Re: [Crash-utility] [PATCH] arm64: fix backtraces of KASAN kernel dumpfile truncated

On 2022/12/02 17:31, dinghui wrote: > On 2022/12/2 15:44, HAGIO KAZUHITO(萩尾 一仁) wrote: >> On 2022/12/01 16:01, Ding Hui wrote: >>> We met "bt" cmd on KASAN kernel vmcore display truncated backtraces >>> like this: >>> >>> crash> bt >>> PID: 4131   TASK: ffff8001521df000  CPU: 3   COMMAND: "bash" >>>    #0 [ffff2000224b0cb0] machine_kexec_prepare at ffff2000200bff4c >>> >>> After digging the root cause, it turns out that arm64_in_kdump_text() >>> found wrong bt->bptr at "machine_kexec" branch. >>> >>> Disassemble machine_kexec() of KASAN vmlinux (gcc 7.3.0): >>> >>> crash> dis -x machine_kexec >>> 0xffff2000200bff50 <machine_kexec>:     stp     x29, x30, [sp,#-208]! >>> 0xffff2000200bff54 <machine_kexec+0x4>: mov     x29, sp >>> 0xffff2000200bff58 <machine_kexec+0x8>: stp     x19, x20, [sp,#16] >>> 0xffff2000200bff5c <machine_kexec+0xc>: str     x24, [sp,#56] >>> 0xffff2000200bff60 <machine_kexec+0x10>:        str     x26, [sp,#72] >>> 0xffff2000200bff64 <machine_kexec+0x14>:        mov     x2, #0x8ab3 >>> 0xffff2000200bff68 <machine_kexec+0x18>:        add     x1, x29, #0x70 >>> 0xffff2000200bff6c <machine_kexec+0x1c>:        lsr     x1, x1, #3 >>> 0xffff2000200bff70 <machine_kexec+0x20>:        movk    x2, #0x41b5, lsl #16 >>> 0xffff2000200bff74 <machine_kexec+0x24>:        mov     x19, #0x200000000000 >>> 0xffff2000200bff78 <machine_kexec+0x28>:        adrp    x3, 0xffff2000224b0000 >>> 0xffff2000200bff7c <machine_kexec+0x2c>:        movk    x19, #0xdfff, lsl #48 >>> 0xffff2000200bff80 <machine_kexec+0x30>:        add     x3, x3, #0xcb0 >>> 0xffff2000200bff84 <machine_kexec+0x34>:        add     x4, x1, x19 >>> 0xffff2000200bff88 <machine_kexec+0x38>:        stp     x2, x3, [x29,#112] >>> 0xffff2000200bff8c <machine_kexec+0x3c>:        adrp    x2, 0xffff2000200bf000 <swsusp_arch_resume+0x1e8> >>> 0xffff2000200bff90 <machine_kexec+0x40>:        add     x2, x2, #0xf50 >>> 0xffff2000200bff94 <machine_kexec+0x44>:        str     x2, [x29,#128] >>> 0xffff2000200bff98 <machine_kexec+0x48>:        mov     w2, #0xf1f1f1f1 >>> 0xffff2000200bff9c <machine_kexec+0x4c>:        str     w2, [x1,x19] >>> 0xffff2000200bffa0 <machine_kexec+0x50>:        mov     w2, #0xf200 >>> 0xffff2000200bffa4 <machine_kexec+0x54>:        mov     w1, #0xf3f3f3f3 >>> 0xffff2000200bffa8 <machine_kexec+0x58>:        movk    w2, #0xf2f2, lsl #16 >>> 0xffff2000200bffac <machine_kexec+0x5c>:        stp     w2, w1, [x4,#4] >>> >>> We notice that: >>> 1. machine_kexec() start address is 0xffff2000200bff50 >>> 2. the instruction at machine_kexec+0x44 store the same value >>>      0xffff2000200bff50 (comes from 0xffff2000200bf000 + 0xf50) >>>      into stack postion [x29,#128]. >>> >>> When arm64_in_kdump_text() search LR from stack, it met >>> 0xffff2000200bff50 firstly, so got wrong bt->bptr. >>> >>> We know that the real LR is always great than the start address >> >> Seems true. >> >> One question, do you see which kernel code stores that value? >> > > Actually, there is no C code stores that value. The source code like this: > > void machine_kexec(struct kimage *kimage) > { >     phys_addr_t reboot_code_buffer_phys; >     void *reboot_code_buffer; >     bool in_kexec_crash = (kimage == kexec_crash_image); >     bool stuck_cpus = cpus_are_stuck_in_kernel(); > >     BUG_ON(!in_kexec_crash && (stuck_cpus || (num_online_cpus() > 1))); >     WARN(in_kexec_crash && (stuck_cpus || smp_crash_stop_failed()), >         "Some CPUs may be stale, kdump will be unreliable.\n"); > ... > > The point is CONFIG_KASAN=y > > I compared the gcc args when compiling machine_kexec.o between kasan eanble [1] and kasan enable but set KASAN_SANITIZE_machine_kexec.o := n [2], the difference is: > > [1]: -fsanitize=kernel-address -fasan-shadow-offset=0xdfff200000000000 --param asan-globals=1   --param asan-instrumentation-with-call-threshold=10000   --param asan-stack=1 > > [2]: -fno-builtin > > If I remove `--param asan-stack=1` but keep other asan args to compile machine_kexec.o, those assembly statement disappear. > I see, thanks. I can see the similar pattern with CONFIG_KASAN=y also on x86_64, which stores the function start address and uses 0xf1f1f1f1 (ASAN_STACK_MAGIC_LEFT in gcc) and etc. (gdb) disas machine_kexec Dump of assembler code for function machine_kexec: 0xffffffff8109b1c0 <+0>: callq 0xffffffff81099e60 <__fentry__ ... 0xffffffff8109b208 <+72>: movq $0xffffffff8109b1c0,0x20(%rsp) 0xffffffff8109b211 <+81>: add %r12,%rax 0xffffffff8109b214 <+84>: movl $0xf1f1f1f1,(%rax) (gdb) disas crash_save_cpu Dump of assembler code for function crash_save_cpu: 0xffffffff8126e7e0 <+0>: callq 0xffffffff81099e60 <__fentry__> ... 0xffffffff8126e817 <+55>: movq $0xffffffff8126e7e0,0x10(%rsp) 0xffffffff8126e820 <+64>: add %rbp,%rax 0xffffffff8126e823 <+67>: movl $0xf1f1f1f1,(%rax) I wondered whether excluding only their start address was enough to fix the issue, but now it seems ok to me.

Acked-by: Kazuhito Hagio <k-hagio-ab(a)nec.com&gt; Let's wait for Lianbo's test and review. Thanks, Kazu