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Message-ID: <ZtV-Lkfrea56egPc@J2N7QTR9R3>
Date: Mon, 2 Sep 2024 09:58:22 +0100
From: Mark Rutland <mark.rutland@....com>
To: Liao Chang <liaochang1@...wei.com>
Cc: catalin.marinas@....com, will@...nel.org, mhiramat@...nel.org,
oleg@...hat.com, peterz@...radead.org, maz@...nel.org,
oliver.upton@...ux.dev, ardb@...nel.org, anshuman.khandual@....com,
ptosi@...gle.com, mcgrof@...nel.org, rppt@...nel.org,
kristina.martsenko@....com, linux-arm-kernel@...ts.infradead.org,
linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH v2] arm64: Replace linked list with switch statement for
breakpoint handlers
On Sat, Aug 31, 2024 at 06:41:41AM +0000, Liao Chang wrote:
> v2->v1:
> Fix a bug of releasing spinlock in kgdb_arch_exit().
>
> As suggested by Mark Rutland [0], this patch remove the linked list used
> for breakpoint handlers and instroduces a switch statement based on the
> immediate value.
>
> To minimize the code duplication, a new enum break_hook_type is defined
> to encapsulate all supported breakpoint types, used as indices in switch
> statement. Macros iterate_break_hook() and __HOOK() generate a series of
> switch statements for getting/setting handlers based on the immediate
> value from ESR or the imm field of struct break_hook.
>
> After that, All breakpint related immediate macros must now end with
> _BRK_IMM, and breakpoint related mask macros must end with _BRK_MASK.
>
> A new user field is added to struct break_hook to separate user and
> kernel breakpint exceptions. This force the callers to be explicit about
> the exception level that they come from [1]. Currently, only the uprobe
> uses this field.
>
> To prevent race condition during break_hook unregistration (primarily by
> KGDB), a spinlock is defined in kgdb code. While there's still a
> potential race where a CPU might be executing KGDB break_hook while
> another CPU unregister it. this issue existed even before this
> refactoring.
>
> [0] https://lore.kernel.org/all/Zs3LnYkXL5sg2yBH@J2N7QTR9R3.cambridge.arm.com/
> [1] https://lore.kernel.org/all/20190301132809.24653-7-will.deacon@arm.com/
>
> Suggested-by: Mark Rutland <mark.rutland@....com>
> Signed-off-by: Liao Chang <liaochang1@...wei.com>
This isn't what I meant; I want to get rid of the break_hook entirely,
and have something like early_brk64() be used consistently, e.g.
| bool __handle_el1_brk(struct pt_regs *regs, unsigned long esr)
| {
| u16 imm = esr_brk_comment(esr);
|
| switch (imm) {
| case BRK_IMM_CFI_BASE ... (BRK_IMM_CFI_BASE | BRK_IMM_CFI_MASK):
| return brk_cfi_handler(regs, esr);
| case BRK_IMM_KASAN_BASE ... (BRK_IMM_KASAN_BASE | BRK_IMM_KASAN_MASK):
| return brk_kasan_handler(regs, esr);
|
| [ ... other BRK IMMs handled here ... ]
|
| default:
| return false;
| }
| }
|
| void handle_el1_brk(struct pt_regs *regs, unsigned long esr)
| {
| if (__handle_el1_brk(regs, esr))
| return;
|
| [ ... handler failure here ... ]
| }
... with a separate path for EL0 which only needs to handle uprobes.
As I mentioned, last I looked at that it required some rework of kgdb
and some larger structural rework. I had meant that KGDB would have to
handle the hooks always being in place.
Overall that should remove code since all the register_*_hook()
functions and related bits could be deleted.
As-is, I don't think this patch is the way to go.
Mark.
> ---
> arch/arm64/include/asm/brk-imm.h | 6 +-
> arch/arm64/include/asm/debug-monitors.h | 7 +-
> arch/arm64/include/asm/esr.h | 2 +-
> arch/arm64/kernel/debug-monitors.c | 110 ++++++++++++++++++------
> arch/arm64/kernel/kgdb.c | 6 ++
> arch/arm64/kernel/probes/kprobes.c | 2 +-
> arch/arm64/kernel/probes/uprobes.c | 3 +-
> arch/arm64/kernel/traps.c | 4 +-
> 8 files changed, 100 insertions(+), 40 deletions(-)
>
> diff --git a/arch/arm64/include/asm/brk-imm.h b/arch/arm64/include/asm/brk-imm.h
> index beb42c62b6ac..a0c2ae4386ab 100644
> --- a/arch/arm64/include/asm/brk-imm.h
> +++ b/arch/arm64/include/asm/brk-imm.h
> @@ -23,7 +23,7 @@
> */
> #define KPROBES_BRK_IMM 0x004
> #define UPROBES_BRK_IMM 0x005
> -#define KPROBES_BRK_SS_IMM 0x006
> +#define KPROBES_SS_BRK_IMM 0x006
> #define KRETPROBES_BRK_IMM 0x007
> #define FAULT_BRK_IMM 0x100
> #define KGDB_DYN_DBG_BRK_IMM 0x400
> @@ -36,7 +36,7 @@
>
> #define CFI_BRK_IMM_TARGET GENMASK(4, 0)
> #define CFI_BRK_IMM_TYPE GENMASK(9, 5)
> -#define CFI_BRK_IMM_BASE 0x8000
> -#define CFI_BRK_IMM_MASK (CFI_BRK_IMM_TARGET | CFI_BRK_IMM_TYPE)
> +#define CFI_BRK_IMM 0x8000
> +#define CFI_BRK_MASK (CFI_BRK_IMM_TARGET | CFI_BRK_IMM_TYPE)
>
> #endif
> diff --git a/arch/arm64/include/asm/debug-monitors.h b/arch/arm64/include/asm/debug-monitors.h
> index 13d437bcbf58..8aedb7c01f69 100644
> --- a/arch/arm64/include/asm/debug-monitors.h
> +++ b/arch/arm64/include/asm/debug-monitors.h
> @@ -41,7 +41,7 @@
>
> /* kprobes BRK opcodes with ESR encoding */
> #define BRK64_OPCODE_KPROBES (AARCH64_BREAK_MON | (KPROBES_BRK_IMM << 5))
> -#define BRK64_OPCODE_KPROBES_SS (AARCH64_BREAK_MON | (KPROBES_BRK_SS_IMM << 5))
> +#define BRK64_OPCODE_KPROBES_SS (AARCH64_BREAK_MON | (KPROBES_SS_BRK_IMM << 5))
> /* uprobes BRK opcodes with ESR encoding */
> #define BRK64_OPCODE_UPROBES (AARCH64_BREAK_MON | (UPROBES_BRK_IMM << 5))
>
> @@ -74,15 +74,12 @@ void register_kernel_step_hook(struct step_hook *hook);
> void unregister_kernel_step_hook(struct step_hook *hook);
>
> struct break_hook {
> - struct list_head node;
> int (*fn)(struct pt_regs *regs, unsigned long esr);
> u16 imm;
> u16 mask; /* These bits are ignored when comparing with imm */
> + int user; /* Breakpoint exception from userspace */
> };
>
> -void register_user_break_hook(struct break_hook *hook);
> -void unregister_user_break_hook(struct break_hook *hook);
> -
> void register_kernel_break_hook(struct break_hook *hook);
> void unregister_kernel_break_hook(struct break_hook *hook);
>
> diff --git a/arch/arm64/include/asm/esr.h b/arch/arm64/include/asm/esr.h
> index 56c148890daf..0fc2710a7154 100644
> --- a/arch/arm64/include/asm/esr.h
> +++ b/arch/arm64/include/asm/esr.h
> @@ -403,7 +403,7 @@ static inline bool esr_is_data_abort(unsigned long esr)
> static inline bool esr_is_cfi_brk(unsigned long esr)
> {
> return ESR_ELx_EC(esr) == ESR_ELx_EC_BRK64 &&
> - (esr_brk_comment(esr) & ~CFI_BRK_IMM_MASK) == CFI_BRK_IMM_BASE;
> + (esr_brk_comment(esr) & ~CFI_BRK_MASK) == CFI_BRK_IMM;
> }
>
> static inline bool esr_fsc_is_translation_fault(unsigned long esr)
> diff --git a/arch/arm64/kernel/debug-monitors.c b/arch/arm64/kernel/debug-monitors.c
> index 024a7b245056..0b92a58682e2 100644
> --- a/arch/arm64/kernel/debug-monitors.c
> +++ b/arch/arm64/kernel/debug-monitors.c
> @@ -276,47 +276,103 @@ static int single_step_handler(unsigned long unused, unsigned long esr,
> }
> NOKPROBE_SYMBOL(single_step_handler);
>
> -static LIST_HEAD(user_break_hook);
> -static LIST_HEAD(kernel_break_hook);
> -
> -void register_user_break_hook(struct break_hook *hook)
> -{
> - register_debug_hook(&hook->node, &user_break_hook);
> -}
> -
> -void unregister_user_break_hook(struct break_hook *hook)
> -{
> - unregister_debug_hook(&hook->node);
> -}
> +enum break_hook_type {
> + KPROBES = 0,
> + UPROBES,
> + KPROBES_SS,
> + KRETPROBES,
> + FAULT,
> + KGDB_DYN_DBG,
> + KGDB_COMPILED_DBG,
> + BUG,
> + KASAN,
> + UBSAN,
> + CFI,
> + NR_BRK,
> +};
> +
> +static struct break_hook *break_hooks[NR_BRK];
> +
> +#define iterate_break_hook(imm) ({ \
> + switch (imm) { \
> + __HOOK(KPROBES) \
> + __HOOK(UPROBES) \
> + __HOOK(KPROBES_SS) \
> + __HOOK(KRETPROBES) \
> + __HOOK(FAULT) \
> + __HOOK(KGDB_DYN_DBG) \
> + __HOOK(KGDB_COMPILED_DBG) \
> + __HOOK(BUG) \
> + __HOOK(KASAN) \
> + __HOOK(UBSAN) \
> + __HOOK(CFI) \
> + default : \
> + break; \
> + } \
> +})
> +
> +#undef __HOOK
> +#define __HOOK(nr) \
> + case nr ## _BRK_IMM: \
> + { return break_hooks[nr]; }
> +
> +static struct break_hook *find_break_hook(unsigned long imm)
> +{
> + iterate_break_hook(imm);
> + return NULL;
> +}
> +
> +#undef __HOOK
> +#define __HOOK(nr) \
> + case nr ## _BRK_IMM: \
> + { break_hooks[nr] = hook; break; }
>
> void register_kernel_break_hook(struct break_hook *hook)
> {
> - register_debug_hook(&hook->node, &kernel_break_hook);
> + iterate_break_hook(hook->imm);
> }
>
> +#undef __HOOK
> +#define __HOOK(nr) \
> + case nr ## _BRK_IMM: \
> + { break_hooks[nr] = NULL; break; }
> +
> void unregister_kernel_break_hook(struct break_hook *hook)
> {
> - unregister_debug_hook(&hook->node);
> + iterate_break_hook(hook->imm);
> }
>
> +#undef __HOOK
> +#define __HOOK(nr) \
> + do { \
> + if (!hook) \
> + hook = find_break_hook(imm & ~(nr ## _BRK_MASK)); \
> + } while (0)
> +
> static int call_break_hook(struct pt_regs *regs, unsigned long esr)
> {
> - struct break_hook *hook;
> - struct list_head *list;
> - int (*fn)(struct pt_regs *regs, unsigned long esr) = NULL;
> + unsigned long imm = esr_brk_comment(esr);
> + struct break_hook *hook = NULL;
>
> - list = user_mode(regs) ? &user_break_hook : &kernel_break_hook;
> + hook = find_break_hook(imm);
> + if (likely(hook))
> + goto call_hook;
>
> - /*
> - * Since brk exception disables interrupt, this function is
> - * entirely not preemptible, and we can use rcu list safely here.
> - */
> - list_for_each_entry_rcu(hook, list, node) {
> - if ((esr_brk_comment(esr) & ~hook->mask) == hook->imm)
> - fn = hook->fn;
> - }
> +#ifdef CONFIG_KASAN_SW_TAGS
> + __HOOK(KASAN);
> +#endif
> +#ifdef CONFIG_UBSAN_TRAP
> + __HOOK(UBSAN);
> +#endif
> +#ifdef CONFIG_CFI_CLANG
> + __HOOK(CFI);
> +#endif
> +
> +call_hook:
> + if (hook && (user_mode(regs) == hook->user))
> + return hook->fn(regs, esr);
>
> - return fn ? fn(regs, esr) : DBG_HOOK_ERROR;
> + return DBG_HOOK_ERROR;
> }
> NOKPROBE_SYMBOL(call_break_hook);
>
> diff --git a/arch/arm64/kernel/kgdb.c b/arch/arm64/kernel/kgdb.c
> index 4e1f983df3d1..e491a5704e61 100644
> --- a/arch/arm64/kernel/kgdb.c
> +++ b/arch/arm64/kernel/kgdb.c
> @@ -304,6 +304,8 @@ static struct notifier_block kgdb_notifier = {
> .priority = -INT_MAX,
> };
>
> +static DEFINE_SPINLOCK(kgdb_hook_lock);
> +
> /*
> * kgdb_arch_init - Perform any architecture specific initialization.
> * This function will handle the initialization of any architecture
> @@ -316,9 +318,11 @@ int kgdb_arch_init(void)
> if (ret != 0)
> return ret;
>
> + spin_lock(&kgdb_hook_lock);
> register_kernel_break_hook(&kgdb_brkpt_hook);
> register_kernel_break_hook(&kgdb_compiled_brkpt_hook);
> register_kernel_step_hook(&kgdb_step_hook);
> + spin_unlock(&kgdb_hook_lock);
> return 0;
> }
>
> @@ -329,9 +333,11 @@ int kgdb_arch_init(void)
> */
> void kgdb_arch_exit(void)
> {
> + spin_lock(&kgdb_hook_lock);
> unregister_kernel_break_hook(&kgdb_brkpt_hook);
> unregister_kernel_break_hook(&kgdb_compiled_brkpt_hook);
> unregister_kernel_step_hook(&kgdb_step_hook);
> + spin_unlock(&kgdb_hook_lock);
> unregister_die_notifier(&kgdb_notifier);
> }
>
> diff --git a/arch/arm64/kernel/probes/kprobes.c b/arch/arm64/kernel/probes/kprobes.c
> index 4268678d0e86..b0e28d48bb19 100644
> --- a/arch/arm64/kernel/probes/kprobes.c
> +++ b/arch/arm64/kernel/probes/kprobes.c
> @@ -360,7 +360,7 @@ kprobe_breakpoint_ss_handler(struct pt_regs *regs, unsigned long esr)
> }
>
> static struct break_hook kprobes_break_ss_hook = {
> - .imm = KPROBES_BRK_SS_IMM,
> + .imm = KPROBES_SS_BRK_IMM,
> .fn = kprobe_breakpoint_ss_handler,
> };
>
> diff --git a/arch/arm64/kernel/probes/uprobes.c b/arch/arm64/kernel/probes/uprobes.c
> index d49aef2657cd..24130161c62d 100644
> --- a/arch/arm64/kernel/probes/uprobes.c
> +++ b/arch/arm64/kernel/probes/uprobes.c
> @@ -190,6 +190,7 @@ static int uprobe_single_step_handler(struct pt_regs *regs,
> static struct break_hook uprobes_break_hook = {
> .imm = UPROBES_BRK_IMM,
> .fn = uprobe_breakpoint_handler,
> + .user = 1,
> };
>
> /* uprobe single step handler hook */
> @@ -199,7 +200,7 @@ static struct step_hook uprobes_step_hook = {
>
> static int __init arch_init_uprobes(void)
> {
> - register_user_break_hook(&uprobes_break_hook);
> + register_kernel_break_hook(&uprobes_break_hook);
> register_user_step_hook(&uprobes_step_hook);
>
> return 0;
> diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
> index 9e22683aa921..6aefb0ba41ec 100644
> --- a/arch/arm64/kernel/traps.c
> +++ b/arch/arm64/kernel/traps.c
> @@ -1025,8 +1025,8 @@ static int cfi_handler(struct pt_regs *regs, unsigned long esr)
>
> static struct break_hook cfi_break_hook = {
> .fn = cfi_handler,
> - .imm = CFI_BRK_IMM_BASE,
> - .mask = CFI_BRK_IMM_MASK,
> + .imm = CFI_BRK_IMM,
> + .mask = CFI_BRK_MASK,
> };
> #endif /* CONFIG_CFI_CLANG */
>
> --
> 2.34.1
>
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