Re: [RFC V1 6/6] powerpc: Enable Generic Entry/Exit for syscalls.

Shrikanth Hegde
2025-04-29T06:12:04Z

On 4/28/25 20:52, Mukesh Kumar Chaurasiya wrote:
> Enable the syscall entry and exit path from generic framework.
>
> Signed-off-by: Mukesh Kumar Chaurasiya <mchauras@linux.ibm.com>

Hi Mukesh. Thanks for working on this. Trying to go through it.

> ---
> arch/powerpc/Kconfig | 1 +
> arch/powerpc/kernel/interrupt.c | 46 +++++++----
> arch/powerpc/kernel/ptrace/ptrace.c | 103 ------------------------
> arch/powerpc/kernel/signal.c | 8 +-
> arch/powerpc/kernel/syscall.c | 117 +---------------------------
> 5 files changed, 38 insertions(+), 237 deletions(-)
>
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index 6722625a406a0..45b70ccf7c89e 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -202,6 +202,7 @@ config PPC
> select GENERIC_CPU_AUTOPROBE
> select GENERIC_CPU_VULNERABILITIES if PPC_BARRIER_NOSPEC
> select GENERIC_EARLY_IOREMAP
> + select GENERIC_ENTRY

IMHO this should be enabled after irqentry_* is supported too.

> select GENERIC_GETTIMEOFDAY
> select GENERIC_IDLE_POLL_SETUP
> select GENERIC_IOREMAP
> diff --git a/arch/powerpc/kernel/interrupt.c b/arch/powerpc/kernel/interrupt.c
> index 7f31f3fb9c1d8..8731064631de0 100644
> --- a/arch/powerpc/kernel/interrupt.c
> +++ b/arch/powerpc/kernel/interrupt.c
> @@ -1,6 +1,7 @@
> // SPDX-License-Identifier: GPL-2.0-or-later
>
> #include <linux/context_tracking.h>
> +#include <linux/entry-common.h>
> #include <linux/err.h>
> #include <linux/compat.h>
> #include <linux/rseq.h>
> @@ -163,15 +164,10 @@ notrace unsigned long syscall_exit_prepare(unsigned long r3,
> unsigned long ret = 0;
> bool is_not_scv = !IS_ENABLED(CONFIG_PPC_BOOK3S_64) || !scv;
>
> - CT_WARN_ON(ct_state() == CT_STATE_USER);
> -
> kuap_assert_locked();
>
> regs->result = r3;
>
> - /* Check whether the syscall is issued inside a restartable sequence */
> - rseq_syscall(regs);
> -
> ti_flags = read_thread_flags();
>
> if (unlikely(r3 >= (unsigned long)-MAX_ERRNO) && is_not_scv) {
> @@ -192,13 +188,27 @@ notrace unsigned long syscall_exit_prepare(unsigned long r3,
> }
>
> if (unlikely(ti_flags & _TIF_SYSCALL_DOTRACE)) {
> - do_syscall_trace_leave(regs);
> ret |= _TIF_RESTOREALL;
> }
>
> - local_irq_disable();
> - ret = interrupt_exit_user_prepare_main(ret, regs);

This loop i dont understand why.

> +again:
> + syscall_exit_to_user_mode(regs);

exit_to_user_mode also calls user_enter_irqoff.
so below user_enter_irqoff may be a nop or wrong.

also, syscall_exit_to_user_mode disables local interrupts first.

is local_irq_enable fixing something here?

> +
> + user_enter_irqoff();
> + if (!prep_irq_for_enabled_exit(true)) {
> + user_exit_irqoff();
> + local_irq_enable();
> + local_irq_disable();
> + goto again;
> + }
> +
> + /* Restore user access locks last */
> + kuap_user_restore(regs);
>
> + if (unlikely((local_paca->generic_fw_flags & GFW_RESTORE_ALL) == GFW_RESTORE_ALL)) {
> + ret |= _TIF_RESTOREALL;
> + local_paca->generic_fw_flags &= ~GFW_RESTORE_ALL;
> + }
> #ifdef CONFIG_PPC64
> regs->exit_result = ret;
> #endif
> @@ -209,6 +219,7 @@ notrace unsigned long syscall_exit_prepare(unsigned long r3,
> #ifdef CONFIG_PPC64
> notrace unsigned long syscall_exit_restart(unsigned long r3, struct pt_regs *regs)
> {
> + unsigned long ret = 0;
> /*
> * This is called when detecting a soft-pending interrupt as well as
> * an alternate-return interrupt. So we can't just have the alternate
> @@ -222,14 +233,23 @@ notrace unsigned long syscall_exit_restart(unsigned long r3, struct pt_regs *reg
> #ifdef CONFIG_PPC_BOOK3S_64
> set_kuap(AMR_KUAP_BLOCKED);
> #endif
> +again:
> + syscall_exit_to_user_mode(regs);
>
> - trace_hardirqs_off();
> - user_exit_irqoff();
> - account_cpu_user_entry();
> + user_enter_irqoff();
> + if (!prep_irq_for_enabled_exit(true)) {
> + user_exit_irqoff();
> + local_irq_enable();
> + local_irq_disable();
> + goto again;
> + }
>
> - BUG_ON(!user_mode(regs));
> + if (unlikely((local_paca->generic_fw_flags & GFW_RESTORE_ALL) == GFW_RESTORE_ALL)) {
> + ret = _TIF_RESTOREALL;
> + local_paca->generic_fw_flags &= ~GFW_RESTORE_ALL;
> + }
>
> - regs->exit_result = interrupt_exit_user_prepare_main(regs->exit_result, regs);
> + regs->exit_result |= ret;
>
> return regs->exit_result;
> }
> diff --git a/arch/powerpc/kernel/ptrace/ptrace.c b/arch/powerpc/kernel/ptrace/ptrace.c
> index c6997df632873..2a2b0b94a3eaa 100644
> --- a/arch/powerpc/kernel/ptrace/ptrace.c
> +++ b/arch/powerpc/kernel/ptrace/ptrace.c
> @@ -21,9 +21,6 @@
> #include <asm/switch_to.h>
> #include <asm/debug.h>
>
> -#define CREATE_TRACE_POINTS
> -#include <trace/events/syscalls.h>
> -
> #include "ptrace-decl.h"
>
> /*
> @@ -233,106 +230,6 @@ static int do_seccomp(struct pt_regs *regs)
> static inline int do_seccomp(struct pt_regs *regs) { return 0; }
> #endif /* CONFIG_SECCOMP */
>
> -/**
> - * do_syscall_trace_enter() - Do syscall tracing on kernel entry.
> - * @regs: the pt_regs of the task to trace (current)
> - *
> - * Performs various types of tracing on syscall entry. This includes seccomp,
> - * ptrace, syscall tracepoints and audit.
> - *
> - * The pt_regs are potentially visible to userspace via ptrace, so their
> - * contents is ABI.
> - *
> - * One or more of the tracers may modify the contents of pt_regs, in particular
> - * to modify arguments or even the syscall number itself.
> - *
> - * It's also possible that a tracer can choose to reject the system call. In
> - * that case this function will return an illegal syscall number, and will put
> - * an appropriate return value in regs->r3.
> - *
> - * Return: the (possibly changed) syscall number.
> - */
> -long do_syscall_trace_enter(struct pt_regs *regs)
> -{
> - u32 flags;
> -
> - flags = read_thread_flags() & (_TIF_SYSCALL_EMU | _TIF_SYSCALL_TRACE);
> -
> - if (flags) {
> - int rc = ptrace_report_syscall_entry(regs);
> -
> - if (unlikely(flags & _TIF_SYSCALL_EMU)) {
> - /*
> - * A nonzero return code from
> - * ptrace_report_syscall_entry() tells us to prevent
> - * the syscall execution, but we are not going to
> - * execute it anyway.
> - *
> - * Returning -1 will skip the syscall execution. We want
> - * to avoid clobbering any registers, so we don't goto
> - * the skip label below.
> - */
> - return -1;
> - }
> -
> - if (rc) {
> - /*
> - * The tracer decided to abort the syscall. Note that
> - * the tracer may also just change regs->gpr[0] to an
> - * invalid syscall number, that is handled below on the
> - * exit path.
> - */
> - goto skip;
> - }
> - }
> -
> - /* Run seccomp after ptrace; allow it to set gpr[3]. */
> - if (do_seccomp(regs))
> - return -1;
> -
> - /* Avoid trace and audit when syscall is invalid. */
> - if (regs->gpr[0] >= NR_syscalls)
> - goto skip;
> -
> - if (unlikely(test_thread_flag(TIF_SYSCALL_TRACEPOINT)))
> - trace_sys_enter(regs, regs->gpr[0]);
> -
> - if (!is_32bit_task())
> - audit_syscall_entry(regs->gpr[0], regs->gpr[3], regs->gpr[4],
> - regs->gpr[5], regs->gpr[6]);
> - else
> - audit_syscall_entry(regs->gpr[0],
> - regs->gpr[3] & 0xffffffff,
> - regs->gpr[4] & 0xffffffff,
> - regs->gpr[5] & 0xffffffff,
> - regs->gpr[6] & 0xffffffff);
> -
> - /* Return the possibly modified but valid syscall number */
> - return regs->gpr[0];
> -
> -skip:
> - /*
> - * If we are aborting explicitly, or if the syscall number is
> - * now invalid, set the return value to -ENOSYS.
> - */
> - regs->gpr[3] = -ENOSYS;
> - return -1;
> -}
> -
> -void do_syscall_trace_leave(struct pt_regs *regs)
> -{
> - int step;
> -
> - audit_syscall_exit(regs);
> -
> - if (unlikely(test_thread_flag(TIF_SYSCALL_TRACEPOINT)))
> - trace_sys_exit(regs, regs->result);
> -
> - step = test_thread_flag(TIF_SINGLESTEP);
> - if (step || test_thread_flag(TIF_SYSCALL_TRACE))
> - ptrace_report_syscall_exit(regs, step);
> -}
> -
> void __init pt_regs_check(void);
>
> /*
> diff --git a/arch/powerpc/kernel/signal.c b/arch/powerpc/kernel/signal.c
> index 719930cf4ae1f..8e1a1b26b5eae 100644
> --- a/arch/powerpc/kernel/signal.c
> +++ b/arch/powerpc/kernel/signal.c
> @@ -6,6 +6,7 @@
> * Extracted from signal_32.c and signal_64.c
> */
>
> +#include <linux/entry-common.h>
> #include <linux/resume_user_mode.h>
> #include <linux/signal.h>
> #include <linux/uprobes.h>
> @@ -22,11 +23,6 @@
>
> #include "signal.h"
>
> -/* This will be removed */
> -#ifdef CONFIG_GENERIC_ENTRY
> -#include <linux/entry-common.h>
> -#endif /* CONFIG_GENERIC_ENTRY */
> -
> #ifdef CONFIG_VSX
> unsigned long copy_fpr_to_user(void __user *to,
> struct task_struct *task)
> @@ -374,11 +370,9 @@ void signal_fault(struct task_struct *tsk, struct pt_regs *regs,
> task_pid_nr(tsk), where, ptr, regs->nip, regs->link);
> }
>
> -#ifdef CONFIG_GENERIC_ENTRY
> void arch_do_signal_or_restart(struct pt_regs *regs)
> {
> BUG_ON(regs != current->thread.regs);
> local_paca->generic_fw_flags |= GFW_RESTORE_ALL;
> do_signal(current);
> }
> -#endif /* CONFIG_GENERIC_ENTRY */
> diff --git a/arch/powerpc/kernel/syscall.c b/arch/powerpc/kernel/syscall.c
> index 9f03a6263fb41..66fd6ca4462b0 100644
> --- a/arch/powerpc/kernel/syscall.c
> +++ b/arch/powerpc/kernel/syscall.c
> @@ -3,6 +3,7 @@
> #include <linux/compat.h>
> #include <linux/context_tracking.h>
> #include <linux/randomize_kstack.h>
> +#include <linux/entry-common.h>
>
> #include <asm/interrupt.h>
> #include <asm/kup.h>
> @@ -21,121 +22,9 @@ notrace long system_call_exception(struct pt_regs *regs, unsigned long r0)
> kuap_lock();
>
> add_random_kstack_offset();
> + r0 = syscall_enter_from_user_mode(regs, r0);
>
> - if (IS_ENABLED(CONFIG_PPC_IRQ_SOFT_MASK_DEBUG))
> - BUG_ON(irq_soft_mask_return() != IRQS_ALL_DISABLED);
> -
> - trace_hardirqs_off(); /* finish reconciling */
> -
> - CT_WARN_ON(ct_state() == CT_STATE_KERNEL);
> - user_exit_irqoff();
> -
> - BUG_ON(regs_is_unrecoverable(regs));
> - BUG_ON(!user_mode(regs));
> - BUG_ON(regs_irqs_disabled(regs));
> -
> -#ifdef CONFIG_PPC_PKEY
> - if (mmu_has_feature(MMU_FTR_PKEY)) {
> - unsigned long amr, iamr;
> - bool flush_needed = false;
> - /*
> - * When entering from userspace we mostly have the AMR/IAMR
> - * different from kernel default values. Hence don't compare.
> - */
> - amr = mfspr(SPRN_AMR);
> - iamr = mfspr(SPRN_IAMR);
> - regs->amr = amr;
> - regs->iamr = iamr;
> - if (mmu_has_feature(MMU_FTR_KUAP)) {
> - mtspr(SPRN_AMR, AMR_KUAP_BLOCKED);
> - flush_needed = true;
> - }
> - if (mmu_has_feature(MMU_FTR_BOOK3S_KUEP)) {
> - mtspr(SPRN_IAMR, AMR_KUEP_BLOCKED);
> - flush_needed = true;
> - }
> - if (flush_needed)
> - isync();
> - } else
> -#endif
> - kuap_assert_locked();
> -
> - booke_restore_dbcr0();
> -
> - account_cpu_user_entry();
> -
> - account_stolen_time();
> -
> - /*
> - * This is not required for the syscall exit path, but makes the
> - * stack frame look nicer. If this was initialised in the first stack
> - * frame, or if the unwinder was taught the first stack frame always
> - * returns to user with IRQS_ENABLED, this store could be avoided!
> - */
> - irq_soft_mask_regs_set_state(regs, IRQS_ENABLED);
> -
> - /*
> - * If system call is called with TM active, set _TIF_RESTOREALL to
> - * prevent RFSCV being used to return to userspace, because POWER9
> - * TM implementation has problems with this instruction returning to
> - * transactional state. Final register values are not relevant because
> - * the transaction will be aborted upon return anyway. Or in the case
> - * of unsupported_scv SIGILL fault, the return state does not much
> - * matter because it's an edge case.
> - */
> - if (IS_ENABLED(CONFIG_PPC_TRANSACTIONAL_MEM) &&
> - unlikely(MSR_TM_TRANSACTIONAL(regs->msr)))
> - set_bits(_TIF_RESTOREALL, &current_thread_info()->flags);
> -
> - /*
> - * If the system call was made with a transaction active, doom it and
> - * return without performing the system call. Unless it was an
> - * unsupported scv vector, in which case it's treated like an illegal
> - * instruction.
> - */
> -#ifdef CONFIG_PPC_TRANSACTIONAL_MEM
> - if (unlikely(MSR_TM_TRANSACTIONAL(regs->msr)) &&
> - !trap_is_unsupported_scv(regs)) {
> - /* Enable TM in the kernel, and disable EE (for scv) */
> - hard_irq_disable();
> - mtmsr(mfmsr() | MSR_TM);
> -
> - /* tabort, this dooms the transaction, nothing else */
> - asm volatile(".long 0x7c00071d | ((%0) << 16)"
> - :: "r"(TM_CAUSE_SYSCALL|TM_CAUSE_PERSISTENT));
> -
> - /*
> - * Userspace will never see the return value. Execution will
> - * resume after the tbegin. of the aborted transaction with the
> - * checkpointed register state. A context switch could occur
> - * or signal delivered to the process before resuming the
> - * doomed transaction context, but that should all be handled
> - * as expected.
> - */
> - return -ENOSYS;
> - }
> -#endif // CONFIG_PPC_TRANSACTIONAL_MEM
> -
> - local_irq_enable();
> -
> - if (unlikely(read_thread_flags() & _TIF_SYSCALL_DOTRACE)) {
> - if (unlikely(trap_is_unsupported_scv(regs))) {
> - /* Unsupported scv vector */
> - _exception(SIGILL, regs, ILL_ILLOPC, regs->nip);
> - return regs->gpr[3];
> - }
> - /*
> - * We use the return value of do_syscall_trace_enter() as the
> - * syscall number. If the syscall was rejected for any reason
> - * do_syscall_trace_enter() returns an invalid syscall number
> - * and the test against NR_syscalls will fail and the return
> - * value to be used is in regs->gpr[3].
> - */
> - r0 = do_syscall_trace_enter(regs);
> - if (unlikely(r0 >= NR_syscalls))
> - return regs->gpr[3];
> -
> - } else if (unlikely(r0 >= NR_syscalls)) {
> + if (unlikely(r0 >= NR_syscalls)) {
> if (unlikely(trap_is_unsupported_scv(regs))) {
> /* Unsupported scv vector */
> _exception(SIGILL, regs, ILL_ILLOPC, regs->nip);