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With CONFIG_CFI_CLANG, assembly functions called indirectly
from C code must be annotated with type identifiers to pass CFI
checking. In order to make this easier, the compiler emits a
__kcfi_typeid_<function> symbol for each address-taken function
declaration in C, which contains the expected type identifier that
we can refer to in assembly code.
Add a typed version of SYM_FUNC_START, which emits the type
identifier before the function. Architectures that support KCFI can
define their own __CFI_TYPE macro to override the default preamble
format.
As an example, for the x86_64 blowfish_dec_blk function, the
compiler emits the following type symbol:
$ readelf -sW vmlinux | grep __kcfi_typeid_blowfish_dec_blk
120204: 00000000ef478db5 0 NOTYPE WEAK DEFAULT ABS
__kcfi_typeid_blowfish_dec_blk
And SYM_TYPED_FUNC_START will generate the following preamble based
on the __CFI_TYPE definition for the architecture:
$ objdump -dr arch/x86/crypto/blowfish-x86_64-asm_64.o
...
0000000000000400 <__cfi_blowfish_dec_blk>:
...
40b: b8 00 00 00 00 mov $0x0,%eax
40c: R_X86_64_32 __kcfi_typeid_blowfish_dec_blk
0000000000000410 <blowfish_dec_blk>:
...
Note that the address of all assembly functions annotated with
SYM_TYPED_FUNC_START must be taken in C code that's linked into the
binary or the missing __kcfi_typeid_ symbol will result in a linker
error with CONFIG_CFI_CLANG. If the code that contains the indirect
call is not always compiled in, __ADDRESSABLE(functionname) can be
used to ensure that the __kcfi_typeid_ symbol is emitted.
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Tested-by: Kees Cook <keescook@chromium.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220908215504.3686827-7-samitolvanen@google.com
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.