symbols in other mach-o modules which themselves contain at
+  least one weak symbol. A consequence is that if your program or dylib contains
+  an intended override of a weak symbol, then it must contain at least one weak
+  symbol as well for the override to be effective. That weak symbol may be the
+  intended override itself, an otherwise usused weak symbol added solely to meet
+  the requirement, or an existing but unrelated weak symbol.
+
+    Examples:
+      __attribute__((weak)) const char * __asan_default_options(void) {...}
+
+      __attribute__((weak,unused)) unsigned __enableOverrides;
+      
+      __attribute__((weak)) bool unrelatedWeakFlag;
 
 Other Changes
 -------------
 
 External Open Source Projects Using LLVM 15
 ===========================================
 
 * A project...