[RFC v2] arm64: kgdb: fix single stepping

From: AKASHI Takahiro
Date: Fri Sep 26 2014 - 07:54:57 EST

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).
(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@xxxxxxxxxx>
---
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);
+
/*
* 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;
-
/*
* 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);
+
kgdb_handle_exception(1, SIGTRAP, 0, regs);
return 0;
}
--
1.7.9.5

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