Re: [PATCH 4/4] arm64: kgdb: Set PSTATE.SS to 1 to reenable single-step

[Doug Anderson]

Hi,

On Sat, May 9, 2020 at 6:49 AM Wei Li <liwei391@xxxxxxxxxx> wrote:
>
> After fixing wrongly single-stepping into the irq handler, when we execute
> single-step in kdb/kgdb, we can see only the first step can work.
>
> Refer to the ARM Architecture Reference Manual (ARM DDI 0487E.a) D2.12,
> i think PSTATE.SS=1 should be set each step for transferring the PE to the
> 'Active-not-pending' state. The problem here is PSTATE.SS=1 is not set
> since the second single-step.
>
> After the first single-step, the PE transferes to the 'Inactive' state,
> with PSTATE.SS=0 and MDSCR.SS=1, thus PSTATE.SS won't be set to 1 due to
> kernel_active_single_step()=true. Then the PE transferes to the
> 'Active-pending' state when ERET and returns to the debugger by step
> exception.
>
> Before this patch:
> * kdb:
> Entering kdb (current=0xffff8000119d2dc0, pid 0) on processor 0 due to Keyboard Entry
> [0]kdb> bp printk
> Instruction(i) BP #0 at 0xffff80001014874c (printk)
>     is enabled   addr at ffff80001014874c, hardtype=0 installed=0
>
> [0]kdb> g
>
> / # echo h > /proc/sysrq-trigger
>
> Entering kdb (current=0xffff0000fa6948c0, pid 265) on processor 3 due to Breakpoint @ 0xffff80001014874c
> [3]kdb> ss
>
> Entering kdb (current=0xffff0000fa6948c0, pid 265) on processor 3 due to SS trap @ 0xffff800010148750
> [3]kdb> ss
>
> Entering kdb (current=0xffff0000fa6948c0, pid 265) on processor 3 due to SS trap @ 0xffff800010148750
> [3]kdb> ss
>
> Entering kdb (current=0xffff0000fa6948c0, pid 265) on processor 3 due to SS trap @ 0xffff800010148750
> [3]kdb>
>
> * kgdb:
> (gdb) target remote 127.1:23002
> Remote debugging using 127.1:23002
> arch_kgdb_breakpoint () at /home/liwei/main_code/linux/arch/arm64/include/asm/kgdb.h:21
> 21              asm ("brk %0" : : "I" (KGDB_COMPILED_DBG_BRK_IMM));
> (gdb) b printk
> Breakpoint 1 at 0xffff80001014874c: file /home/liwei/main_code/linux/kernel/printk/printk.c, line 2076.
> (gdb) c
> Continuing.
> [New Thread 277]
> [Switching to Thread 276]
>
> Thread 171 hit Breakpoint 1, printk (fmt=0xffff8000112fc130 "\001\066sysrq: HELP : ")
>     at /home/liwei/main_code/linux/kernel/printk/printk.c:2076
> 2076    {
> (gdb) stepi
> 0xffff800010148750      2076    {
> (gdb) stepi
> 0xffff800010148750      2076    {
> (gdb) stepi
> 0xffff800010148750      2076    {
> (gdb)
>
> After this patch:
> * kdb:
> Entering kdb (current=0xffff8000119d2dc0, pid 0) on processor 0 due to Keyboard Entry
> [0]kdb> bp printk
> Instruction(i) BP #0 at 0xffff80001014874c (printk)
>     is enabled   addr at ffff80001014874c, hardtype=0 installed=0
>
> [0]kdb> g
>
> / # echo h > /proc/sysrq-trigger
>
> Entering kdb (current=0xffff0000fa800040, pid 264) on processor 2 due to Breakpoint @ 0xffff80001014874c
> [2]kdb> ss
>
> Entering kdb (current=0xffff0000fa800040, pid 264) on processor 2 due to SS trap @ 0xffff800010148750
> [2]kdb> ss
>
> Entering kdb (current=0xffff0000fa800040, pid 264) on processor 2 due to SS trap @ 0xffff800010148754
> [2]kdb> ss
>
> Entering kdb (current=0xffff0000fa800040, pid 264) on processor 2 due to SS trap @ 0xffff800010148758
> [2]kdb>
>
> * kgdb:
> (gdb) target remote 127.1:23002
> Remote debugging using 127.1:23002
> arch_kgdb_breakpoint () at /home/liwei/main_code/linux/arch/arm64/include/asm/kgdb.h:21
> 21              asm ("brk %0" : : "I" (KGDB_COMPILED_DBG_BRK_IMM));
> (gdb) b printk
> Breakpoint 1 at 0xffff80001014874c: file /home/liwei/main_code/linux/kernel/printk/printk.c, line 2076.
> (gdb) c
> Continuing.
> [New Thread 281]
> [New Thread 280]
> [Switching to Thread 281]
>
> Thread 174 hit Breakpoint 1, printk (fmt=0xffff8000112fc138 "\001\066sysrq: HELP : ")
>     at /home/liwei/main_code/linux/kernel/printk/printk.c:2076
> 2076    {
> (gdb) stepi
> 0xffff800010148750      2076    {
> (gdb) stepi
> 2080            va_start(args, fmt);
> (gdb) stepi
> 0xffff800010148758      2080            va_start(args, fmt);
> (gdb)
>
> Fixes: 44679a4f142b ("arm64: KGDB: Add step debugging support")
> Signed-off-by: Wei Li <liwei391@xxxxxxxxxx>
> ---
>  arch/arm64/include/asm/debug-monitors.h | 2 ++
>  arch/arm64/kernel/debug-monitors.c      | 2 +-
>  arch/arm64/kernel/kgdb.c                | 3 ++-
>  3 files changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/include/asm/debug-monitors.h b/arch/arm64/include/asm/debug-monitors.h
> index b62469f3475b..a48b507c89ee 100644
> --- a/arch/arm64/include/asm/debug-monitors.h
> +++ b/arch/arm64/include/asm/debug-monitors.h
> @@ -78,6 +78,8 @@ struct step_hook {
>         int (*fn)(struct pt_regs *regs, unsigned int esr);
>  };
>
> +void set_regs_spsr_ss(struct pt_regs *regs);
> +
>  void register_user_step_hook(struct step_hook *hook);
>  void unregister_user_step_hook(struct step_hook *hook);
>
> diff --git a/arch/arm64/kernel/debug-monitors.c b/arch/arm64/kernel/debug-monitors.c
> index 25ce6b5a52d2..7a58233677de 100644
> --- a/arch/arm64/kernel/debug-monitors.c
> +++ b/arch/arm64/kernel/debug-monitors.c
> @@ -141,7 +141,7 @@ postcore_initcall(debug_monitors_init);
>  /*
>   * Single step API and exception handling.
>   */
> -static void set_regs_spsr_ss(struct pt_regs *regs)
> +void set_regs_spsr_ss(struct pt_regs *regs)
>  {
>         regs->pstate |= DBG_SPSR_SS;
>  }
> diff --git a/arch/arm64/kernel/kgdb.c b/arch/arm64/kernel/kgdb.c
> index 3910ac06c261..093ad9d2e5e6 100644
> --- a/arch/arm64/kernel/kgdb.c
> +++ b/arch/arm64/kernel/kgdb.c
> @@ -230,7 +230,8 @@ int kgdb_arch_handle_exception(int exception_vector, int signo,
>                         kernel_prepare_single_step(&per_cpu(kgdb_ss_flags,
>                                         raw_smp_processor_id()), linux_regs);
>                         kernel_enable_single_step(linux_regs);
> -               }
> +               } else
> +                       set_regs_spsr_ss(linux_regs);

One slight nit is that my personal preference is that if one half of
an "if/else" needs braces then both halves should have braces.  I
don't know what Catalin and Will's policies are, though.

Other than that, this seems right to me.  I will leave it to the
Catalin and Will folks to say if they'd rather have this call made
from a different place or if they're happy with where you've put it.

Reviewed-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
Tested-by: Douglas Anderson <dianders@xxxxxxxxxxxx>