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When UBSAN is enabled, we get a very large stack frame for __serpent_setkey, when the register allocator ends up using more registers than it has, and has to spill temporary values to the stack. The code was originally optimized for in-order x86-32 CPU implementations using older compilers, but it now runs into a highly suboptimal case on all CPU architectures, as seen by this warning: crypto/serpent_generic.c: In function '__serpent_setkey': crypto/serpent_generic.c:436:1: error: the frame size of 2720 bytes is larger than 2048 bytes [-Werror=frame-larger-than=] Disabling -fsanitize=alignment would avoid that warning, presumably the option turns off a optimization step that is required for getting the register allocation right, but there is no easy way to do that on gcc-7 (gcc-8 introduces a function attribute for this). I tried to figure out a way to modify the source code instead, and noticed that the two stages of the setkey() function (keyiter and sbox) each are fine by themselves, but not when combined into one function. Splitting out the entire sbox into a separate function also happens to work fine with all compilers I tried (arm, arm64 and x86). The setkey function uses a strange way to handle offsets into the key array, using both negative and positive index values, as well as adjusting the array pointer back and forth. I have checked that this actually makes no difference to modern compilers, but I left that untouched to make the patch easier to review and to keep the code closer to the reference implementation. Link: https://patchwork.kernel.org/patch/9189575/ Signed-off-by: Arnd Bergmann <arnd@arndb.de> Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. 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.