Alignment Fault =============== terminology ----------- - aligned memory access & unaligned memory access - `Linux Kernel Doc - Unaligned Memory Access `_ - `Linux Kernel Doc - arm/mem_alignment `_ - Alignment Fault: `ARM9 對 Alignment Fault 的定義 `_ ARM CPU ------- 滿多 RISC 的 CPU 並不完整支援 unaligned memory access, 比如說 ARM9 所有的 memory access instruction 都不支援. 有一說是 ARMv5 (e.g. ARM9) 跟以前的 ARM CPU 不支援 unaligned memory access, ARMv6 以後 default 支援大部份的 aligned memory access: - ``arch/arm/mm/alignment.c``: safe_usermode():: ARMv6 and later CPUs can perform unaligned accesses for most single load and store instructions up to word size. LDM, STM, LDRD and STRD still need to be handled. - ``arch/arm/mm/alignment.c``: cpu_is_v6_unaligned():: return cpu_architecture() >= CPU_ARCH_ARMv6 && get_cr() & CR_U; - For ARMv6 and latter CPUs, it has CP15 c1 register U field. which represent "Enables unaligned data access operations for mixed little-endian and big-endian operation". ref: `ARM11_CP15_c1 `_ - ``arch/arm/include/asm/cp15.h`` ARMv8 的 normal memory 可以支援 unaligned memory access, 但 device memory 只支援 aligned memory access. - `[Linux Kernel patch] pstore/ram: Use memcpy_toio instead of memcpy `_ - `ARM Cortex-A Series Programmer’s Guide for ARMv8-A - Ch5.1 Addressing `_ - **Unaligned address support** Except for exclusive and ordered accesses: all loads and stores support the use of unaligned addresses when accessing normal memory. This simplifies porting code to A64. - `Porting Linux to a new Architecture - ARMv8 AArch64 Software Bring-up `_ p.23 Compiler/Toolchain ------------------ 我撞到的狀況 - ``strncpy()`` 跟 ``memset()`` 會碰到 alignment fault, ``memcpy()`` 似乎不會. Detail - strncpy() 的 len 為 5, 6, 7 或其他大於 8 但非 8 的倍數的常數時. 會碰到 alignment fault. - strncpy() len <= 8: gcc builtin strncpy():: strncpy(str, "hello world\n", 5); 400be4: f9401ba2 ldr x2, [x29,#48] 400be8: 90000000 adrp x0, 400000 <_init-0x620> 400bec: 9135e001 add x1, x0, #0xd78 400bf0: aa0203e0 mov x0, x2 400bf4: b9400022 ldr w2, [x1] 400bf8: b9000002 str w2, [x0] # aligned write to x0[0:4] 400bfc: b8401021 ldur w1, [x1,#1] 400c00: b8001001 stur w1, [x0,#1] # unaligned write to x0[1:5] - strncpy() len > 8: `glibc strncpy() `_ - 問題點似乎在 ``strncpy()`` 裡的 ``memset()``, 而且是 glibc aarch64 優化版的 ``memset()`` - gdb 收到 SIGBUS 時在 strncpy() 的 memset 裏面 (``../sysdep/aarch64/memset.S``) - ``memset(str+13, '\0', 3);`` 可以重現 alignment fault. reference --------- - `ARM64 环境中的对齐异常 (alignment fault) 问题 `_ - memset 的 glibc 实现由于性能需要，使用了 DC ZVA 指令, 而 ARM64 架构中，DC ZVA 指令不能用于 device 类型的内存，否则就会触发 alignment fault 。 - `arm64:lib: Use Linaro's memset routine to avoid DC instruction `_ - `GCC builtin function `_ - ``-fno-builtin`` disable it.