[RFC v2] arm64: kgdb: fix single stepping
Vijay Kilari
vijay.kilari at gmail.com
Mon Sep 29 04:58:53 PDT 2014
Hi Akashi,
On Fri, Sep 26, 2014 at 5:24 PM, AKASHI Takahiro
<takahiro.akashi at linaro.org> wrote:
> I tried to verify kgdb in vanilla kernel on fast model, but it seems that
> the single stepping with kgdb doesn't work correctly since its first
> appearance at v3.15.
>
> On v3.15, 'stepi' command after breaking the kernel at some breakpoint
> steps forward to the next instruction, but the succeeding 'stepi' never
> goes beyond that.
> On v3.16, 'stepi' moves forward and stops at the next instruction just
> after enable_dbg in el1_dbg, and never goes beyond that. This variance of
> behavior seems to come in with the following patch in v3.16:
>
> commit 2a2830703a23 ("arm64: debug: avoid accessing mdscr_el1 on fault
> paths where possible")
>
> This patch
> (1) moves kgdb_disable_single_step() from 'c' command handling to single
> step handler.
> This makes sure that single stepping gets effective at every 's' command.
> Please note that, under the current implementation, single step bit in
> spsr, which is cleared by the first single stepping, will not be set
> again for the consecutive 's' commands because single step bit in mdscr
> is still kept on (that is, kernel_active_single_step() in
> kgdb_arch_handle_exception() is true).
Have you please check the functionality by running KGDB test suit
with multicores?
> (2) removes 'enable_dbg' in el1_dbg.
> Single step bit in mdscr is turned on in do_handle_exception()->
> kgdb_handle_expection() before returning to debugged context, and if
> debug exception is enabled in el1_dbg, we will see unexpected single-
> stepping in el1_dbg.
> (3) masks interrupts while single-stepping one instruction.
> If an interrupt is caught during processing a single-stepping, debug
> exception is unintentionally enabled by el1_irq's 'enable_dbg' before
> returning to debugged context.
> Thus, like in (2), we will see unexpected single-stepping in el1_irq.
>
> Basically (1) is for v3.15, (2) and (3) with (1) for v3.16.
>
> With those changes, we will see another problem if a breakpoint is set
> at interrupt-sensible places, like gic_handle_irq():
>
> KGDB: re-enter error: breakpoint removed ffffffc000081258
> ------------[ cut here ]------------
> WARNING: CPU: 0 PID: 650 at kernel/debug/debug_core.c:435
> kgdb_handle_exception+0x1dc/0x1f4()
> Modules linked in:
> CPU: 0 PID: 650 Comm: sh Not tainted 3.17.0-rc2+ #177
> Call trace:
> [<ffffffc000087fac>] dump_backtrace+0x0/0x130
> [<ffffffc0000880ec>] show_stack+0x10/0x1c
> [<ffffffc0004d683c>] dump_stack+0x74/0xb8
> [<ffffffc0000ab824>] warn_slowpath_common+0x8c/0xb4
> [<ffffffc0000ab90c>] warn_slowpath_null+0x14/0x20
> [<ffffffc000121bfc>] kgdb_handle_exception+0x1d8/0x1f4
> [<ffffffc000092ffc>] kgdb_brk_fn+0x18/0x28
> [<ffffffc0000821c8>] brk_handler+0x9c/0xe8
> [<ffffffc0000811e8>] do_debug_exception+0x3c/0xac
> Exception stack(0xffffffc07e027650 to 0xffffffc07e027770)
> ...
> [<ffffffc000083cac>] el1_dbg+0x14/0x68
> [<ffffffc00012178c>] kgdb_cpu_enter+0x464/0x5c0
> [<ffffffc000121bb4>] kgdb_handle_exception+0x190/0x1f4
> [<ffffffc000092ffc>] kgdb_brk_fn+0x18/0x28
> [<ffffffc0000821c8>] brk_handler+0x9c/0xe8
> [<ffffffc0000811e8>] do_debug_exception+0x3c/0xac
> Exception stack(0xffffffc07e027ac0 to 0xffffffc07e027be0)
> ...
> [<ffffffc000083cac>] el1_dbg+0x14/0x68
> [<ffffffc00032e4b4>] __handle_sysrq+0x11c/0x190
> [<ffffffc00032e93c>] write_sysrq_trigger+0x4c/0x60
> [<ffffffc0001e7d58>] proc_reg_write+0x54/0x84
> [<ffffffc000192fa4>] vfs_write+0x98/0x1c8
> [<ffffffc0001939b0>] SyS_write+0x40/0xa0
>
> Once some interrupt occurs, a breakpoint at gic_handle_irq() triggers kgdb.
> Kgdb then calls kgdb_roundup_cpus() to sync with other cpus.
> Current kgdb_roundup_cpus() unmasks interrupts temporarily to
> use smp_call_function().
> This eventually allows another interrupt to occur and likely results in
> hitting a breakpoint at gic_handle_irq() again since debug exception is
> always enabled in el1_irq.
>
> We can avoid this issue by specifying "nokgdbroundup" in kernel parameter,
> but this will also leave other cpus be in unknown state in terms of kgdb,
> and may result in interfering with kgdb activity.
>
> Signed-off-by: AKASHI Takahiro <takahiro.akashi at linaro.org>
> ---
> arch/arm64/kernel/entry.S | 1 -
> arch/arm64/kernel/kgdb.c | 29 +++++++++++++++++++----------
> 2 files changed, 19 insertions(+), 11 deletions(-)
>
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index fdd6eae..a935d5f 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -325,7 +325,6 @@ el1_dbg:
> mrs x0, far_el1
> mov x2, sp // struct pt_regs
> bl do_debug_exception
> - enable_dbg
> kernel_exit 1
> el1_inv:
> // TODO: add support for undefined instructions in kernel mode
> diff --git a/arch/arm64/kernel/kgdb.c b/arch/arm64/kernel/kgdb.c
> index 75c9cf1..f1fc1d8 100644
> --- a/arch/arm64/kernel/kgdb.c
> +++ b/arch/arm64/kernel/kgdb.c
> @@ -22,6 +22,7 @@
> #include <linux/irq.h>
> #include <linux/kdebug.h>
> #include <linux/kgdb.h>
> +#include <asm/percpu.h>
> #include <asm/traps.h>
>
> struct dbg_reg_def_t dbg_reg_def[DBG_MAX_REG_NUM] = {
> @@ -95,6 +96,8 @@ struct dbg_reg_def_t dbg_reg_def[DBG_MAX_REG_NUM] = {
> { "fpcr", 4, -1 },
> };
>
> +static DEFINE_PER_CPU(unsigned int, kgdb_pstate);
> +
> char *dbg_get_reg(int regno, void *mem, struct pt_regs *regs)
> {
> if (regno >= DBG_MAX_REG_NUM || regno < 0)
> @@ -176,18 +179,14 @@ int kgdb_arch_handle_exception(int exception_vector, int signo,
> * over and over again.
> */
> kgdb_arch_update_addr(linux_regs, remcom_in_buffer);
> - atomic_set(&kgdb_cpu_doing_single_step, -1);
> - kgdb_single_step = 0;
> -
> - /*
> - * Received continue command, disable single step
> - */
> - if (kernel_active_single_step())
> - kernel_disable_single_step();
>
> err = 0;
> break;
> case 's':
> + /* mask interrupts while single stepping */
> + __this_cpu_write(kgdb_pstate, linux_regs->pstate);
> + linux_regs->pstate |= (1 << 7);
Hard coded values.
> +
> /*
> * Update step address value with address passed
> * with step packet.
> @@ -198,8 +197,6 @@ int kgdb_arch_handle_exception(int exception_vector, int signo,
> */
> kgdb_arch_update_addr(linux_regs, remcom_in_buffer);
> atomic_set(&kgdb_cpu_doing_single_step, raw_smp_processor_id());
> - kgdb_single_step = 1;
why kgdb_single_step is not set?
> -
> /*
> * Enable single step handling
> */
> @@ -229,6 +226,18 @@ static int kgdb_compiled_brk_fn(struct pt_regs *regs, unsigned int esr)
>
> static int kgdb_step_brk_fn(struct pt_regs *regs, unsigned int esr)
> {
> + unsigned int pstate;
> +
> + kernel_disable_single_step();
> + atomic_set(&kgdb_cpu_doing_single_step, -1);
> +
> + /* restore interrupt mask status */
> + pstate = __this_cpu_read(kgdb_pstate);
> + if (pstate & (1 << 7))
> + regs->pstate |= (1 << 7);
> + else
> + regs->pstate &= ~(1 << 7);
> +
Same as above comment
> kgdb_handle_exception(1, SIGTRAP, 0, regs);
> return 0;
> }
> --
> 1.7.9.5
>
>
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