ubsan: disable UBSAN_DIV_ZERO for clang

Building with UBSAN_DIV_ZERO with clang produces numerous fallthrough
warnings from objtool.

In the case of uncheck division, UBSAN_DIV_ZERO may introduce new
control flow to check for division by zero.

Because the result of the division is undefined, LLVM may optimize the
control flow such that after the call to __ubsan_handle_divrem_overflow
doesn't matter.  If panic_on_warn was set,
__ubsan_handle_divrem_overflow would panic.

The problem is is that panic_on_warn is run time configurable.  If it's
disabled, then we cannot guarantee that we will be able to recover
safely.  Disable this config for clang until we can come up with a
solution in LLVM.

Link: https://github.com/ClangBuiltLinux/linux/issues/1657
Link: https://github.com/llvm/llvm-project/issues/56289
Link: https://lore.kernel.org/lkml/CAHk-=wj1qhf7y3VNACEexyp5EbkNpdcu_542k-xZpzmYLOjiCg@mail.gmail.com/
Reported-by: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Acked-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/lib/Kconfig.ubsan b/lib/Kconfig.ubsan
index a9f7eb04776851ac8e5aaeb9d5919fed167636ee..fd15230a703b76e846ec90a2ade25c148d252bee 100644 (file)
--- a/lib/Kconfig.ubsan
+++ b/lib/Kconfig.ubsan
@@ -84,6 +84,9 @@ config UBSAN_SHIFT
 config UBSAN_DIV_ZERO
        bool "Perform checking for integer divide-by-zero"
        depends on $(cc-option,-fsanitize=integer-divide-by-zero)
+       # https://github.com/ClangBuiltLinux/linux/issues/1657
+       # https://github.com/llvm/llvm-project/issues/56289
+       depends on !CC_IS_CLANG
        help
          This option enables -fsanitize=integer-divide-by-zero which checks
          for integer division by zero. This is effectively redundant with the