Re: [PATCH v15 10/11] arm64: entry: Convert to generic entry

[Ada Couprie Diaz <ada.coupriediaz@arm.com>]

On 11/05/2026 10:21, Jinjie Ruan wrote:
> Implement the generic entry framework for arm64 to handle system call
> entry and exit. This follows the migration of x86, RISC-V, and LoongArch,
> consolidating architecture-specific syscall tracing and auditing into
> the common kernel entry infrastructure.
If I understand correctly, as Syscall User Dispatch is gated being
`CONFIG_GENERIC_ENTRY` only and handled via ptrace, this patch
effectively enables Syscall User Dispatch for arm64.
I think it would be great to mention it here explicitly !
>
> [Background]
> Arm64 has already adopted generic IRQ entry. Completing the conversion
> to the generic syscall entry framework reduces architectural divergence,
> simplifies maintenance, and allows arm64 to automatically benefit from
> improvements in the common entry code.
>
> [Changes]
>
> 1. Kconfig and Infrastructure:
> - Select GENERIC_ENTRY and remove GENERIC_IRQ_ENTRY (now implied).
>
> - Migrate struct thread_info to use the syscall_work field instead
>    of TIF flags for syscall-related tasks.
>
> 2. Thread Info and Flags:
> - Remove definitions for TIF_SYSCALL_TRACE, TIF_SYSCALL_AUDIT,
>    TIF_SYSCALL_TRACEPOINT, TIF_SECCOMP, and TIF_SYSCALL_EMU.
>
> - Replace _TIF_SYSCALL_WORK and _TIF_SYSCALL_EXIT_WORK with the
>    generic SYSCALL_WORK bitmask.
>
> - Map single-step state to SYSCALL_EXIT_TRAP in debug-monitors.c.
>
> 3. Architecture-Specific Hooks (asm/entry-common.h):
> - Implement arch_ptrace_report_syscall_entry() and _exit() by
>    porting the existing arm64 logic to the generic interface.
>
> - Add arch_syscall_is_vdso_sigreturn() to asm/syscall.h to
>    support Syscall User Dispatch (SUD).
Related to the above : I feel this is missing an important information.
Given that SUD is only controlled by `CONFIG_GENERIC_ENTRY`,
converting to generic entry _requires_ supporting SUD, so we do it here.
I think this would be important to mention, as I otherwise felt like this
change did not belong in this patch.

General question that follows : does it make sense to require an arch
to support Syscall User Dispatch to be able to convert to generic entry ?
(I assume not really, given that only `arch_syscall_is_vdso_sigreturn()` is
required on the arch side, but I am curious)

>
> 4. Cleanup and Refactoring:
> - Remove redundant arm64-specific syscall tracing functions from
>    ptrace.c, including syscall_trace_enter(), syscall_exit_work(),
>    and related audit/step helpers.
>
> - Update el0_svc_common() in syscall.c to use the generic
>    syscall_work checks and entry/exit call sites.
>
> [Why this matters]
> - Unified Interface: Aligns arm64 with the modern kernel entry standard.
>
> - Improved Maintainability: Bug fixes in kernel/entry/common.c now
>    apply to arm64 automatically.
>
> - Feature Readiness: Simplifies the implementation of future
>    cross-architecture syscall features.
>
> [Compatibility]
> This conversion maintains full ABI compatibility with existing
> userspace. The ptrace register-saving behavior, seccomp filtering, and
> syscall tracing semantics remain identical to the previous implementation.
I agree, would it make sense to mention that there is no change related
to RSEQ as arm64 does not have `HAVE_GENERIC_TIF_BITS` ? As that is
part of generic entry, but is indeed a no-op for us.
>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Thomas Gleixner <tglx@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Reviewed-by: Linus Walleij <linusw@kernel.org>
> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Reviewed-by: Yeoreum Yun <yeoreum.yun@arm.com>
> Reviewed-by: Kevin Brodsky <kevin.brodsky@arm.com>
> Suggested-by: Kevin Brodsky <kevin.brodsky@arm.com>
> Suggested-by: Mark Rutland <mark.rutland@arm.com>
> Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
> ---
> [...]
> diff --git a/arch/arm64/kernel/debug-monitors.c b/arch/arm64/kernel/debug-monitors.c
> index 29307642f4c9..e67643a70405 100644
> --- a/arch/arm64/kernel/debug-monitors.c
> +++ b/arch/arm64/kernel/debug-monitors.c
> @@ -385,11 +385,18 @@ void user_enable_single_step(struct task_struct *task)
>   
>   	if (!test_and_set_ti_thread_flag(ti, TIF_SINGLESTEP))
>   		set_regs_spsr_ss(task_pt_regs(task));
> +
> +	/*
> +	 * Ensure that a trap is triggered once stepping out of a system
> +	 * call prior to executing any user instruction.
> +	 */
I was a bit confused by the comment in isolation at first : we already
have a signal that we are stepping and would need a trap, `TIF_SINGLESTEP`.
Would it make sense to mention here that this is for/handled by the generic
entry code ?
Something along the lines of "[...], as the generic entry code does not
check for `TIF_SINGLESTEP`.", or "Ensure that the generic entry code
triggers a trap [...]", if you think its useful ?
> +	set_task_syscall_work(task, SYSCALL_EXIT_TRAP);
>   }
>   NOKPROBE_SYMBOL(user_enable_single_step);
>   
>   void user_disable_single_step(struct task_struct *task)
>   {
>   	clear_ti_thread_flag(task_thread_info(task), TIF_SINGLESTEP);
> +	clear_task_syscall_work(task, SYSCALL_EXIT_TRAP);
>   }
>   NOKPROBE_SYMBOL(user_disable_single_step);

Apart from my minor nitpicks :

Reviewed-by: Ada Couprie Diaz <ada.coupriediaz@arm.com>

Thanks,
Ada