On Mon, Sep 14, 2020 at 03:13:14PM -0700, Kees Cook wrote:

On Mon, Sep 14, 2020 at 05:27:42PM +0000, George-Aurelian Popescu wrote:

quoted

From: George Popescu <redacted>

When the kernel is compiled with Clang, UBSAN_BOUNDS inserts a brk after
the handler call, preventing it from printing any information processed
inside the buffer.
For Clang -fsanitize=bounds expands to -fsanitize=array-bounds and
-fsanitize=local-bounds, and the latter adds a brk after the handler
call

That sounds like a compiler bug?

Actually in clang 12 documentation is written that -fsanitize=bounds
expands to that. GCC  doesn't have those two options, only the
-fsanitize=bounds which looks similar to -fsanitize=array-bounds from
clang. So I don't see it as a compiler bug, just a misuse of flags.

quoted

Signed-off-by: George Popescu <redacted>
---
 scripts/Makefile.ubsan | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/scripts/Makefile.ubsan b/scripts/Makefile.ubsan
index 27348029b2b8..3d15ac346c97 100644
--- a/scripts/Makefile.ubsan
+++ b/scripts/Makefile.ubsan

@@ -4,7 +4,14 @@ ifdef CONFIG_UBSAN_ALIGNMENT
 endif
 
 ifdef CONFIG_UBSAN_BOUNDS
-      CFLAGS_UBSAN += $(call cc-option, -fsanitize=bounds)
+      # For Clang -fsanitize=bounds translates to -fsanitize=array-bounds and
+      # -fsanitize=local-bounds; the latter adds a brk right after the
+      # handler is called.
+      ifdef CONFIG_CC_IS_CLANG
+            CFLAGS_UBSAN += $(call cc-option, -fsanitize=array-bounds)

This would mean losing the local-bounds coverage? Isn't that for locally
defined arrays on the stack?

This would mean losing the local-bounds coverage. I tried to  test it without
local-bounds and with a locally defined array on the stack and it works fine
(the handler is called and the error reported). For me it feels like
--array-bounds and --local-bounds are triggered for the same type of
undefined_behaviours but they are handling them different.

quoted

+      else
+            CFLAGS_UBSAN += $(call cc-option, -fsanitize=bounds)
+      endif
 endif
 
 ifdef CONFIG_UBSAN_MISC
-- 
2.28.0.618.gf4bc123cb7-goog

--

Thanks,
George

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