From de78d6ca0a5d399c55f9bae489239f5542d20fe7 Mon Sep 17 00:00:00 2001 From: Zihao Yu Date: Sat, 1 Jun 2024 11:02:06 +0800 Subject: [PATCH] x86-qemu,boot: fix strict aliasing bug by union * In gcc 12, by the strict-aliasing rule, gcc will consider elf32 and buf during calling copy_from_disk() point to different memory area. This will let gcc move the checking of EM_X86_64 ahead of copy_from_disk(), causing incorrectly parsing the x86_64 ELF header as x86_32. --- am/src/x86/qemu/boot/main.c | 20 +++++++++++--------- 1 file changed, 11 insertions(+), 9 deletions(-) diff --git a/am/src/x86/qemu/boot/main.c b/am/src/x86/qemu/boot/main.c index 13910d1..86bc56d 100644 --- a/am/src/x86/qemu/boot/main.c +++ b/am/src/x86/qemu/boot/main.c @@ -64,27 +64,29 @@ static void load_elf32(Elf32_Ehdr *elf) { } void load_kernel(void) { - Elf32_Ehdr *elf32 = (void *)0x8000; - Elf64_Ehdr *elf64 = (void *)0x8000; + union { + Elf32_Ehdr elf32; + Elf64_Ehdr elf64; + } *u = (void *)0x8000; int is_ap = boot_record()->is_ap; if (!is_ap) { // load argument (string) to memory copy_from_disk((void *)MAINARG_ADDR, 1024, -1024); // load elf header to memory - copy_from_disk(elf32, 4096, 0); - if (elf32->e_machine == EM_X86_64) { - load_elf64(elf64); + copy_from_disk(u, 4096, 0); + if (u->elf32.e_machine == EM_X86_64) { + load_elf64(&u->elf64); } else { - load_elf32(elf32); + load_elf32(&u->elf32); } } else { // everything should be loaded } - if (elf32->e_machine == EM_X86_64) { - ((void(*)())(uint32_t)elf64->e_entry)(); + if (u->elf32.e_machine == EM_X86_64) { + ((void(*)())(uint32_t)(u->elf64.e_entry))(); } else { - ((void(*)())(uint32_t)elf32->e_entry)(); + ((void(*)())(uint32_t)(u->elf32.e_entry))(); } }