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?
Thanks,
Kazu
> of a function, so let's fix it by change the search conditon to
> (*ptr > xxx_start) && (*ptr < xxx_end).
>
> Signed-off-by: Ding Hui <dinghui@xxxxxxxxxxxxxx>
> ---
> arm64.c | 18 +++++++++---------
> 1 file changed, 9 insertions(+), 9 deletions(-)
>
> diff --git a/arm64.c b/arm64.c
> index c3e26a3..7e8a7db 100644
> --- a/arm64.c
> +++ b/arm64.c
> @@ -3479,7 +3479,7 @@ arm64_in_kdump_text(struct bt_info *bt, struct arm64_stackframe *frame)
> ms = machdep->machspec;
> for (ptr = start - 8; ptr >= base; ptr--) {
> if (bt->flags & BT_OPT_BACK_TRACE) {
> - if ((*ptr >= ms->crash_kexec_start) &&
> + if ((*ptr > ms->crash_kexec_start) &&
> (*ptr < ms->crash_kexec_end) &&
> INSTACK(*(ptr - 1), bt)) {
> bt->bptr = ((ulong)(ptr - 1) - (ulong)base)
> @@ -3488,7 +3488,7 @@ arm64_in_kdump_text(struct bt_info *bt, struct arm64_stackframe *frame)
> fprintf(fp, "%lx: %lx (crash_kexec)\n", bt->bptr, *ptr);
> return TRUE;
> }
> - if ((*ptr >= ms->crash_save_cpu_start) &&
> + if ((*ptr > ms->crash_save_cpu_start) &&
> (*ptr < ms->crash_save_cpu_end) &&
> INSTACK(*(ptr - 1), bt)) {
> bt->bptr = ((ulong)(ptr - 1) - (ulong)base)
> @@ -3498,14 +3498,14 @@ arm64_in_kdump_text(struct bt_info *bt, struct arm64_stackframe *frame)
> return TRUE;
> }
> } else {
> - if ((*ptr >= ms->machine_kexec_start) && (*ptr < ms->machine_kexec_end)) {
> + if ((*ptr > ms->machine_kexec_start) && (*ptr < ms->machine_kexec_end)) {
> bt->bptr = ((ulong)ptr - (ulong)base)
> + task_to_stackbase(bt->tc->task);
> if (CRASHDEBUG(1))
> fprintf(fp, "%lx: %lx (machine_kexec)\n", bt->bptr, *ptr);
> return TRUE;
> }
> - if ((*ptr >= ms->crash_kexec_start) && (*ptr < ms->crash_kexec_end)) {
> + if ((*ptr > ms->crash_kexec_start) && (*ptr < ms->crash_kexec_end)) {
> /*
> * Stash the first crash_kexec frame in case the machine_kexec
> * frame is not found.
> @@ -3519,7 +3519,7 @@ arm64_in_kdump_text(struct bt_info *bt, struct arm64_stackframe *frame)
> }
> continue;
> }
> - if ((*ptr >= ms->crash_save_cpu_start) && (*ptr < ms->crash_save_cpu_end)) {
> + if ((*ptr > ms->crash_save_cpu_start) && (*ptr < ms->crash_save_cpu_end)) {
> bt->bptr = ((ulong)ptr - (ulong)base)
> + task_to_stackbase(bt->tc->task);
> if (CRASHDEBUG(1))
> @@ -3566,7 +3566,7 @@ arm64_in_kdump_text_on_irq_stack(struct bt_info *bt)
>
> for (ptr = start - 8; ptr >= base; ptr--) {
> if (bt->flags & BT_OPT_BACK_TRACE) {
> - if ((*ptr >= ms->crash_kexec_start) &&
> + if ((*ptr > ms->crash_kexec_start) &&
> (*ptr < ms->crash_kexec_end) &&
> INSTACK(*(ptr - 1), bt)) {
> bt->bptr = ((ulong)(ptr - 1) - (ulong)base) + stackbase;
> @@ -3576,7 +3576,7 @@ arm64_in_kdump_text_on_irq_stack(struct bt_info *bt)
> FREEBUF(stackbuf);
> return TRUE;
> }
> - if ((*ptr >= ms->crash_save_cpu_start) &&
> + if ((*ptr > ms->crash_save_cpu_start) &&
> (*ptr < ms->crash_save_cpu_end) &&
> INSTACK(*(ptr - 1), bt)) {
> bt->bptr = ((ulong)(ptr - 1) - (ulong)base) + stackbase;
> @@ -3587,7 +3587,7 @@ arm64_in_kdump_text_on_irq_stack(struct bt_info *bt)
> return TRUE;
> }
> } else {
> - if ((*ptr >= ms->crash_kexec_start) && (*ptr < ms->crash_kexec_end)) {
> + if ((*ptr > ms->crash_kexec_start) && (*ptr < ms->crash_kexec_end)) {
> bt->bptr = ((ulong)ptr - (ulong)base) + stackbase;
> if (CRASHDEBUG(1))
> fprintf(fp, "%lx: %lx (crash_kexec on IRQ stack)\n",
> @@ -3595,7 +3595,7 @@ arm64_in_kdump_text_on_irq_stack(struct bt_info *bt)
> FREEBUF(stackbuf);
> return TRUE;
> }
> - if ((*ptr >= ms->crash_save_cpu_start) && (*ptr < ms->crash_save_cpu_end)) {
> + if ((*ptr > ms->crash_save_cpu_start) && (*ptr < ms->crash_save_cpu_end)) {
> bt->bptr = ((ulong)ptr - (ulong)base) + stackbase;
> if (CRASHDEBUG(1))
> fprintf(fp, "%lx: %lx (crash_save_cpu on IRQ stack)\n",
--
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