objtool: Documentation: document UACCESS warnings

Compiling with Clang and CONFIG_KASAN=y was exposing a few warnings:

  call to memset() with UACCESS enabled

Document how to fix these for future travelers.

Link: https://github.com/ClangBuiltLinux/linux/issues/876Suggested-by: Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>
Suggested-by: Matt Helsley <mhelsley@vmware.com>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Suggested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

tools/objtool/Documentation/stack-validation.txt

@@ -289,6 +289,32 @@ they mean, and suggestions for how to fix them.
       might be corrupt due to a gcc bug.  For more details, see:
       https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70646
 
+9. file.o: warning: objtool: funcA() call to funcB() with UACCESS enabled
+
+   This means that an unexpected call to a non-whitelisted function exists
+   outside of arch-specific guards.
+   X86: SMAP (stac/clac): __uaccess_begin()/__uaccess_end()
+   ARM: PAN: uaccess_enable()/uaccess_disable()
+
+   These functions should be called to denote a minimal critical section around
+   access to __user variables. See also: https://lwn.net/Articles/517475/
+
+   The intention of the warning is to prevent calls to funcB() from eventually
+   calling schedule(), potentially leaking the AC flags state, and not
+   restoring them correctly.
+
+   It also helps verify that there are no unexpected calls to funcB() which may
+   access user space pages with protections against doing so disabled.
+
+   To fix, either:
+   1) remove explicit calls to funcB() from funcA().
+   2) add the correct guards before and after calls to low level functions like
+      __get_user_size()/__put_user_size().
+   3) add funcB to uaccess_safe_builtin whitelist in tools/objtool/check.c, if
+      funcB obviously does not call schedule(), and is marked notrace (since
+      function tracing inserts additional calls, which is not obvious from the
+      sources).
+
 
 If the error doesn't seem to make sense, it could be a bug in objtool.
 Feel free to ask the objtool maintainer for help.