On Sun, May 28, 2023 at 04:40:31PM +0000, Joan Bruguera Micó wrote: > I can also reproduce the problem with Arch's linux-next-git, see config: > https://aur.archlinux.org/cgit/aur.git/tree/config?h=linux-next-git&id=f9a384e1c582321651fb613782ebc5a581146af0 > > I've bisected it to df8fc4e934c1 ("kbuild: Enable -fstrict-flex-arrays=3"), > which explains why it only happens on GCC13. Okay, this was a wild ride. Bottom line, -fstrict-flex-arrays=3 means that CONFIG_FORTIFY_SOURCE wrappers will be included in new places now that trailing arrays aren't ignored any more. The trailing array in question was for struct sha256_state: struct sha256_state { u32 state[SHA256_DIGEST_SIZE / 4]; u64 count; u8 buf[SHA256_BLOCK_SIZE]; }; And this "buf" is a memcpy() destination, it was being runtime bounds checked, which means FORTIFY might emit wrappers, which will call the WARN wrappers, which will hit this asm, which isn't supported in purgatory. > > The problematic expansion that causes the error seems to be this fragment > from `_BUG_FLAGS` in `arch/x86/include/asm/bug.h`: > asm(".long %c0 - .\n": : "i" (__FILE__)); > > Along with the fact that this file is built with `-mcmodel=large` > (see `PURGATORY_CFLAGS` in `arch/x86/purgatory/Makefile`). So, we can either disable fortify (which seems a big hammer) or disable the warning. Disabling the warning kind of hides the problem, though that seems to be the intention of purgatory.c's empty warn() implementation. :P I think it's best to disable fortify, though, as we're in a DISABLE_EXPORTS state, and probably others are going to need it too, as most other have already done so: arch/arm64/kernel/pi/Makefile: -D__DISABLE_EXPORTS -ffreestanding -D__NO_FORTIFY \ arch/riscv/kernel/pi/Makefile:CFLAGS_cmdline_early.o += -D__NO_FORTIFY arch/riscv/kernel/pi/Makefile:CFLAGS_lib-fdt_ro.o += -D__NO_FORTIFY drivers/firmware/efi/libstub/Makefile: -D__NO_FORTIFY \ These should probably gain -D__NO_FORTIFY: arch/riscv/purgatory/Makefile:CFLAGS_sha256.o := -D__DISABLE_EXPORTS arch/riscv/purgatory/Makefile:CFLAGS_string.o := -D__DISABLE_EXPORTS arch/riscv/purgatory/Makefile:CFLAGS_ctype.o := -D__DISABLE_EXPORTS arch/s390/purgatory/Makefile:CFLAGS_sha256.o := -D__DISABLE_EXPORTS arch/x86/purgatory/Makefile:CFLAGS_sha256.o := -D__DISABLE_EXPORTS I'll send patches. -Kees -- Kees Cook
