On Wed, Sep 16, 2020 at 11:48:42PM +0100, Andrew Cooper wrote: > Every day is a school day. Tell me about it... > This is very definitely one to be filed in obscure x86 corner cases. > > The code snippet above is actually wrong for the kernel, as it uses one > slot of the red-zone. Recompiling with -mno-red-zone makes something > which looks safe stack-wise, give or take this behaviour. Right, we recently disabled red zone in the early decompression stage, for SEV-ES: https://git.kernel.org/tip/6ba0efa46047936afa81460489cfd24bc95dd863 I probably should go audit that for similar funnies: $ objdump -d arch/x86/boot/compressed/vmlinux | grep -E "pop.*\(%[er]?sp" $ Nope, nothing. Because building your snippet with: $ gcc -Wall -O2 -mno-red-zone -o flags{,.c} still does use that one slot: 0000000000001050 <main>: 1050: 48 83 ec 18 sub $0x18,%rsp 1054: 48 8d 3d a9 0f 00 00 lea 0xfa9(%rip),%rdi # 2004 <_IO_stdin_used+0x4> 105b: 31 c0 xor %eax,%eax 105d: 9c pushfq 105e: 8f 44 24 08 popq 0x8(%rsp) 1062: 48 8b 74 24 08 mov 0x8(%rsp),%rsi Wonder if that flag -mno-red-zone even does anything... -- Regards/Gruss, Boris. https://people.kernel.org/tglx/notes-about-netiquette
