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Subject[PATCH] 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.

Suggested-by: Peter Zijlstra <>
Signed-off-by: Nick Desaulniers <>
.../Documentation/stack-validation.txt | 20 +++++++++++++++++++
1 file changed, 20 insertions(+)

diff --git a/tools/objtool/Documentation/stack-validation.txt b/tools/objtool/Documentation/stack-validation.txt
index de094670050b..156fee13ba02 100644
--- a/tools/objtool/Documentation/stack-validation.txt
+++ b/tools/objtool/Documentation/stack-validation.txt
@@ -289,6 +289,26 @@ they mean, and suggestions for how to fix them.
might be corrupt due to a gcc bug. For more details, see:

+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_enable()
+ These functions should called to denote a minimal critical section around
+ access to __user variables. See also:
+ 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.
+ To fix, either:
+ 1) add the correct guards before and after calls to low level functions like
+ __get_user_size()/__put_user_size().
+ 2) add funcB to uaccess_safe_builtin whitelist in tools/objtool/check.c, if
+ funcB obviously does not call schedule().

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.
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