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Kernel: Fix UB caused by taking a reference to a packed struct's member

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Taking a reference or a pointer to a value that's not aligned properly
is undefined behavior. While `[[gnu::packed]]` ensures that reads from
and writes to fields of packed structs is a safe operation, the
information about the reduced alignment is lost when creating pointers
to these values.

Weirdly enough, GCC's undefined behavior sanitizer doesn't flag these,
even though the doc of `-Waddress-of-packed-member` says that it usually
leads to UB. In contrast, x86_64 Clang does flag these, which renders
the 64-bit kernel unable to boot.

For now, the `address-of-packed-member` warning will only be enabled in
the kernel, as it is absolutely crucial there because of KUBSAN, but
might get excessively noisy for the userland in the future.

Also note that we can't append to `CMAKE_CXX_FLAGS` like we do for other
flags in the kernel, because flags added via `add_compile_options` come
after these, so the `-Wno-address-of-packed-member` in the root would
cancel it out.
This commit is contained in:
Daniel Bertalan 2021-08-01 20:30:43 +02:00 • committed by Andreas Kling
parent 90caebe96a
commit fa8507d1ce
3 changed files with 45 additions and 25 deletions
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4
Kernel/CMakeLists.txt
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@ -351,6 +351,10 @@ set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fstack-protector-strong")
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fno-exceptions")
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -nodefaultlibs -nostdlib")
if (USE_CLANG_TOOLCHAIN)
add_compile_options(-Waddress-of-packed-member)
endif()
# Apply any flags that are only available on >= GCC 11.1
if (NOT USE_CLANG_TOOLCHAIN AND CMAKE_COMPILER_IS_GNUCXX AND CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 11.1)
# Zero any registers used within a function on return (to reduce data lifetime and ROP gadgets).